Contract H0308004 

A.R.F. PRODUCTS, IWC. 

BUREAU OF MCNES 

UNITED STATES QEPARTMEMT OF THE lNTzERlOW 
The views and conclusions contained in this document are 

those of the authors and should not be interpreted as necessarily representing the official policies or recommendations 

of the Interior Department's Bureau of Mines or of the U.S. 

Government. 
7. 

Dr. 

- 

Larry G. Stolarczyk 

i~o~pltp(wn~lrr-- ZO.-Wsw?m-rsr I 

, -KT DOCUMEMATION 2. RfCbwr I I L-8-m 

1 PMiL 

A.R.F. Products, Inc. 

Gardner Road 

Raton, New Mexico 87740 

A Medium Frequency Wireless ~ommunicatio~ System for 

Underground Mines 

ilkarroao 

,Se~t.l984(Date Publi 

L 

=-wmmsoo..r 

This report deals with information regarding a new Medium Frequency (MF) 

Wireless Communication System for underground mines. This new telecommunication 

system works on low loss electromagnetic wave propagation modes which allow it to 

5 

rZ*---1~~m~ 

provide quality radio communications almost anyw-here in a mine. 

Tests show the MF system can provide improved safety and managerial control, 

increased productivity, better organizational team-work and reduced maintenance 

cost; furthermore, MF1s unique design allows for easy installation at a minimum 

(0 

1Lmd-&-r 

Final - 28 DEC. 1979 

31 MAY, 1984 

cost. 

This paper covers the following: 1) MF design plan, 2) Overview of mine radio 

communications systems, 3) Preparations needed for designing an MF system, 4) MF 

equipment, 5) MF installation, 6) MF design considerations, 7) MF design specifics 

tions, and 8) Recommendations dealing with MF system. 

Experiments in four mines are illustrated to demonstrate the success and viability of the MF system. The mines range from small (500 tonslday) to large - 

(67,000 tonslday) capacity and represent several different mining methods. The 

MF Wireless Communication System represents an advancement in mining communicatior 

and the groundwork for other advanced mine systems. 

=-I. a,- 

Mines; radio communications; communications; coal mines; metallnon metal mines; 

radio propagation; tunnel mode, coal seam mode; wave propagation; underground 

Un-I fled . . I I Release Unlimited &auv-~rnyc.b) 

, Unclassified 

--a -(-alo- - - ZYXf'i 

b.mrr3- 

-I.lhe 

- - 

mine; mine rescue radio; trapped miners; radio vest transceiver; underground 

mines; medium frequency; existing wireplant; loop antennas, parasitic coupling; 

r-T-line couplers; radio, health and safety enchancement. 

I 
This report was prepared by A.R.F. Products, Inc., Raton, 

New Mexico, under USBM contract number H0308004. The contract was initiated under the Mine Health and Safety Program. It was administered under the technical direction of 

the Pittsburgh Research Center with Harry Dobroski acting as 

the Technical Project Officer. Patrick Neary was the Contract Administrator for the Bureau of Mines. 

This report is a summary of the work recently completed as a 

part of this contract during the period January 1980 to 

May 31, 1983. This report was submitted by the authors on 

August 31, 1984. 
CONTENTS 

. 

Page 

Illustrations ......................................... 10 

Tables ............................................... 14 

I . Executive summary .............................. 16 

Objective .................................... 16 

Summary of previous work ...................... 20 

MF equipment design .......................... 22 

Findings and conc'lusions ..................... 25 

Results of in-mine system tests ............ 25 

MF communication quality and range ......... 27 

Improvements in vehicular antenna 

technology ................................ 29 

Mobile transceivers ....................... 30 

RF line coupler design ..................... 31 

Cost/benef i t analysis ...................... 31 

Other important uses of -the MF system ...... 31 

1 . Determining communication requirements 

in underground mines ......................... 

Introduction ............ .................... 

Communication needs ......................... 

Organizational theory supporting 

communication needs ........................ 

Mine Organization . .......................... 

Information flow ............................ 

Benefit analysis ............................ 
CONTENTS.. Continued 

Page 

111 . .Bescription of the MF radio communication 

system ........................................ 

Introduction ................................. 

MF architecture .............................. 

Equipment .................................... 

Mobile transceivers ......................... 

Vest transceiver ....................... 

Vehicular transceiver ................... 

Fixed 1ocation.repeaters ................. 

Base station ............................. 

Coupling devices ........................... 

RF line coupler ........................... 

Antenna ................................... 

MF installation ...........................C.. 

IV . Preinstallation preparations ................. 

Introduction .............................. 

Initial radio system planning meeting ........ 

Defining communication needs ................. 

Selecting needed networks if MF radio 

appears to be the solution .................. 70 

Maintenance and production networks ........ 70 

Hoist and haulage networks ................. 71 

Emergency communications network ........... 72 
CONTENTS.. Continued 

Preparing a radio coverage area map ........... 

Insuring usage and maintenance of the system .. 

V . MF design considerations ........................ 

Introduction .................................. 

Signal propagation modes ...................... 

Coal seam propagation mode .................. 

Tunnel propagation modes .................... 

Modulation processes .......................... 

Coupling MF signals to the wireplant .......... 

Transmitting loop antenna and two-wire 

transmission line coupling ................. 

Transmitting loop antenna to receiving loop 

antenna via transmission line coupling ..... 

Attenuation rate ........................... 

Loop antenna design considerations ............ 

Theoretical basis for loop antenna design ... 

Design optimization process ................. 

Design for .operation in hazardous 

atmospheres ................................ 

Vehicular antenna area constraint ........... 

Personnel antenna area constraint ........... 

RF line coupler design considerations ......... 

Considerations of acoustical device 

selection ................................... ; . 

Microphones ................................. 

Page 

74 

75 

79 

79 

83 

83 

86 

88 

94 
CONTENTS.. Cont i nued 

Page . 

Noise cancelling microphones ............. 140 

Vest microphones ......................... 141 

VI . MF system design specifications .............. 143 

Introduction ............... ............... 143 

General system specifications .............. 144 

Standard module specifications ............. 147 

Base station and vehicular transceiver 

specifications .............................. 148 

Standard transceiver ..................... 149 

Remote control terminal (RCT) unit ....... 153 

Remote audio control head (RCH) .......... 156 

Remote control console (RCC) ............... 157 

!Vest transceiver specifications ............ 159 

Permissible battery pack ................. 165 

ART - 29 vest transceiver ............... 167 

Repeater transceiver specifications ........ 168 

RF line coupler and antenna specifications . 170 

VII . Field evaluations ............................ 172 

Introduction ............................... 172 

San Manuel copper mine (MAGMA) ............. 173 

MAGMA'S initial communications system .... 178 

Installation of MF equipment ............. 183 
CONTENTS.. Continued 

Page 

MF system tests and evaluation ............. 185 

Test no . 1 . communications from battery 

powered locomotives ..................... 185 

Test no . 2 . hoist communications test/ 

ba~se station in hoist control center .... 185 

Test no . 3 . hoist communications test/ 

base station at the collar .............. 189 

Test no . 4 . vest communications to 

audio room base station from grizzly 

and production levels ................... 190 

Conclusion ................................. 191 

Escalante silver mine ........................ 192 

Escalantels initial communication system .. : 195 

. Communication needs ........................ 195 

Installation of the MF radio system ........ 196 

MF system tests and evaluation ............. 198 

Conclusions ................................ 201 

York Canyon coal mine ...................... 201 

York Canyon mine's initial communications 

system ................................. 203 

Communications needs ........................ 209 

Installation of MF equipment ............... 204 

Test results and evaluations ............... 205 

Conclusions .......... ...................... 208 
CONTENTS.. Continued 

Page 

VIII . 

Star Point coal mine ........................ 209 

Star Point's initial communication system . 21 0 

Communication needs ....................... 210 

Installation of MF equipment .............. 211 

Test results and evaluation ............... 213 

Conclusion .............................. 214 

Recommendations ............................... 216 

ILLUSTRATIONS 

The MF underground radio system (tunnel 

mode) ......................................... 

The cellular repeater (seam mode) ............. 

Formulation of the system design 

specifications ............................... 

Typical organizational chart .................. 

Typical production profile .................... 

Radio coverage cells .......................... 

Coupling of electromagnetic signals to 

electrical conductors ........................ 

Vest radio communications with the telephone 

network or other distant vest transceiver 

via the cellular repeater .................... 

MF system architecture ........................ 

Vest transceiver .............................. 

Control head .................................. 
ILLUSTRATIONS.. Cont i nued 

Vehicular transceiver ......................... 

Remote control ................................ 

Remote control console unit ............... ... . 

Base station .................................. 

Tuned loop antenna ............................ 

Seam mode field components .................... 

Signal current (I) flow on conductors ......... 

Typical modulation process gain ............... 

Geometries for a vertical transmission line 

and transmitting loop antenna ............... 

Equivalent transmission line network for 

induced voltages and currents ................ 

Signal coupling to a receiving loop 

antenna ...................................... 

Page 

62 

64 

64 

65 

66 

84 

86 

90 

Setup for measuring induced line current ...... 

Induced 1 ine current versus vehicular 

transceiver distance from base ............... 102 

Transmitting Loop Antenna Excitation 

current and magnetic moment .................. 103 

Twin-coil transmitting loop antennas .......... 106 

Equivalent circuit of a single-coil 

loop antenna ................................ 106 
5-12. Unloaded Q versus frequency for a flexible 

rectangular tubular lexan antenna single 

rectangular coil ............................. 108 

5-13. Unloaded Q versus frequency f0r.a flexible 

rectangular planar lexan antenna............. 109 

5-14. Typical matching network..................... 110 

5-1 5. Inductive coupling in the twin-coil 

loop 

5-16. Equivalent circuit of the twin-coil loop 

antenna..................... ................. 116 

5-17. RF line coupler ............................... 126 

5-18. Vireplant signal coupling to the toroidal 

coupler ...................................... 127 

5-19. Test setup for measuring induced current as 

a function of line resistance.. . . . . . . . . . . . . . . 129 

5-20. Over the earth multiple wire coupling ......... 131 

5-21. Acoustical noise comparisons .................. 132 

5-22. Normalized noise frequency response of a 

. gradient microphone .......................... 136 

5-23. PVF2 boon microphone and circum-aural 

earcup speakers .............................. 142 

6-1. Base Station and vehicular transceiver 

block diagram ................................ 152 
ILLUSTRATIONS.. Cont i nued 

6.2 . Remote control terminal (RCT) block 

diagram ....................................... 155 

6.3 . Remote control head (RCH) block diagram ........ 158 

6.4 . Remote control console (RCC) block diagram ..... 160 

Vest transceiver block diagram ................. 165 

Permissible battery pack block diagram ......... 166 

Vest transceiver ............................... 167 

Internal tuned loop sewn' into the back of 

vest in such a way that it is not exposed ..... 168 

Repeater transceiver block diagram ............. 169 

Field test locations ......................... 173 

Isometric d-iagram of the blockcave mining 

method ........................................ 174 

Train loading .................................. 176 

Planeview of track installed on the 2375 

haulage level ................................. 177 

MAGMA'S communications system .................. 179 

Scale model of haulage system .................. 182 

Hoist communications system .................... 188 

Cross section of ore body ...................... 193 

Side view of the vertical crater retreat 

mining method ... .............*................ 194 

Telephone cable in the Redco silver mine 

haulage drifts ................................ 197 
ILLUSTRATIONS.. Continued 

Page 

Measured level 0% carrier signal from vest 

and haulage vehicles .......................... 199 

Test setup For measuring current induced in 

blasting cable ................................ 200 

York Canyon mine ............................... 202 

Cross section of entry (looking inby) .......... 203 

Measured line current as a function of the 

vehicular distance From the base station ...... 206 

Simplified drawing of mains in Star Point 

mine ...................................... 211 

TABLES 

Productivity multiplier work sheet ............. 49 

MF equipment requirements for a longwall coalmine .......................................... 73 

Approximate mobile transceiver (tunnel mode) 

talk-back range ............................... 75 

Comparison of single and twin-coil vehicular 

antenna characteristics ....................... 119 

RF line coupler transfer impedance ............. 129 

Dependence and induced line current on line 

resistance .................................... 130 

Measured sound pressure level of mining 

machines ...................................... 132 

Measured articulation index .................... 141 
Page 

144 

147 

151 

154 

156 

159 

162 

171 

171 

General system specifications.. ............... 

Standard module specifications. ............... 

Standard transceiver specifications.. ......... 

Remote control terminal (RCT) specifications.. 

Remote control head (RCH) specifications.. .... 

 emote control, console (RCC) specifications.. . 

Vest transceiver specifications ............... 

RF line coupler specifications................ 

Antenna specifications........................ 

Vest and vehicular communications to no. 5 

shaft base station. .......................... 

Hoist communications signal level (cage to 

base at the drum) ............................ 

Hoist communications signal levels (cage to 

base at the collar) .......................... 

Vest transceiver communications to audio 

room base station ............................ 
I. EXECUTIVE SUMMARY 

OBJECTIVE 

The objectives of this work were to develop and evaluate a 

medium frequency (MF) radio communications system for use in 

coal and metal/non-metal (M/NM) underground mines which 

would: 

. Use radio communications to minimize miner exposure to 

undergsound hazards. 

. Provide the same communication advantages, reliability 

and operational characteristics as a surface radio 

system. 

. Be installed in any mine within a few hours, so it 

could be evaluated psios to purchase. 

Achieve a balance between humanism and technology 

as well as survive the hostile mine environment. 

. Make work tasks easier without creating additional 

maintenance problems that could not be solved by 

existing maintenance personnel. 

Have equipment that was simple enough for maintenance 

personnel with good electrical skills to understand 

and troubleshoot to the board level. 

To meet these objectives, a program was setup to: (1) 

develop design guidelines for a system and equipment, with 

the assistance of mine personnel; (2) develop mock-up models 

of the equipment; (3) evaluate user acceptance in the mining 
environment; (4) insure maintainability of the equipment by 

working with mine maintenance personnel; (5) determine the 

system's cost/benefit in various types of mines with the 

help of the mine engineering department; (6) build a working 

model of the system and evaluate it in the mine environment; 

and (7) correct any difficiency in the design and then 

evaluate performance in different mine environments over a 

period of time. The objectives were achieved with the MF 

radio system illustrated in figure 1-1. 

$-? transceiver 

Local mine wiring ---- 

~/f, V T/f, or f, 

l t mnsceiverl 

R/ f, t 

?.-Cj Console 

R/ 1, I 

y Antenna 9 RF L~ne Coupler 

repeater 

FIGURE 1-1. - The MF underground radio system (tunnel 

mode ) . 

The system consists of base stations, repeaters and line 

couplers, vehicular transceivers and antennas, and personal transceivers configured into a vest design. Existing mine 

electrical conductors serve not only as the transmission 

L Base 

station 
lines but also the distribution antennas in the ontryways. 

Unit to unit communications are conducted on frequency F1. 

Standard repeater action is conducted by use of frequency 

F2. The use of a base station and repeater results in very 

long range. The base stations and repeaters use torroidal 

radio frequency (RF) line couplers to efficiently couple 

signals into and receive signals from all types of electrical conductors. 

The radio sys tem provides a high qua1 i ty communications 

link between the surface complex, all types of underground 

vehicles, and roving miners. With the help of the mine's 

maintenance personnel, the equipment could be installed 

within a few hours due to the unique mechanical design of 

the antenna and coupling devices. The radio coverage area 

includes almost all entryways with electrical conductors. 

Conductorless entries require the installation of a wire 

pair. Communication range was measured in miles. With the 

MF system, maintenance, haulage and production personnel 

could be reached within a matter of seconds thereby reducing 

downtime and mining costs . Misunders tandings were mi nimi zed 

because first hand information was received by keymen, and 

safety was improved since miners were immediately advised of 

unexpected changes in the mining conditions. 

Unexpected changes expose miners to hazardous situations. 

Many injury scenarios could be avoided with the use of such 

a radio system. For instance, in a hoist system, if the cage 

stops, personnel must sometimes climb down a ladder in the 
shaft to determine what is wrong. Reliable radio links 

bet.ween shaft inspectors, hoist operators, and miners in 

the cages would have eliminated this. In train and belt 

conveyor haulage systems, serious accidents are of ten caused 

by unexpected changes. For example, without warning a train 

may move backward resulting in injury to an off train miner. 

Other objectives were also achieved. The intrinsic safety 

design features of the personnel carried transceiver laid 

the groundwork for more advanced mine rescue and recovery 

communication systems. The vest transceiver also allows 

miners to safely work alone because emergency signalling can 

be used to summon help. The vest transceiver design holds 

promise for trapped or injured miner location. The system 

can also be used for wireless sensors that can both monitor 

and control equipment anywhere in the mine. Tests in deep 

shafts indicated that MF can even be used as a reliable 

hoist control and communications system. 

An important hurnan factor problem was solved by designing 

the personnel transceiver in a vest configuration. By placing radio circuit modules in pockets on the vest, the 

weight and bulk of the transceiver has been evenly distributed on the wearer. The vest contains a conveniently 

located hinged control head and integrated antenna. The 

light weight design allows the wearer to maneuver in tight 

quarters and perform normal mining tasks without interf erence. The vest transceiver output power is less then that 
of the vehicular 'transceiver, 4 and 20 watts respectively. 

The vests are able to communicate almost anywhere in the 

mine with the use of specially designed repeaters which are 

easily installed. The system also features the use of 

narrow band FM and highly sensitive superhetrodyne receivers 

that achieve excellent noise and unwanted signal rejection. 

The use of common modules greatly simp1 i f i e s the maintenance 

task in the mine. 

SUMMARY OF PREVIOUS WORK 

As early as 1922, Bureau experiments showed that radio 

propagation in mines was possible, but not practical. 1 n 

1948, an important contribution to underground communications was made by the Chamber of Mines of South Africa when 

they began programs to develop radio systems for deep mines, 

primarily gold mines. The result was that by 1973, an 

advanced 9 Watt single sideband (SSB) portable radio system 

had been developed. Although the SSB system was reported to 

work effectively in South African mines, its performance was 

not satisfactory in the highly electrified U.S. coal and 

M/NM mines. The Bureau's analysis of the underground radio 

communications problem indicated that further theoretical 

research was required to understand signal propagation mechanisms in underground coal and M/NM mines. 

The Bureau's approach was to first determine the actual 

propagation characteristics of electromagnetic signals in 
U.S. mines and then to relate the propagation characteristics to the underground environment such as the geology, 

entry size, and the effects of existing electrical conductors. Several exhaustive in-mine measurement and analysis 

programs were conducted. They formed the foundation for the 

first true understanding of how electromagnetic signals 

propagate in stratified geologies and down entryways in 

underground mines. Theoretical research and actual measurements in coal and M/NM mines indicated that the communication range was maximum in the MF band (300 kHz - 3 MHz) 

and peaks around 500 kHz. 

Underground tests proved that electromagnetic signals in 

the MF band couple into and reradiate from continuous electrical conductors in such a way that these conductors become 

the transmission lines and antenna system for the signals. 

The existance of electrical conductors in the entryway provides the means for what is called the "tunnel moden of 

radio signal propagation in an underground mine. Testing 

also showed that MF signals propagating on one conductor 

would, by magnetic induction, induce signal current flow on 

other nearby conductors. Thus all of the entryway conductors 

and the magnetic coupling mechanism between conductors provided a means of minewide signal distribution in an underground mine. Further Bureau testing showed that radio signal 

propagation was possible in llnatural waveguidesH (coal, 

trona and potash seams that are surrounded by more conductive rock) exis ting in certain layered formations. Because 
of' signal propag.3tion in the waveauide (seam mode), communication links can be established through coal pillars and 

can be extended to the "working face" (the focal point of 

most mining activity). Testing also showed that signal 

propagation was possible around corners and through barricades in the mine. 

Although Very High frequency (VHF) and Ultra High Frequency (UHF) leaky feeder communication techniques provided tun-, 

nel coverage, coverage in the face area of the room and 

pillar and longwall mines was not provided without deploying special cable into the working area. Since face areas 

are always advancing, deploying special cables is an impractical solution to the area coverage problem. By way of 

contrast, MF signals propagate well in tunnels as well as in 

coal seams thus providing a practical solution to the minewide radio communications problem in underground mines. 

MF EQUIPMENT DESIGN 

The MF communications system design was based upon prior 

fundamental research in MF. The equipment was designed to 

operate in both the tunnel and seam modes. For the tunnel 

mode, radio coverage to all vehicle and vest transceivers 

was provided by magnetic induction (inductively coupling) of 

radio signals into local electrical conductors. The electrical conductors include AC power cable, telephone cable, 

metal, water and air pipes, and wire rope. Base station 
signals are coupled to the conductors via an RF line coupler 

(see figure 1-1). The coupler induces carrier current flow 

in the conductors which produces local magnetic fields. The 

mobile vest and vehicular transceivers communicate with the 

base via magnetic coupling to the conductors. The transceivers use tuned loop antennas to receive and generate the 

magnetic fields. The base station is centrally located in 

the radio coverage area. Passageway repeaters extend the 

operating range of the base station, vest and vehicular 

transceivers. RF line couplers are used to couple the repeaters and base stations to the conductors. . The same repeater can be equipped with tuned loop antennas to radiate and 

receive seam mode signals as illustrated in figure 1-2. 

1 transceiver] 

Pager phone 1in.e (existing) A a. 

7 

T/f, 

R/f2 , y Transmitter 

I Vehicular l1 

fir 

1 transceiver 1 

FIGURE 1-2. - The cellular repeater (seam mode). 

Cellular 

repeater ' 7 Receiver 
Since the repeater services a "cell" (local working area) 

- in the mine,- it is Called a cellular repeater. This repeater may be connected to the local pager telephone system to 

enable communications with other distant miners and with the 

surface communications center. Vhen repeaters are used to 

extend the operating range, a second operating frequency is 

assigned to the radio system. A transmission frequency of 

F2 causes repeater action to occur. The repeater retransmits 

the message at the frequency of F1 (a high power signal). 

All receivers are tuned to the F1 frequency; base stations 

and the mobile transceivers are designed to transmit on F1 

or F2 frequencies. A direct communications link between the 

base and mobile transceivers is created by a transmission 

frequency of Fl; and repeater action extends the operating 

range by using F2 transmissions from the base or mobile 

transceivers. The repeaters are essential in large mines 

since they enable vest transceivers to communicate over 

ranges which are similar to the higher output power vehicubar and base transcei uers . 

The system has been designed to operate on several selectable frequencies in the MF band. Separate communication 

networks (cells) can be set up by assigning a unique set of 

frequencies (F1 and F2) to each network which may independently serve the communication needs of maintenance, production, haulage and supervision. Additional networks are 

created by installing other base stations and repeaters. 

The base stations, repeaters and mobile transceivers are all 
tuned to the assigned unique network frequencies. Because 

digital frequency synthesizers are used in the equipment 

design, it is possible for mobile transceivers to monitor 

and communicate with other networks in the mine. 

FINDINGS AND CONCLUSIONS 

Results of In-Mine System Tests 

The MF communications system was installed and evaluated 

in four underground mines. The mines ranged in size from 

small (500 tons/day) to large (67,000 tons/day) capacity and 

represented several different mining methods. Two of the 

mines were medium sized coal mines (approximately 1 MTPY). 

These mines use continuous mining equipment to develop longwall panels and belt haulage systems to transport coal out 

of themine. The third mine was a small silver mine which 

uses the vertical crater retreat mining method. Diesel 

trucks haul approximately 500 tons of silver ore per day up 

a 7,000 foot long decline to the adit. The fourth mine was a 

large copper mine which uses the multiple level block caving 

method to extract 67,000 tons of ore (muck) per day. The 

mine uses an extensive rail haulage system to transport muck 

to the muck bins where hoists are used to lift it to the 

surf ace. 

These mines offered an excellent. opportunity to evaluate 

numerous aspects of the system's features in a wide variety 

of mining condi tions . In-mine tests were conducted to 
measure: (1) the MF signal carrier to noise ratio along the 

entire length of conductors in several entryways; (2) the 

signal attenuation rate of the MF signal along the conductors; (3) the receive and "talk-back" (transmit) range from 

mobile transceivers to the base station; (4) electrical and 

acoustical noise levels in the mine; and (5) receiver quieting. Additional experimental testing involved a simulated 

rescue and recovery operation and a simulated locating of a 

trapped miner. 

Analysis of the in-mine test results showed that the ME 

communication system performed equally well in each mine. 

Because the performance was consistant and predictable in 

these mines, underground radio system engineering can predict performance in other mines. Furthermore, the in-mine 

evaluation showed that the system provided high quality 

radio coverage in most work areas and entryways with electrical conductors; the cellular repeater offers excellent 

coverage in the face areas of both continuous and longwall 

mining; Veach timen can be decreased from an average of 35 

minutes (with the pager telephone system) to seconds; teamwork improves by allowing first hand information to reach 

key personnel in the mine; equipment downtime is reduced due 

to quicker communications between the production and maintenance crews; and safety and productivity are enhanced with 

the continued use of the system. 
- MF Communication Quality - and Range 

MF signals can be received almost everywhere electrical 

conductors exist in the mine. The talk-back range between a 

mobile transceiver and the base station or repeater depends 

upon the distance of the radiating antenna from the conductors and the type of nearby conductors. For radiating 

antenna-to-cable separation of approximately 7 ft, the vehicular to base communications range exceeded 30,000 ft 

along unshielded single-pair cables. At 520 kHz the attenuation rate was 2.4 dB/1 ,000 ft. It was only 1 dB/1 ,000 ft at 

350 kHz. The range along the shielded 3-phase AC power 

cable exceeded 10,000 ft. At each point where the primary 

power cable connected to a power center, the signal loss 

increased by 8 to 12 dB. At 400 kHz, the attenuation rate 

was approximately 4 dB/1 ,000 ft. The vest talk-back range 

exceed 16,000 ft. In a layered formation like coal, the 

vest talk-back range includes adjacent conductorless entries 

for a distance of 5,000 ft from the base station. The seam 

mode radio coverage cell exceeded a radius of 500 ft. This 

provided an excellent communication link between a roving 

miner at the face and the cellular repeater at the power 

center. 

The MF carrier to noise (C/N) ratio was measured at each 

base station in the four mines. The measured C/N ratio often 

exceeded 50 dB (30 dB above the good intelligibility level) 

when a mobile transceiver was a mile or more from the base 
station. 

In conductorless entryways, a low cost twin-lead cable was 

installed to extend radio coverage in one mine. One end of 

the cable was placed inside an RF line coupler to enable 

coupl.ing to the entire cable. The cable provided excellent 

coverage in the entryway for the entire distance of the 

cable (more than 10,000 ft). 

Additional in-mine testing of the communications range 

showed: 

. Vest to vest communication range can exceed 11,000 ft 

in an entryway with conductors when the vest-to-cable 

distance was less than 7 ft. 

. Vehicular transceivers, antennas and coupler used in 

hoist communications achieved a link C/N ratio of 

60 dB. 

. A vehicular transceiver can communicate with a vest 

transceiver for distances greater than 15,000 ft 

when the vest and vehicular antennas are less than 7 

ft away from each other. 

The acoustical noise levels near operating mine equipment 

are extremely high. Noise cancelling microphones and circum- aural ear cup speakers are required to maintain acceptable intelligibility 1,evels in the speech channels. 
Improvements - In Vehicular Antenna Technology 

A loop antenna featuring a twin-coil planar structure was 

developed and evaluated for use on vehicles. The antenna 

achieved high magnetic moment, a fundamental requirement for 

inducing. high level carrier currents in nearby electricaJ 

conductors and was also highly sensitive in receiving low 

level magnetic signals. This antenna design offers the following advantages: 

. The antenna has a self-resonant frequency of 2.8 MHz 

as compared to 1 .5 MHz in the single coil. 

. Because the twin coils are connected in parallel, the 

terminal inductance of the coil is less than a single 

coil antenna with the same number of turns. Thus, 

the transmitting twin-coil loop produces less electric 

field stress across the structural components. By 

way of contrast, a transmit single coil antenna can 

develop more than 5,500 volts across the coil when 

excited by a 20 watt transmitter. This can lead to 

electrical st,ress on the tuning components. Laboratory tests show that undesirable corona discharge 

effects in the antenna structure result from the 

single coil antenna. 

. The radio frequency (RF) voltage of the twin-coil design is approximately 55 percent less than in a single 

coil antenna operating at the same magnetic monent. 
The vest transceiver uses a single loop antenna for use 

in hazardous atmospheres. To achieve this, two design constraints were adopted. First, the stored energy within the 

antenna inductances and capacitances must be less than 0.3 x 

loo3 joules. Second, localized electric fields developed in 

the structure cannot be of sufficient strength to cause corona discharge effects or dielectric breakdown in the antenna 

tuning elements. 

Mobile Tpansceivers 

The vest and vehicular transceiver designs were optimized 

for use with tuned loop antennas. Impedance matching networks were designed to maintain antenna tuning when the 

transceiver is receiving. 

The MF transceive~s are designed with highly sensitive 

superhetrodyne receive~s. The receiver design includes 

filtering technology to reject unwanted signals and noise. 

This prevents unwanted signals from desensitizing the receiver. 

The vest and vehicular transcei ve~s have been designed to 

operate at a power level of 4 and 20 watts into 50 ohms, 

respectively. In the magnetic induction (inductive) cornmunications system, . it is the magnetic moment of the antenna 

that is important in determining operating range. These 

power levels produce a magnetic moment of about 2.0 and 6.0 

ampere-turn-meter in the vest and vehicular antennas. 
RF Line Cou~ler Design 

The RF line coupler is a tuned multiple turn toroidal 

(doughnut shaped) aircore current transformer which can be 

easily clamped around local conductors. In transmit mode 

the coupler provides efficient signal coupling to the mine 

electrical conductors. Since it is a tuned network, it is a 

preselector filter for the sensitive receivers used in the 

base station and repeater transceivers. 

Cost/Benef i t Analysis 

Cost/benefit studies were used to determine the value of 

an MF system to underground mining. The cost with 20 transceivers, a base station, and repeaters will range from 

$50,000 to $80,000. A benefit analysis was conducted in a 1 

MTPY coal mine, and it indicated that the radio sytem could 

reduce mining costs by 13 percent. This cost savings will 

represent a return of 2.6 times the initial equipment investment in the first year. 

Other Important ---- Uses of the MF System 

The MF system performance has been evaluated inside a 

high-densi ty concrete and steel nuclear reactor containment 

vessel, a large thermo-electric power generating station, 

and a fire fighting training structure. A large diameter 

loop antenna placed on the gangway inside of the containment 
vessel was excited by a base station via an RF line coupler. 

In another series of tests, loops were placed around the 

outside of the electric power generating station and the 

f-ire fighting training structures. As in the containment 

vessel test, the loops were excited by RF line couplers. In 

these tests , commercially available handheld transceivers 

operating in the ultra high frequency (UHF) band and MF vest 

transceivers were compared on the basis of radio coverage. 

Although UHF transceivers perf ormed well on all 1 ine-of -si te 

paths, poor coverage existed in elevators, and behind steel 

reinforced walls. In contrast, MF coverage included these 

areas. The conclusion that can be drawn from this work is 

that MF inductive communications provides radio coverage 

where surface communication technology now fails. 
11. DETERMINING COMMUNICATION REQUIREMENTS 

IN UNDERGROUND MINES 

INTRODUCTION 

The task of designing and introducing a high technology 

radio system in underground mining requires extensive organizational, marketing and economic research. The system and 

equipment design must achieve a balance between innovative 

technology and communication needs in mining. 

This task was addressed for several reasons: (1) organizational theory research results are useful in determining 

how radio (increased information flow in the organization) 

can increase the effectiveness of the mining organization 

thereby making it safer and more productive, (2) market 

research determines the radio communication needs as perceived by the mining community .as we1 1 as factors inf luencing the introduction of high technology radio communication 

products in the mining industry, (3) economic research was 

useful in determining the cost saving benefit of using rapid 

Communications in a1 1 phases of the mining process. This 

work had two fundamental purposes. First, it involved the 

mining community in identifying its own cor~lmunications requirements. Secondly, it insured that the system and equipment design would focus on solving communication problems 

with the greatest interest to mining. The research studies 

clearly identified how' radio could im.prove ~af ety and lower 
mining cost. With this important background information, 

reliable design specifications were formulated for the system and equipment . The market-engineering research plan 

which was used in formulating the system and equipment 

design specifications is shown in figure 2-1. 

/ PRELIMINARY DRAFT 

REVISIONS I 

FIGURE 2-1. - Formulation of the system design 

specifications. 

The process begins with the Research and Engineering (R 

and E) group formulating a preliminary draft of the system's operational specification. With the use of 35 mm 

slides of the system concept, mock-up models of personnel 
transceivers, etc., the design was presented to various 

levels of management and operating personnel. During each 

presentation, mine personnel are asked to cooperate in making a forecast of the overall benefits of using radio, 

exploring saf ety issues and determining possible improvement 

production rates (tons/hour) . With this information the R 

and E group was able to tailor the system's design to maximi ze benefits. 

COMMUNICATION NEEDS 

Communication systems in underground mines have remained 

relatively unchanged over the past two decades. It is 

interesting to observe that this occurred during a period of 

high technological change in extractive technology and 

equipment. The reason , for this is threefold. First, management and mine equipment manufacturers understand that 

bigger and better automated mining equipment improves the 

machine component of productivity. Second, managers perceive that they have maximized the human component of 

productivity. Finally, most managers are not communications 

specialists, so they fail to perceive how radio can be a 

productivity multiplier. 

Although the communication requirements in coal and M/NM 

mines are different, the following needs were found to be 

common in both types of mining: 
. A radio communications link between the maintenance 

foreman and his crew so radio communications can be 

provided to the point of equipment breakdown or 

points of danger. 

. Improved communications between key personnel to 

better coordinate the production task. 

. Communication networks which can operate either 

dependently or independently of each other. 

Radio equipment which withstands the hostile mine 

environment and is not cumbersome to use. 

ORGANIZATIONAL THEORY SUPPORTINCaCOMMUNICATION NEEDS 

The major factors which concern mine managers and engineers are'productivity and safety because they realize that 

a safe mine is a productive mine. In the mine environment 

productivity and safety are greatly influenced by (1) the 

geological formation; (2) the mining process; (3) the type 

of mining equipment and its maintainability; (4) the attitudes of the work force; and (5) the effectiveness of the 

communication sys tem. 

When considering productivity issues, Pugh (1) - states that 

managers tend to concentrate on three factors: 

1. The application of advanced technology and improvement skills. 

2. The organizational structure of mine operations. 

3. Teamwork. 
The first factor interests most managers. In order to 

keep informed of the latest technological advances, managers 

and engineers read trade pub1 icat ions and attend mining 

equipment shows. They also spend a great deal of time 

evaluating technological improvements in the mining process 

and relating them to productivity, The second factor is 

we1 1 developed in most organi zations. In general, managers 

are able to organize the physical plant and processes for 

near optimum operations. The third factor, by comparison to 

the other two, is almost completely neglected. Yet, teamwork 

is important to productivity. This was proven by Mayo and 

Bavelas. Mayo (2) - through his Hawthorne case study proved 

that a close relationship between the working group and 

management was a key factor in improving productivity. 

Bavslas (9, in 1948 , studied the link between communication patterns and the behavior of miners within vcells" 

(small working groups). The purpose of the work was to 

consider the psychological conditions that are imposed upon 

the group members by various communication patterns and the 

effects of these conditions on the organization and behavior 

of its members (4). The concept of centrality became the - 

main determinant of behavioral differences because it ref Lects the closeness of a miner to other group members and 

the extent to which a miner is strategically located rela-. 

ti ve to his source of information. Individuals who could 

rapidly col lect information were viewed differently than 

those to whom information was not accessible. The result 
was decreased teamwork which 1 ead to decreased product i vi ty. 

An organization's overall success depends on the balance 

of the three .key productivity factors. The first two factors (technology and organizational structure) operate to 

make an organization effective; the third factor (teamwork) 

makes it efficient. 

MINE ORGANIZATION 

Mining companies are generally organized into pyramidal 

structures similar in many ways to those found in industrial 

firms producing a constant product over a period of time. 

An organizational chart for a typical mine is given in 

figure 2-2 . 

1 GENERAL MANAGER 

PRODUCTION 

'OREMAN FOREMAN 7 F 

I MAINTENANCE 

SUPERINTENDENT 

MECHANICAL 

FOREMAN 

FIGURE 2-2. - Typical organizational chart. 

38 
The miners are organi zed into smal 1 semi-autonomous 

working groups (crews). Each crew is responsible for a 

specific task and located accordingly. For example, a production crew spends most of its time in the working section 

(face, stope, etc.), whereas a maintenance crew moves to 

various sections of the mine. Each crew is supervised by a 

foreman. The foreman works under a superintendent who coordinates and directs the mining activity. The organizational 

efficiency is measured in terms of product tonnage produced 

per man-shift. 

: INFORMATION FLOW 

In present day mining organizations, information flows by 

several communication methods. At the beginning of each 

shift, the superintendent and foremen meet to outline the 

work task, objectives and priorities, then foremen relay 

vital information to members of their crews. During the 

working shift, the mine telephone system usually becomes 

the primary communications means. It is sometimes used to 

change crew members work schedules and priorities. 

To obtain the highest possible level of productivity 

through information flow, it is important that central ity 

exists. This has been confirmed in the course of this work 

by observing group management activities during shift changes in several coal and M/NM mines. 

In high productivity coal mines (greater than 25 tonlrnan- 
shift), meetings were held at, the beginning of each shift. 

The meetings included a1 1 supe~intendents and foremen belonging to the in-and-out-going shifts. Some of the crew 

members were also present. Members of the management team 

freely discussed problems encountered on the prior shift. 

The likelihood of the problems emerging again on the next 

shift were also discussed. By their presence, maintenance 

crews tlere made aware of possible problems. Repaf rs were 

scheduled to avoid production downtime. In one mine a 

wcrashtf vehicle containing emergency repair parts was 

stationed undergpound and used in emergencies by the 

maintenance crew. It was interesting to note that the face 

production crew on the out-going shift left the mining 

equipment in position to reduce start up time f o r t h e next, 

crew. 

In lower productivity mines, the meetings at the beginning 

of a shift were noteworthy in the sense that they tend to be 

"by chancen one-on-one encounters between the superintendent 

and his own foremen. Thus, lower centrality occurs in a low 

productivity mine. 

Although management and other mine personnel have wished 

to use radio to improve the production rate, the pager 

telephone system of ten remained the on1 y avai lable communications resource. Lack of coverage in the mining complex 

was management's most frequent criticism of the system - 

timely messages needed for adequate coordination of ten f ai 1 
to reach keymen.l Roving miners were isolated from the 

information source most of the time thus becoming independent. The price paid for independence is low centrality and 

productivity . 

Things do not have to be this way in underground mining. 

Radio can be used to increase centrality, replacing independence with synergism2 in the working group. The availability of vital information, through the radio communication 

resource, will enable the miners to coordinate their act ivity and create a definite sense of "belongingnessM in the 

working group and the mining organization. 

BENEFIT ANALYSIS 

In both coal and M/NM mines, production and safety are the 

key factors which are involved in a benefit analysis. 

In coal, product ion depends largely on geological conditions and the availability of face equipment. Potentially 

more coal can be mined than can be transported out of the 

l~his is a key disadvantage of any pager telephone or 

carrier telephone system. That is, it is "station to 

stationft not "person to person." 

'synergism - the cooperative act ion of indi vidual working 

groups such that the total effect is greater than the sum 

of the two effects taken independently. 
mine. The Jet Propulsion Laboratory (JPL) and others have 

e x t e n s i v e l y s t u d i e d the effects of equipment downtime on 

productivity (5) - (6). - Their work shows that the downtime 

period tends to increase with the comp.Lexity of the mining 

equipment. Increased complexity also requires greater proficiency on the part of the maintenance crew members. In 

some mines, the proficiency of new maintenance crew members 

is suspect and repair burdens have been found to be shifted 

to others (ski 1 led repairmen and supervisors). The average 

availability time of 35 continuous miners studied in the JPL 

work was 73 percent of the "face time" (production shifts 

less travel time). The availability of the longwall section 

was found to be near 76 percent. These figures exclude time 

lost to travel, scheduled maintenance, moving equipment, 

etc, Hence the availability is often less than 50 percent. 

In most M/NM mines, production depends largely on the 

haulage system. It is the haulage equipment breakdown factor that tends to dominate productivity. The effect of 

mining equipment downtime on productivity is shown in 

figure 2-3. 

Productivity is naturally lost during scheduled start-ups, 

lunch breaks and shift change activities. As figure 2-3 

shows, unscheduled downtime has a significant effect on 

productivity . 
Enter Leave TIME, hours 

FIGURE 2-3. - Typical production profile. 

The cost of downtime depends upon the failure point in the 

mining process. Downtime in a working section may "range 

from a few thousand dollars per hour to tens of thousands of 

do1 lars per hour on the main haulageway system. Thus the MF 

system cost can be justified by saving only a few production 

hours. 

Economic analysis conducted in coorperation with mine personnel indicates that the use of MF communication systems 

can enhance safety and productivity in the following ways: 

1. Decreased reach time - Presently in both coal and 

M/NM mines it takes approximately 35 minutes to reach key 
personnel with the pagsr telephone system.3 With the use 

of radios, such as MF, reach time could be reduced from 35 

minutes to a few seconds. 

Reduction of reach time i% especially important during 

mining equipment breakdowns since 10s t time means decreased 

productivity which could translate into a loss 'of thousands 

0% dollars. With the use of the MF radio system, the 

communication needed between roving maintenance personnel 

and production foremen at the face or haulageway breakdown 

location could be significantly reduced. 

2. Improved accuracy of information - Because of the 

lengthy reach time involved when using the non-radio sys tem, 

critical messages are frequently relayed to keymen. Through 

this relay process keymen often receive inaccurate information which costs mines more than most realize. In many 

instances a wrong part is ordered from the warehouse or a 

part is delivered to the wrong location. Either way productivity decreases and repair costs increase. For example, in 

one minePs cleanup operation all the wayside litter was 

gathered and shipped out of the mine, It was later discovered that 75 percent of the litter was vital repair parts. 

3~his statistic was established by placing calls (paging) 

to mine superintendents and foremen and then measuring the 

reply time interval. 
The use of MF radio eliminates the relay system of delivering messages since keymen can talk directly to each other. 

This decreases the probability of inaccurate information 

reaching the keymen, thereby decreasing the overall cost of 

repairs and downtime. 

3. Increased centrality - The direct dial private line 

(star and branch networks) telephone systems have lead to 

decreased centrality in the maintenance group a1 though supervisors may often prefer the private lines. In the private line system, messages tend to be directed to specific 

maintenance personnel. Whereas, in a party line communication system all members are aware of problems in the mine. 

Since each maintenance person is aware of the others1 problems, they tend to cooperate in solving maintenance problems. 

4. Reduces the probability of an accident occurring - 

Poor communications can cause serious accidents and fatalities in underground mines. Most mine accidents are the 

result of unexpected changes in the mining process. When 

unexpected changes occur, miners are frequently exposed to 

hazardous situations. Studies of incidents relating to accidents reveals that improved communications would have prevented many accidents. For instance, an off-train-man 

expects the train to move forward but instead it moves 

backward and seriously injures the snapper. The hoist cage 

was expected to go up, but instead proceeds downward and 
'strikes a maintenance man. A radio link in these cases would 

greatly increase safety. 

5. Increased efficiency in mine rescues - In the event 

of a mine disaster such as an explosion, cave-in, fire, 

etc., where miners may be trapped underground, efficient 

rescue efforts are essential. Rapid and efficient operations not only enhance the possibility of achieving a 

successful rescue, but also reduce the risk to the rescue 

team. 

Teams equipped with personal ?IF radios can greatly enhance 

mine rescue. In practice, rescue teams from surrounding 

mines cooperate during a disaster. To insure communicat ions 

compatability among rescue teams, a common national rescue 

frequency (415 kHz) can be assigned. To insure that the 

rescue radio equipment will be properly maintained, the 

rescue radios can also be used in the maintenance net. 

6. Improved communications technology in hoist systems 

In deep shafts, electronic equipment in cages is subjected 

to high vibrations, rapid temperature change, and condensation. All these factors degrade the reliability of the 

existing carrier current radio equipment. With an MF system 

the antennas or couplers are capable of efficiently using 

the wire rope signal transmission medium. A base station 

and antenna can be located at the collar or in the hoist 

room. This enables the surface MF equipment to operate in a 

less harsh environment. It also permits maintenance to be 
performed at ground level, thus reducing the chance of an 

accident occurring. The improved RF line coupler greatly 

increases the signal coupling efficiency to the wire rope 

which in return allows for excellent communication in the 

shaft. By way of contrast, the existing shaft communications requires mounting the antenna on top of the hoist next 

to the sheave wheel; this increases the probability of 

having equipment damaged. 

7. Improved coordination of underground mining act ivity - In coal mining the transportation of supplies must be 

coordinated with production needs. The foreman must be able 

to communicate with service vehicle motormen to insure timely delivery of supplies; he must also be able to adjust the 

production rate to match the belt haulage rate. In vertical 

crater retreat (VCR) and blockcave mining processes, it is 

necessary to coordinate haulage and mucking operations 

which occur at different levels of the mine. If the production foreman can be made aware of wfullll transfer raises 

or draw points mucking can be efficiently directed. 

As a practical matter, roving mucking and production foremen cannot use telephones to efficiently coordinate haulage. 

Use of MF radio is expected to increase haulage by five 

percent since it allows for communication between different 

levels of operation, person to person. 
By considering the seven factors above, the radio productivity multiplier for any mining organization can be 

determined. Table A illustrates the expected production 

improvement when u s i n g a n MF system in a medium sized coal 

mine. Line item B shows the average face production and 

belt haulage in a typical one million ton per year (1MTPY) 

mine. The data f 01 lows from actual production statistics 

(prior 12 months). Line item C shows the, downtime cost in 

tonslhour. In coal and M/NM mining, haulsgeway disruption 

is exceedingly expensive. The expected production increases 

due to the use of MF radio is shown in Line item D. It. is 

assumed that equipment breakdowns will occur in at least one 

section during a shift. Product flow along the haulageway 

will be disrupted. Radio will eliminate the keypersonnel 

reach time (35 minutes) associated with each production 

disruption. 

Face and haulage system production rates can increase as 

shown. Use of MF radio by supervisors is expected to 

increase mining efficiency by five percent. The radio productivity mu1 tiplier for a one million ton/year (IMTPY) coal 

mine significantly increases product i vi ty by 13.5 percent 

(135,000 tons/year). 

The expected return on investment is noteworthy. The cost 

of the radio equipment including maintenance and replacement 

cost is approximate1 y $53,900. Assuming a $35/ton revenue 
TABLE A. - Productivity Multiplier Work Sheet. 

Mine 

Example Forecast 

A. Actual Production Statistics 

Annual Production (tons) 

Production Days 

Shifts Per Day 

Operating Sections 

' B. Production Tonage Per Shift 

Each Face 

Haul ageway 

Feeder 

Main 

C. Downtime Cost Per Hour (Tons/Hour) 

Face 

Haulageway 

Feeder 

Main 

D. Increase In Production Per Shift 

(Tons/Shift ) 

Reduced Breakdown Time 

Face 

Haulage 

Better Coordination 

Total 

E. Increase In Production 

( Per Year ) 

with a three percent operating margin, the first year return 

on investment Is about: 
The cost of the equipment can be recovered 2.63 times in one 

year. The present value (P'Ji-1 2%) of the five year production increases are: 

The above productivity and financial analysis are conservati ve in the sense that safety enhancement expectations 

were not included. Poor communications is a factor in many 

accidents. Cost includes damaged equipment, disruption in 

the mining process, and most of all, the loss of an injured 

employee's services. It is not unreasonable to expect an 

even greater ROI. 
111. DESCRIPTION OF THE MF RADIO COMMUNICATION SYSTEM 

INTRODUCTION 

The philosophy of the M,F system is illustrated in figures 

3-1 and 3-2. Figure 3-1 shows the radio coverage areas 

(cells) possible and figure 3-2 shows how existing mine 

conductors (the wireplant) are used to carry and radiate the 

MF signals throughout the mine. 

Existing conductors in entryways provide a means of dist r i b u t i n g t h e MF radio signals in the mine. These signals 

can be transmitted from one connected set of conductors to 

another via inductive coupling. These conductors (called a 

wireplant) include electrical power cable, control 1 ines, 

telephone cable, metal water and air pipes, etc. 

Signal coupling to the wireplant occurs when the transmit 

antenna produces a "neartf magnetic field (H,) signal that 

is intercepted by the wireplant. The signal magnetically 

couples to the wireplant inducing current flow which 

produces a local magnetic field (HZ) as it travels (propagates). At a distant location, the local field inductively 

couples the MF signal to an antenna or 1 ine coupler associated with a receiving transceiver. 

Inducti ve coupl ing of signals into existing mine conductors is necessary to insure extensive radio coverage. 

Because of magnetic coupl ing and the existing wireplant, 

usually no special communications cable need be instal led. 
Cellular 

communications 

FIGURE 3-1. 
I 

y-'\ fl *. \. r \ < 5 I 

\ 

\ 

I I- I I 

\ \ I Hz 

LINE CURRENT Y' 

TRANSCEIVER 

STATION 

n TUNED LOOP 

ANTENNA 

FIGURE 3-2. - Coupling of electromagnetic signals to 

electrical conductors. 

Signals can also be coupled to the wireplant conductors 

with RF line couplers. Like a current probe, RF line couplers can be easily clamped around electrical conductors 

where they induce a greater amount of line current than that 

produced by an antenna. In fact, couplers increase the 

operating range by miles. 

Base stations and repeaters, as previously illustrated in 

figure 1-1 enable roving miners wearing mobile transceivers 

(vests) to cornmuni cate great distances. Base stat ions can 

be controlled remotely by a single pair of conductors connected to a control console. The control console can be 

placed in a surface building for convenience, and the base 
transceiver is placed in the mine where it can more effectively couple signals to the wireplant. The global 

repeaters are used to extend the system's operating range in 

two 'ways: (1 1 by transmitting on frequency (F2), the base 

is able to access the global repeater and reach a distant 

mobile transceiver, and (2) by transmitting on frequency 

(F2), a mobile transceiver can access the repeater and reach 

a distant mobile transceiver. 

The system may also use repeaters to increase the range 

between mobile transceivers In the face area of a coal mine. 

This type OF repeater is known as a 9gce1$ular repeater" 

because it illuminates only a 91cel19s or area of the mine. A 

distinguishing feature of this repeater is that it uses 

longwire loop antennas to excite both local conductors and 

the coal seam propagation mode in the working area. The 

antennas used are dual-loop antennas (for transmitting and 

reeei-ving) attached to the rib or posts. The transmitting 

and receiving antennas are similar; both produce a large 

magnetic moment that provides the signal for local cellular 

coverage which is usually aided by local in'ductive coupling 

effects. The repeater can be connected to the section pager 

telephone system just like a pager phone. Wireless signal 

paths exist between the section repeater and roving miners. 

Figure 3-3 illustr'ates the mobile telephone nature of the 

working sect ion communications system when configured in 

this manner. 
TELEPHONE PAIR f CELLULAR 1 

1 1 REPEATER I R/ f2 

TRANSCEIVER to VEST 

TRANSCEIVER 

FIGURE 3-3. - Vest radio communications with the telephone network or other distant vest transceivers via the 

cellular repeater. 

The personnel transcei vers ut i 1 ize a uni que vest concept 

that is convenient to wear and permits excellent communication from almost anywhere in a mine. Miners (production 

foremen and maintenance crew) can wear vest transceivers to 

communicate with the cellular repeater. The repeater transmi ts the telephone message to other vests using frequency 

(F1 ). The vest transcei vers are tuned to recei ve the frequency (F1) and are able to create a local mobile-to-mobile 

network by transmitting on (F1). The vest transceivers can 
also transmit on (F2) in order to access the cel lular repeater and communicate with the pager phone network. In this 

instance, the cel lul ar repeater enables the vest transcef ver 

(using F2) to reach a distant vest transceiver in the working area by repeater action, 

The' vehicular transceiver is connected to a tuned loop antenna of advanced design enclosed in high strength exa an^ to 

protect it. The transceiver is connected to the vehicular 

power by power cable. 

NF ARCHITECTURE 

The architecture of the who$ e mine radio communications 

system is shown in figure 3-4. 

/ UNDERGROUND 

FIGURE 3-4. - MF system architecture. 

4~eneral Electric trade name for polycarbonate. 
The MF system was archi tectual ly designed following the 

recommendations of Bavelas (3). Cel 1s of radio coverage 

were created by assigning a specific frequency to the radio 

equipment within each working section. The cells are designed so roving miners within a working group can communicate in a party line sense, thus allowing every miner within 

the group to receive the same information. The cell's 

coverage overlay each other in such a way that messages in 

one cell are not heard in other cells, yet the cells can be 

interconnected to permit the flow of i nf ormation throughout 

the organization. 

The maintenance cell extends from the point of equipment 

breakdown to the surface shop and supply areas. The haulage 

cell covers the loading and dumping areas and the haulageway; this a1 lows motormen (dri vers) to communicate with the 

dispatcher and of f-train men. The maintenance cell extends 

throughout the entry drifts and face area. The production 

cell includes coverage of the work area and entry drifts so 

key production personnel can coordinate haulage of critical supplies and production products (muck, coal, etc.). In 

most mines, the maintenance and production cells can be 

merged into a singlecell so that production supervisors can 

communicate with maintenance supervisors. 
EQUIPMENT 

The MF equipment was designed for "near1' magnetic field 

communications and is divided into mobile and fixed Location 

catagories as shown below: 

Mobile 

Vest Transceiver 

Vehicular Transceiver 

Fixed Location 

Base Station 

Global Repeater 

CelPu.lar Repeater 

Coupling Devices 

RF Line Coupler 

Antennas 

Mobile Transceivers 

Two types of' mobile transceivers have been developed: 

vest units for individuals and vehicular units $or vehicles. 

Functionally the two are identical differing only in output 

power levels and physical configuration. 

Vest Transceiver 

The vest transceiver has been designed for operation in 

gassy atmospheres (CFR, Title 30, Part 23-Telephones and 

Signal l ing Devices) and is shown in figure 3-5. 
FIGURE 3-5. - Vest transceiver. 
phone, the voice message frequency modulates the carrier. 

Depressing the signalling (signal) button modulates the 

transmitter with a 1,000 Hz audible tone. This can be used 

for emergency signalling in the event that a miner is unable 

to speak. 

The cellular repeater design includes switching circuitry 

to allow the vest to operate like a mobile telephone. Depressing the paging (page) switch keys the vest transmitter 

to the carrier frequency (F2). This signal is modulated 

with a 100 Hz subaudible tone. The tone signal is decoded 

in the cellular repeater causing the vest voice signal to be 

switched onto the local pagsrphone network. The pagerphone 
network can key the repeater to the carrier frequency (F1) 

for return message transmission into the radio coverage 

cell. 

An important human factor problem was solved in the vest 

design. By distributing the bulk and weight of the transceiver of the vest, the radio becomes an ifitegral part of 

the users garment (7). - He is free to move in tight quarters and perform normal mining tasks without catching the 

radio on obstructions. 

V e h i c u l a r T r a n s c e i v e r 

The vehicular transceiver is shown in figure 3-7. The 

transceiver includes a control head which enables the user 

convenient access to the microphone and volume control. It 

is connected to an advanced design tuned loop antenna which 

is enclosed in high strength Lexan to protect it from being 

damaged in the mine environment. 

The vehicular transceiver is connected to the vehicle's 

power by power cable to permit operation from the vehicular 

electrical power system.. The antenna, control head and vehicular transceiver are designed with mounting brackets (that 

include weld bars) for easy mounting on most vehicular 

equipment. 
. Observe the color codes where the black wire is 

connected to the positive supply voltage and the 

white wire is connected to the ground. 

. Mount the cables in a way which best avoids abuse. 

. Mount the vehicular transceiver where it receives 

some p r o t e c t i o n . 

. Mount the antenna horizontally or vertically. 

. Mount the control head in a convenient location. 

Fixed Location-Repeaters 

The global and cellular repeaters are identical. They can 

operate with RF line couplers (as illustrated in figure 1- 
1 ), or with antennas (as illustrated in figure 1-2), or as 

part of a pager telephone system (as illustrated in figure 

1-2). 

The receiver section of the repeater demodulates the received carrier signals (F2), recovering the base and F2 m'esSage and the 100 Hz subaudible tone. The .recovered message 

signals may be used for two purposes. First, the message 

signal keys the repeater's transmitter section to produce a 

high power retransmission signal at the F1 frequency. The 

message appears as FM modulation on the F1 signal. Second, 

when the subaudi bl e tone is present, the repeater decodes 

the 100 Hz tone and switches the message to the mine telephone network enabling telephone communications. The repeater design includes filter networks to prevent recei ver 

desensitization. 

The repeaters and the base station can be operated from 

remote control console(s) which can be located anywhere in 

the mine. A pair of wires connect the console to the 

repeater as shown in figure 3-8. 

Multiple Remote Control Console (RCC) units, base stations and repeaters can be connected to t.he Remote Control 

(RC) cable (wire pair). The maximum control signalling 

distance over the RC cable is 5 miles. The RCC unit is 

shown in figure 3-9. 
FIGURE 3-9. - Remote control console unit. 

- Base Station 

The base station permits communication with mobile radio 

equipment via the RF line couplers and the mine's electrical 

conductors. 
VEHICULAR TRANSCEIVER 

FIGURE 3-10. - Base station. 

Coupling Devices 

The coupling devices used in the systpm include RF line 

couplers, and tuned loop antennas. 
FIGURE 3-11. - Tuned loop antenna. 

The antenna may be mounted on a vertical or horizontal 

mounting surface; however, to avoid undesirable influence of 

nearby metal surfaces, the antenna should be mounted at 

least 6 inches away from metal surfaces. 
ilF INSTALLATION 

The MF radio communication system features installation 

versatility as well as simplicity and reliability in actual 

operation. The system has been successf ul-1 y instal Led and 

demonstrated in four underground mines 1 isted in Chapter 

VII. These four mines are representative of the mining 

processes commonly used in the U.S. In the course of the 

installation work, various problems were encountered and 

solved, a few of which were: 

Physical space problems when mounting larger vehicular antennas to certain types of mobi 1 e mining equi pment. Smaller antennas were made to facilitate such 

installation requirements. 

The damage of cable. When possible protection was 

pro vi ded . 

Highly reflective paint on the vehicular antennas 

tended to blind the operator in areas of darkness so 

the antennas were painted with a dull finish. 

Preinstal lation guide1 ines have been developed for easy, 

successfu1 installation of tne MF system. The guide1 ines 

will be described in Chapter IV. 
IV. PREINSTALLATION PREPARATIONS 

INTRODUCTION 

The ultimate goab sf any communizations System is to make 

underground mining safer and easier without reducing the 

productivity of the mining process. To achieve this goab it 

is important that the installation of a communications system be well planned in advance. This section describes the 

preparations which should be followed before an MF system is 

installed in order to assure the system will work to its 

full potential. 

INITIAL RADIO SYSTEM PLANNING MEETING 

The primary goal of the planning meeting is to involve 

management and other personnel, at the earliest possible 

time, in understanding the principles and features of MF 

communications. The material presented in Chapter 111 is 

an overview of the MF system and serves as the briefing 

handout for the meeting; mine personnel should read the 

handout before attending the meeting. This material will 

acquaint then with the communication system so they may help 

identify areas where radio communication can reduce mining 

costs and increase safety. The expectation of cost reduction can be developed for the mine by preparing a cost/ 

benefit analysis similar to that developed in Chapter 11. 
The analysis should be used to pinpoint communication needs 

with the greatest payback potential in the near term. 

The meeting should also address the concerns of the work 

crews and establish the communication requir.ements for the 

entire mine. These requirements should be broken into near 

and far term objectives. The near term should concentrate 

on only as m'uch equipment as necessary to provide the 

maximum benefit to the mine. The equipment costs of this 

system should be recovered in the first few months of operation through savings in production cost. The far term 

objectives should be more general. 

DEFINING COMMUNICATION NEEDS 

In considering where communication is needed, it is important to remember that a safe, smooth running mine has 

good communications in all work areas and frequently traveled passageways, and insures that all foremen can easily 

communicate with each other. 

When determining a mine's communication needs, the following questions should be considered: 

. Who must communicate (net together)? 

. Will communications be required from vehicles? 

. Will communications be required from roving 

miners? 

. Will communications equipment be given to 

miners that work alone? 
. How many independent communication networks 

are needed for near and far term objectives? 

. What type of mining problems can be 

solved with good communica&ions? 

. Will a communications center be established? 

. How can the system be used, to improve safety 

and reduce mining costs? 

. Will the mine be willing to commit resources 

to the maintenance and well being of the 

system? 

What type of communication system'is needed? 

The answers to these questions will establish the basic 

communications requirements for the mine. 

SELECTING NEEDED NETWORKS IF MF RADIO APPEARS TO BE THE 

SOLUTION 

If MF is selected, the following networks should be considered for use in a miners communication system: 

Supervision Network 

Maintenance Network 

Production Network 

Haulage Network 

Hoist System Network 

Emergency Netwo~k 

Maintenance and Production Networks 

Since miners often prefer party line to private Line 

communication networks, some of the networks will merge 
together. For example, the maintenance and product ion networks can be operated as a single netwsrk to improve teamwork and the supervision network can be operated as a single 

network for management which prefers to have a private 

channel. 

To establish the network, vehicular transceivers should be 

installed on all service and foreman vehicles and vest 

transceivers should be assigned to keypersonnel who are 

frequently on foot during the course of their work. This 

enables keypersonnel to maintain close contact at all times. 

The base station should be installed near the center of tho 

radio coverage area, and the underground base remote control 

unit(s) should be located in the shop, warehouse, and 

supervision (production and maintenance) off ices. A remote 

control console(s) should be installed in the dispatcher's 

off ice or communications center. 

In very large mines, a separate supervision network nay be 

considered for the transmission of sensitive information 

(safety matters, accident prevention information, labor 

matters, etc.). In some mines, a direct dial pager phone 

system serves this communication need. 

Hoist - and Haulage Networks 

Hoist and haulageway communications networks are important 

in reducing the chances of accident and serious injury in 

mining. To avoid accidents, information must be instantly 
available and non-conf using. 

In haulageway networks motormen should receive only rail 

traffic information and be able to communicate with off 

train men. To achieve this a separate base station and set 

of operating frequencies should be assigned to the haubageway network. 

Vest transceivers can be assigned to all off-train men 

(loader, snapper, chute tappers, etc. ) and shaft inspectors. 

In larger mines, the dispatcher's office is the communications control center for the haulage networks. The base 

station remote control console centers are usually located 

in the communications center. In small mines, the haulage 

communication systems are most efficiently self-dispatched. 

In belt haulage systems, beltmen can be assigned vest transceivers to communicate on the maintenance network. In the 

hoist network, separate frequencies should be assigned to 

each shaft. 

Emergency Communications Network 

To create the emergency network, rescue teams could be 

equipped with vests tuned to an emergency network frequency. 

Underground ambulances could also be equipped with vehicular transceivers tuned to an emergency frequency. The emergency network could use the base station designated for the 

maintenance network. The emergency equi pment could use the 

f reql~encies assigned to the maintenance network. 
The amount of radio equipment recommended for use in each 

network depends upon the size of the mine. Table B describes the equipment used in maintenance and product ion 

networks in a medium sized coal mine. 

TABLE B. - MF equipment requirements for a longwall coal 

mine longwall coal mine ( 1MTPY). 

Product ion/Maintenance Network 

Fixed Equipment Mobile Equipment 

Key personnel Base Repeater Vest Vehicular 

(CONTINUOUS MINING (DEVELOPMENT) SECTIONS) 

For emen 1 

Roving Electrician 

Roving Mechanics 

Fire Boss 

Service Vehicles 

(LONGWALL MINING SEC'I'ION) 

For eman 

Coordinator 

Shearer Operator 

Mechani c 

Shield Operator 

For eman 

Be1 tmen 

For eman 

Motormen 

Off Train Personnel 

(BELT HAULAGEWAY) 

1 1 

(VEHICULAR HAULAGE 
PREPARING A RADIO COVERAGE AREA MAP 

The mine map can be used to indicate radio areas in the 

mine. Included on the map should be a routing of electrical 

cables (telephone and AC power cable, control lines, belt 

fire monitoring lines, etc.) and all AC switch locations. A 

color code should be used to distinguish cable types. The 

map should also show the approximate geometrical center of 

the coverage area and the base station(s) and repeaterb) 

locations. The base station should be placed at the center 

sf the radio coverage area. It must be located near 11 7 V 

AC power to enable charging of the base's standby batteries. 

When mine ventilation failure occurs in a gassy mine, an 

MSHA approved (see local MSHA district off ice) procedure 

must be used to turn off the battery supply. NOTICE: All 

installations must be coordinated with and approved by MSHA 

and state officials responsible for the inspection and 

safety sf the mine(s) in question. Base station and passageway repeater signals in coal and M/NM mines can usually 

be received almost everywhere electrical conductors exist in 

the entryways; however, the talk-back range depends upon the 

distance of the radiating antenna from the conductors and 

the type of conductors in the entryway. The talk-back range 

that can usually be expected in the entry with different 

types of conductors is shown in table C. 

Tunnel mode signal propagation enables the vehicular to 

base communication range to exceed 30,000 ft along unshield- 
sd single-pair telephone cable. The range along the shielded 

3-phase AC cable will usually exceed 10,000 ft. At each 

point where the primary power cable connects to a substation, the cable signal path loss will decrease range by 

approximately 2,000 ft. If AC switches are used in the power 

distribution system, an open switch will dramatically reduce 

the operating range. 

TABLE C. - Approxinate mobile transceiver (tunnel mode) 

talk-back range. 

(Radiating antenna at 7 ft or less from cable-425 kHz) 

TYPE OF CONDUCTOR 

Dedicated two-wire cables 

Telephone cable* 

AC power cable 

Steel cable 

Pipe 

TALK-BACK RANGE 

(feet 

* If shielded, range may be less than 20,000 feet. 

In coal mines, seam mode signal propagation enables the 

coverage area to extent to adjacent conductorless entries. 

Seam mode coverage should also be sketched on a mine map 

for each base station and passageway repeater. The t a1 kback range from a vest or vehicular transceiver to a base 

station or passageway repeater depends on the distance of 

the transmitting vest antenna from the cable and the range 

to the base station or repeater along the cable. At a 

range of 1,000 ft, the vest talk-back distance can be as 
much as 80 ft from the cable. It decreases to about 20 ft 

at a range of 5,000 ft. In a conductor free area the cellular repeater provides a vest talk-back range often exceeding 

a radial distance of 500 ft from the cellular repeater 

antenna. 

Fringe zones exist beyond the coverage areas. At distances beyond 15,000 ft from the base, the zones exist beyond 8 

f t from the conductor. Since communications received in 

these zones is low quality, passageway repeaters may be 

required to achieve acceptable coverage. 

To avoid complaints of poor cornmunieation coverage, it is 

necessary to install a wire pair in all frequently traveled 

conductorless entries. Signs should be posted indicating 

the fringe zone limit. If the received communication sounds 

llscratchyu, the miner should step toward the cable. 

Figure 1-1 (see page 17) illustrates the repeater used to 

increase the operating range of the lower power vest transcei ver . 

The repeater can be used to increase the operating range 

between mobile transceivers and the network base station. To 

increase the range between mobile transceivers requires 

that the transceivers operate in the F1 /F2 signalling mode. 

In a coal mine, cellular repeaters which use the 'lcoal seam 

moden of propagation should be used to extend radio comrnunications to the face. 
ENSURING USAGE AND MAINTENANCE OF THE SYSTEM 

Once the system is installed and checked, management must 

assure it is used. To encourage use, in and out shift meetings should be required of all keypersonnel so mining activities can be set up and coordinated with radio in mind. 

Keypersonnel should be trained in the design and usage of 

the system. The training should take place in an area equipped with operating radio equipment to -enable first hand 

experience with the equipment. It is suggested that keypersonnel be trained in the following areas: 

. The operational features of the MF communications 

systemCharacteristics of the System 

1. Component parts of the system 

2. Various circumstances applicable to the use of 

the equipment 

3. How to troubleshoot the equipment 

. Underground communications environment. 

. How to operate the equipment. 

. Theory of operation. 

. The loop antenna. 

The RF line coupler. 

. How to maintain the equipment. 

The speech and signalling procedures. 

The reliability of the radio system will depend upon adequate maintenance. Maintenance has been simp1 if ied since 

the radio equipment is designed around a standard set of 
interchangeable plugLin circuit modules. All the equt ~ment 

uses similar performing modules, thus all transceiver specif ications are similar. - The standa-rd receiver, synthe3izer, excites and transmitter modules are used in the vehicular' transceiver, base station and repeaters. The vest 

transceiver modules have been designed for use Ln hazardous 

atmospheres per Underwriters Laboratory Document UL 9 1 3 

(MSHA requirements). The vest modules perform functions 

simf Lar to standard modules; however, their design is substantially different. 

Because of the equipment s design, maintenance requires 

repairs only to the module level; nevertheless, daily maintenance checks are required on the radios and a complete 

system check is recommended every three months. 
V. MF DESIGN CONSlDERATIONS 

INTRODUCTION 

This chapter descri bes'the theoretical methods used in the 

design of the medium frequency communications system. 

The operating range of a medium frequency (MF) underground 

communications system depends upon many technical factors, 

including the characteristics of the noise generated in 

electrical power systems, signal propagation modes, the 

modulation-demodulation process, the efficiency of the couplers and the susceptibility of the receiver to desensitization by impulsive mine noise. Considerations of the.se 

factors provide additional insight into the expected system 

performance and a conclusion that narrow-band FM is the 

preferable modulation process. 

Some of the problems which arise in the design are similar 

to those encountered in surface radio systems; therefore, 

solutions to these problems follow from a vast body of the 

communications knowledge developed over the past few years. 

Todayf s advanced radio communication technology is largely 

based upon the propagation of radiated electromagnetic 

fields that travel great distances through atmospheri c constituents. While it is true that these signals provide 

re1 iable communications over free space paths, waves encountering a contrasting medium are subjected to change in the 

wave propagation constant causing reflection, diffraction, 
and absorption of the signal energy. The subterranean environment is extremely hostile to these signals. It was found 

that the signals from VHF and UHF transceivers are severely 

weakened when passing near rail haulage equipment, around 

corners or through ventilation barricades in the mine. As a 

result, communications fail (go below threshold) exactly 

where qua1 ity voice communications are needed the most. 

Leaky coaxial transmission 1 ines excited by high power transmi tters improve tunnel communications; however, old problems persist and new problems emerge. Communications through 

venti Eation barricades and to the point of equipment breakdown were still not reliable. Further, the practicability 

of installing coaxial, cable in the working area of the mine 

seems questionable". 

Theoretical and experimental, work by the USBM (8) - and 

others have shown that MF signals propagate through natural 

. media (rock, coal, etc.) and great distances down passageways in the underground mining complex. Emslie and Lagace 

(9) - (10) - established that a loop antenna would excite a 

coal seam mode enabling the propagation of MF signals 

through a coal seam. Other work (2) considered the excitation of monof ilar and bif ilar propagation modes on a transmission line in a passageway. Low loss transmission 1 ine 

propagation modes were discovered. 

Whole mine radio signal transmission and distribution 

depe,nds upon the model excitations described above. One of 

the advantages of MF signals over VHF/UHF planar wave sig- 
nals is that MF signals are not subjected to extremely high 

path loss when passing near mining equipment, through vent ilation barricades and around corners. This is especially 

important when communicating in a confined mine area or when 

repairing mining equipment at the point of equipment breakdown. 

The operating range of an MF underground radio communication system depends upon many factors including: 

. antenna magnetic moment (transmitted output power), 

. the signal propagat ion mechanism (modes), 

. carrier signal coupling efficiency, 

. the system modulation-demodulation processes, and 

. susceptibility of the receiver to desensitization by 

electrical noise and other interfering/jamming 

signals. 

In a communication system, operating range is a strong 

function of the signal propagation mechanism and medium, and 

antenna coup1 ing efficiency between other antennas and 

transmission lines in the signal distribution system. These 

factors have been extensively studied in the context of the 

subterranean communication problem (10) - as well as in the 

related terrestrial communication 1 iterature. With respect 

to these factors, range optimization procedures are well 

known. 

Another dominating factor in the design of any communication system is the radio frequency interference (RFI) generated by the mine electrical system. A review of the 
measured noise data (12) - (13) - reveals that the noise field 

strength varies over an enormous range (by several orders of 

magnitude - 60 to 100 dB variation). The noise is greatest 

near AC to BC converters (rectifiers) and along the trolley 

conductor in a rail haulage system. Away from the haulageways, the electrical noise level is as much as 40 to 50 dB 

below the haulageway level. The noise level also changes 

over a large range during a typical working shift. The 

magnitude of the noise spectrum is a function of frequency. 

For rrequencies below 50 kHz Its magnitude increases with 

decreasing frequency at an average rate of approximately 14 

dB per octave. Above 100 kHz, the noise density decreases 

at the rate of 6 dB per octave. 

Receivers used in the communication system must be designed to operate in high electrical nofse environments. High 

noiase pulses .cause ringing to occur in filter networks 

associated with IF amplifier and discriminator circuits. The 

ringing interf ere8 with signal processing. Ringing can be 

minimized by selecting filter transfer functions with approximately linear phase response. The RF section must discriminate against the noise as much as possible and then 

amplify the modulated signal and noise in a linear way 

before converting the signal and noise to the proper intermediate frequency (IF) and subsequent signal processing. 
SIGNAL PROPAGATION MODES 

Coal seam and tunnel signal propagation modes were evaluated in a series of underground tests conducted by the 

Bureau (1 -- 41 5 The test results indicated that the coal 

seam mode enabled maximum commun.ication8 range in the frequency band extending from 400 to 700 kHz. This band is a 

subset of the medium frequency, (300 kHz to 3000 kHz) 

portion of the electromagnetic spectrum, Further, as predicted by theory (u), low loss signal propa'gation modes 

existed in entryways with electrical conductors. In-mine 

testing confirmed the existence of low loss signal ' propagation in the entries of both coal and M/NM mines. 

-- Coal Seam Propagation - Mode 

The theory of electromagnetic wave propagation in layered 

formations has been rigorously developed by Wait (16). Figure 5-1 illustrates the model used to analytically determine 

the wave propagation characteristics in a coal seam. 

The coal seam is bound by two conducting half spaces with 

the conductivity of the roof rock (opr) being several orders 

of magnitude more than the conductivity of coal (0,) . Along 

the transmission path (r), the vertical orientation of the 

transmitting loop antenna (source) produces a horizontal 
B Coal 

0 r 

entry 

transmi tting 

antenna I 

FIGURE 5-1. - Seam mode field components. 

magnetic field H and a vertical electrical field Eze ThBse 

fields are relatively constant over the height of, the coal 

seana5. It has been shown (17) - that the fields die off 

exponentially in the rockd. At large radial distances (r) 

from the antenna, the fields decay exponentially. The rate 

of this exponential decay is determined by an effective 

attenuation constant which depends on losses in the coal and 

rock, and on the coal's dielectric constant. There is also a 

r -'I2 factor at large radial distances because of the 

wave1 s cylindrical spreading. The zero-order TEM magnetic 

field H in the transmitting loop plane is given by: 

H+ = (iPIk2/8 be) HI ' (kr) 

5~ith perfectly conducting boundaries, when (err) is finite, 

the zero-order mode field varies with z and r owing to wave 

propagat ion across the boundaries . 

6~kin depth in rock is 0.69 meter, (err) =I mho, 520 kHz. 
where M = the magnetic moment of the transmitting loop 

antenna , 

be? = b + 1 /2 is the effective half-height of the 

coal seam, 

6, = the z direction skin depth in the rock, 

k=B = the complex propagation constant in the radial direction, and 

(2)' H1 (kr) - the derivative of the first-order Hankel Function for an outgoing wave. 

On taking the asymptotic form of the Hankel Function, the 

azimuthal component of the magnetic field is: 

The attenuation and phase constants are given by: 

and 

where 2(&-2~fc,) 

P = 

2 2 

b - 4.n f ]lo€, 

(5) 

and 

'be may be complex. 

8~alid at ranges kr < 1. 
The zero-order TEM wave described by equation (1) has been 

compared with experimental measurements (1 - 8). These measurements were found to be in close agreement with equation 

(1). Equations (3) (5) and (6) are useful in determining 

the dependence of the wave propagation constant on seam 

parameters. 

The attenuation rate in coal is approximately 4 dB/100 

f t (6c= 1.4 x 1 oW4 MHO/m, &,=7, (a,) =ls/m, b= lm,350 kHz). The 

attenuation rate is expected to increase with decreased coal 

seam thickness and f ncreased conducti vi ty. The rate decrease3 with increased rock conductivity. The rate also 

depends upon frequency. 

Tunnel Propagation Modes 

Bif ilar and monofilar signal propagation modes exist on a 

pair of wires in the wireplant. The current flow of each 

mode is shown in Figure 5-2. 

Roof 

- Conductors 

" Floor 

A Bifilar (differential) mode 8 Monofilar (common) mode 

FIGURE 5-2. - Signal current (I) flow on conductors .9 

9~he number of possible modes is n-1 where n is the number 

of electrical conductors in the entryway. One of the conductors may bethe conductingwall of the tunnel. 
The excitation of monofilar and bif ilar modes on a tunnel's transmission line have been examined (1 - 1). In the 

monofilar mode Calso c a l l e d ' t h e balanced, differential, 

coaxial, or symmetrical mode) the current flows in the same 

direction in each conductor and returns as a surface current 

in the mine tunnel. In fact, only one. conductor is actually 

needed. This mode is easily excited by a magnetic coupling 

from a loop antenna anywhere in the mine tunnel, but it 

suffers high attenuation because the return current flows in 

the lossy tunnel wall. 

I n t h e bif ilar mode which requires two conductors (also 

called the unbalanced, asymmetrical or differential mode), 

the forward current in one conductor returns through the 

other conductor. This transmission line mode has low attenuation because the return current flows on the second wire 

rather than through the surrounding rock. Excitation of 

this mode depends upon magnetic flux threading the area 

between the transmission line conductors. 

Random imperfections in the wireplant and mine tunnels 

cause mode conversion to occur on a two-wire transmission 

1 ine. Non-uniformi ties in the tunnel cross section, cable 

sagging with respect to the roof, and incidental changes in 

conductor spacing cause the characteristic impedance (zO) 

of the cable to change along the line. Changes in t h i s l i n e 

characteristic impedance cause radiation and reflection of 

the signal energy thereby causing monof ilar and bif ilar mode 

interchange conversions to occur a1 1 along a 1 ine. 
Radio coverage in conductorless entries can be provided by 

installing a dedicated cable such as two-wire telephone 

cable. It is easy to instal1 and supports both the monofilar 

and bif ilar modes of signal propagation. The MF antenna-tocable coupling efficiency is high due to monofilar coupling and the operating range is great due to the bif ilar mode 

propagation. 

MODULATION PROCESSES 

The quality of a communication system is judged by analyzing the recovered audio signal at the output of the receive~. The measurement of r e c o v e r e d a u d i o o u t p u t signal to 

noise ratio (SIN), has come to be a reliable quality standard. It is made at the output of the discriminator in an FM 

receiver system and at the output of the product detector in 

an SSB r'eceiver.' 

The output signal to noise ratio (SIN), may be maximized 

by enhancing the signal in the presence of noise. Predetection filtering is useful in discriminating against noise 

that occurs outside the occupied bandwidth of the modulated 

signal. Noise discrimination improves by selecting higher 

operating frequencies (Fo) and by using the narrowest post 

detection bandwidth that is compatible with the modulated 

signal. 

Spread spectrum modulating processes can increase the 

output signal to noise ratio (SIN), by providing "process 
gainn (the result of subsequent bandwidth spreading and despreading) in the communication channel. One of the most 

familiar spread spectrum modulation processes is seen in 

conventional frequency modulation (FM) where the modulation 

process produces a much wider occupied bandwidth than the 

information bandwidth. When discussing the signal-to-noise 

ratio, using frequency modulation at the output of the 

receiver, it is customary to relate the signal-to-noise 

ratio at the output of the receiver (S/N), to the carrierto-noise ratio (C/N) at its input. Both ratios are expressed in terms of powers or in a corresponding decibel ratio; 

The output signal-to-noise ratio (S/N), represents the 

ratio of the power of the recovered signal (typically audio 

signal) to the power of the electrical noise collected over 

the bandwidth of the receiver. At the input of a receiver 

the signal power is represented by the power of the incoming 

carrier. To form the carrier-to-noise power ratio (designated C/N) we relate the carrier power at the input to the 

noise power at the input collected over the bandwidth of the 

receiver. 

Figure 5-3 shows the process gain characteristics for FM, 

SSB, and AM modulation processes. 

FM process gain increases with the modulation index (B) 

given by: 
CARRIER YO NOISE RATIO (c/H) 

FIGURE 5-3.- Typical modulation process gain. 

where F = the peak frequency deviation, 

and f, = the highest frequency in the modulating 

signal spectrum. 

The break in the slope of the FM cha~aeteristics marks the 

threshold in process gain. For carrier to noise (C/N), 

ratios below the threshold value, the recovered signal to 

noise (s/N), ratio rapidly degrades. In contrast to FM, 

single sideband (SSB) does not exhibit a threshold. 

When considering process gain, FM has an advantage in a 

communication system that can be operated with a large 

carrier to noise (C/N) ratio. The advantage, of course, 

depends upon the modulation index and the carrier to noise 

(C/N) ratio. In the near field communications system, the 

combined effects of near field propagation boss and noise 

force the carrier to noise (C/N) ratio to be low at times. 

In some cases, SSB might be preferable since it does not 
exhibit a threshold. Thus, SSB signals can be understood 

better below threshold. 

In today's congested-band terrestrial radio services, SSB 

is favored because the RF occupied bandwidth is the same as 

the information bandwidth. This means that more user service 

channels can co-inhabi t the assigned service channel. On 

the other hand, the occupied bandwidth of an FM signal given 

by: 

or 

BW = 2 Af where B < 1 

is much greater than the information bandwidth. 

The operating range of the MF communication system is 

optimized when the magnetic moment (M) of the antenna is 

maximized. For a given transmitter output power and loop 

area, reducing the bandwidth (BW) of the loop increases the 

magnetic moment as shown in equation (7). 

The occupied bandwidth of the modulated loop current signal must be within the bandwidth (BW) of the tuned loop. 

The tuned loop antenna attenuates signal components that 

occur near the band edge frequencies of the loop passband. 

The loop antenna will not reproduce magnetic moments for 

these band edge signal components. When the loop bandwidth 

is less than the occupied bandwidth, the modulation index B 

is reduced in the carrier signal. The modulation carrier 

signal occupied bandwidth determines the circuit Q(QC~~) of 
the loop antenna as: 

n W~ 

In practice, Q~~~ is bounded by considerations of whether or not it can be built. The higher bound is determined 

by unloaded Q(Q,) of the antenna structure. The lower bound 

is determined by the available transcei ver power supply 

voltage and the current drive capability of the' power amp1 ifier. The lower Q bound militates against the use of wideband FM in the communication system. 

SSB and narrowband FM can be compared on the basis of' 

occupied bandwidth. The SSB circuit bandwidth can be onehalf that required for narrowband FM. With respect to 

equivalent magnetic moment generated by the transmitting 

loop antenna, the SSB transmitter power can be 3 dB less 

than the FM transmitter power. On the other hand, an SSB 

loop antenna matching network requires a higher transfer 

ratio (n). The higher ratio makes the SSB antenna more 

sensitive to loading caused by nearby metal objects. Loading 

will cause the terminal impedance of the antenna to change. 

The Automatic Level Control (ALC) associated with the SSB 

transmitter will cause output power to vary with loading. 

By way of contr'ast, an FM transmitter's final amplifier can 

be designed to produce almost constant load plane power over 

a wade range of' impedance. Thus a narrowband FM transmitter 

will produce constant power over a .large range of incidental antenna loading conditions. 
From the standpoint of recei ver i'mpulse noise succeptabi lity, narrowband FM would appear t.0 have an advantage over 

SSB because of limiting in the FM receiver. However, repet i t i ve impulse noise causes the predetection circuits, to 

ring in both FM and SSB receiver9. This degrades the recovered (S/N), ratio. In an SSB receiver, each noise burst 

causes the SSB automatic gain control (AGC) circuit to 

immediate1 y desensitize the recei ver for the release period 

(time constant) of the AGC. Because of the way AGC control 

signals are developed, decreasing the release period below a 

certain value causes instability in the AGC control circuit. 

While impulse noise degrades the FM receiver recovered audio 

signal quality, an SSB receiver is desensitized for the AGC 

release period which dramatically reduces the intelligibility of the voice signal. 

The operating range of prototype 1 Watt FM transceivers 

was compared to 1 Watt SSB transceivers in the York Canyon 

mine near Raton, New Mexico. Before the field test, laboratory measurements were made on the FM and SSB transceivers 

to ensure that the output powers were identical. 

Along the longwall, the FM and SSB medium frequency (520 

kHz) transcei vers provided excellent coverage. In the entryways, the SSB transceiver range was about 400 ft while 

the FM transcei ver provided entryway coverage exceeding one 

mile. 

The above factors indicate that narrowband FM modulation 

has some important advantages over SSB. Narrowband FM was 
selected for use in underground wireless communication systems. 

COUPLING MF SIGNALS TO THE WIREPLANT 

This section discusses the coupbiwg of signals between 

transmitting loop antennas and the wirepbant. The equations 

define biff bar mode coupling to cables in free space. Monofilar coupl ing also occurs, but the signal level depends 

upon the tunnel surface impedance. 

Transmitting - Loop Antenna - and Two-Wire Transmission 

Coupling 

Planning of the communication sys.temls design is aided by 

theoretical equations that relate the bifilar current signal 

bevel induced in a wireplant transmission 1 ine to the magnetic moment (M~) of the loop antenna operating frequency (f), ' 

loop to line distance (R) and conductor separation (b) in 

the line. 

Equation (9) shows that coupl ing improves with frequency, 

the first power of magnetic moment, and an increased separation between 1 ine conductors. Coupl ing improves with the 

second power by distance from the loop to the line as the 

loop approaches the 1 ine. 

Coupl ing also depends upon the geometrical relationships 

between the transmitting loop antenna and the two-wire 

transmission line. Some of the possible geometrical rela- 
tionships are illustrated in figure 5-4. 

TWO WIRE TRANSMISSION LINE 

rTRANSM ITTlNG COOP 1;' ANTENNA t%f 

b.) ALIGNED ; EDGE ON 

c.) COPLANAR 

FIGURE 5-4. - Geometries for a vertical transmission line 

and transmitting loop antenna. 

In the aligned case, shown in figure 5-4 (a), the loop 

antenna and the line conductors lie in a vertical plane. 

The distance R separates the planes. The coaxial configuration occurs when the center line of the transmission line 

coincides wi th the axis of the transmitting loop antenna. 

This geometry is frequently encountered when vest and vehicular transceivers are in an entryway. The axis of the loop 

may also be below the center line of the transmission line. 

This a1 ignment induces strong bif ilar currents and weaker 

monofilar elements in the line. The aligned, edge-on case 
of figure 5-4 (b) induces stronger monofilar and weaker 

bif ilar currents. The edge-on situation produces unfavorable coupling. The coplanar case in figure 5-4 (c) produces 

both monof ilar and -bif ibar currents. 

Equations have been developed that define the induced 

current and voltage in a transmission cabbe. The two-wire 

signal transmission medi urn is represented as a transmission 

line which is terminated at both ends by load impedances (zl 

and 9) that are equal to the characteristic impedance of 

the line. The line is shown in figure 5-5. 

The conductor spacing is represented by b. The field 

produced by the loop threads the area enclosed by the conductor spacing and a length (1) along the line. Since the 

field between the lines does not have a uniform magnitude, 

( 1 ) r e p r e s e n t s t h e length of the line giving it an equivalent total threading flux, as if the field were uniform 

- v fi 

w 

CENTER 

=I f LINE 

R 

I 

I 

FIGURE 5-5. - Equivalent transmission line network for 

induced voltages and currents (Z1 =Z2=Zo)- 
within the length (1) and zero elsewhere. The length (1) is 

a simp1 if ied mathematical model; however, if more elaborate 

mathematics were used, (1) would approximately equal R. When 

2 .rr R<<X, the induced voltage is given by: 

vOfMTb 

v = R~ 

where f = the operating frequency, 

p0 = the magnetic permeability of free space 

R = the loop to line distance, 

b = the separation between conductors, 

and MT = the loop magnetic moment. 

The current flowing in the 1 ine is given by: 

In equations (9) and (lo), the voltage and current signals 

induced in the line are directly proportional to the magnetic moment of the radiating loop antenna. Maximizing the 

!magnetic moment for a given transmi tter power will maximize 

the induced signal and the system's operating range. Increasing the transmission 1 ine conductor spacing decreases 

the line current since the line characteristic impedance 

(Zo)increases as shown by equation (11). 

The voltage (V) depends on the first power of the conductor separation b, and Z0 depends on its logarithm. By 

maximizing b, there is some improvement in the obtainable 
operating range. Coup1 ing efficiency increases with the 

first power of operating frequency (f ); furthermore, the 

line attenuation rate increases with frequency. Because of 

line loss, the use of lower operating Frequencies is advisable. Yet, though transmission losses are lower at lower 

operating frequencies, increased mine generated electrical 

noise (EMI) causes the carrier-to-noise ratio (C/N) to degrade. This occurs because the EM1 increases as frequency 

decreases. Mine tests indicate that the carrier to noise 

ratio (C%N) is optimized around the operating band of 250 

and 500 kHz. 

Transmiting - Loop Antenna To Receiving Loop Antenna Via - 

Transmission - Line coupling 

The coupling of magnetic fields from current carrying 

conductors to distant receive antennas is shown in Figure 5- 

6. 

CABLE 

CARRYING 

MF BlFILAR 

SIGNALS IVING LOOP 

LR,J 

FIGURE 5-6. - Signal coupling to a receiving loop antenna. 
The bif ilar line current in the cables near the receiving 

loop produces a magnetic field which is mathematically represented by: . pofMT b2 

The electromotive force (EMF) produced in the recei0ving loop 

is gi ven by Maxwell's first equation: 

where B = the flux density threading the loop area A, 

n = the unit vector of surface threaded by B, 

and N = the number of turns in the receiving loop. 

The equation simp1 if ies to: 

emf = upo HSARNR 

By combining equations (1 2) and (1 41, the direct relationship between the transmitting antenna moment (MT) and the 

receiving loop voltage (VL) is obtained by: 

Equation 16 is important in considering loop to loop via 

line coup1 ing in the radio system. From the system design 

point of view, the voltage induced in a distant loop in- 
creases with the second power of the transmission line 

conductor separation (b), When installing transmission 

lines for the purpose of increasing radio coverage in entryways, ruggedized twin lead cable is recommended since it has 

good conductor separation, Given the antenna physical mounting constraints (height a.nd length) , the receiving loop 

voltage increases with the number of turns, thereby, increasing the coupling efficiency of the antenna. Coupling 

increases with the second power of operating frequency. 

Attenuation Rate 

The attenuation rate on twisted telephone cable and AC 

power- cable was measured in coal and M/NM mines. Accurate 

measu~ements are extremely difficult to make because (1 ) 

the distance variation between the transmitting/receiving 

loop antennas and the signal transmission line in the 

entryways causes uncontrollable second power of distance 

change effects in induced line currents; (2) there is an 

elevation deviation of the wire in the entry; (3) the shield 

conduction of the telephone cable may not be continuous 

through a telephone line junction box; (4) standing waves 

caused by discontinuities resulting Prom open or short circuits of the transmission lines may exist; and (5) mining 

operations limit vehicular transportation and prohibit mining processes to be disrupted for measurement procedures. 

Even with these difficulties, average attenuation rates were 
measured in four mines. 

The rates were found to be similar in both coal and M/NM 

mines. The induced monofilar current flowing on the transmission line at the base station was measured with a calibrated current transformer (probe). 

Figure 5-7 shows a source (transmitting loop antenna) 

mounted on a mine service vehicle. The induced current was 

measured at the portal as the vehicle moved away. The 

measured data is shown in figure 5-8. The attenuation rate 

was approximately 2.4 dB/1 ,000 f t . 

VOLTMETER 

SINGER 

MODEL NM-25T 1 

FIGURE 5-7. - Setup for measuring induced line current. 

With a vehicle-to-base station separation range of 5,000 

f t , the induced line current was approximately 707 microamperes. The mine generated noise was also measured during 

each field test. The induced noise level usually measured 

2.2 microamperes in a medium sized coal mine with conveyor 

haulage. The carrier to noise ratio was found to exceed 50 

dB. 
The horizontal dashed line (C/N = 20 dB) represents the' 

I1threshold line currentv needed by the system for good 

communications. The intersection between the dashed line 

and the projection of induced line current (not shown) 

forecast that the maximum communications range along telephone cable will exceed 20,080 ft at 520 kHz. 

V I i 1 I 1 I 1 I I 1 I i 

12 3 4 5 6 7 8 91011 121314 

MAN WAY DISTANCE FROM BASE (THOUSAND FT.) 

Figure 5-8. - Induced line current versus vehicular transceiver distance from base. 

LOOP ANTENNA DESIGN CONSIDERATIONS 

The underground communication range depends upon the abil- 
ity of the transmitting and receiving antennas to efficiently couple coal seam mode and transmission line mode signals. 

Equations (9) and (16) show that the level of signals induced on a transmission line, or at the receiving antenna 

output terminal depend upon the first power of the transmitting antenna magnetic moment (MT)* The recei ving signal 

also depends upon the first power of turns-area product 

(N~A~) of the receiving antenna. The transmitting loop 

magnetic moment (MT) is produced by an excitation current 

(I) flowing through the transmitting coil shown in figure 5- 

9. 

FIGURE 5-9.- Transmitting loop antenna excitation current 

and magnetic moment. 10 

''The direction of the magnetic moment can be determined by 

the "rlght hand rulef1 whereby the fingers of the right hand 

polnt in the direction of the magnetic field when the 

conductor 1s grasped such that the rlght thumb points in the 

direction of the current. 
The magnetic moment is defined as: -b I M~ I = NIA ampere turn meters 

where A = the area of the. loop in'square meters, 

1 = the level of the excitation current in amperes, 

and N the number of turns in the transmitting loop. 

The magnetic moment is aavector that is perpendicular to 

the plane of the loop. The transmitting loop tusns-cusrentarea product (NIA) is a scaler quantity. The loop antenna 

produces a horizontal magnetic dipole (HMD) when the transmitting loop antenna lies in a vertical plane and a vertical 

magnetic dipole (VMD) when in a horizontal plane. 

Theoretical Basis -- For Loop Antenna Design 

The theoretical basis for the design and optimization of a 

tuned loop antenna is presented in this section. It wil b be 

shown that the use of a twin parallel connected coil instead 

of a conventional single coi 1 occupying the same cons trained 

antenna area offers some important advantages. First, since 

each unit of the twin-coil is physically smaller, its selfresonant frequency is greater than that of a larger single 

coil design. Secondly, the unloaded Q of the twin-coil is 

greater enabling the receiving response to a given magnetic 

field strength to be greater than in a single coil loop. 

Finally, because the twin-coils are connected in parallel, 

the terminal inductance of the coil is less than a single 

coil antenna with the same number of turns. Thus, the twin- 
coil loop produces less electric field stress across the 

antenna to coil and tuning capacitor. The magnetic moment 

of any conceivable system of current loops in any given 

geometrical configuration is mathematically represented by: 

where vectorial integr.ation is conducted for each coil (I) 

and all its turns, 

i = running subscript identifying the coils, 

Ii= current the ith coil, 

-9 

r a position vector with respect to any selected 

origin. The result does not depend on the 

selection of the origin, provided each loop is 

geometrically closed, 

-9 

and dl = incremental vector, tangential to the loop of 

each coil in the direction of current flow. 

The concept of magnetic moment can be used to compute electric and magnetic fields at remote locations, The def inition of "remoten is a large distance as compared with the 

dimensions of the coil, For coplanar coils, the magnetic 

moment is simplified to: 

-C 

where, piis a unit vector perpendicular to the plane of the 

ith loop, Its direction is determined by the "right handw 

rule. 
The excitation currents flowing in a twin-coil transmitting loop antenna are shown in figure 5-10. 

FIGURE 5-10. - Twin-coil transmitting loop antennas. 

The twin-coil magnetic moment is mathematical 1 y represented by: 

The equivalent circuit of a single coil loop antenna is 

shown in figure 5-11. 

FIGURE 5-1 1. - Equivalent circuit of a single-coil loop 

antenna . 

The inductance (L) represents the apparent i nductance of the 

coil- used in the construction of the loop antenna. It is an 

apparent inductance because distributed capacitance (not 
shown) between coil turns tune out some of the real inductance. The inductance of a planar air-core coil is mathematical 1y approximated by: 

where A = the area in square meters, 

N = the number of turns, 

and KO= the form factor. 

The inductance form factor depends on the ratio of the 

diameter (d) to the width (W) of a single layer winding. 

The incidental dissipation loss associated with the coil is 

represented as the AC ohmic resistance which is determined 

by equation 22. 

where, Q, is the unloaded Q of the coil. The capacitance 

(CO) and loop inductance are resonant at the operating 

frequency. This is represented by equation 23. 

The typical unloaded Q of tubular and planar antenna coils 

are shown in figures 5-12 and 5-1 3. In t h e t u b u l a r antenna 

design, the antenna inductor was wound inside a 1 1 /4 inch 

Lexan pipe. This allowed the starting and ending turns of 

the wire to be physically closer together which in turn 

lowered the unloaded Q. In a planar antenna inductor design 

a flat coil is placed between Lexan plates. Comparison of 
the figures shows that the planar coil has significantly 

greater unloaded Q. Figure 5-12 shows the family of unloaded Q curves for a tubular single-coil antenna structure. 

The unloaded Q in the MF band degrades as turns are increased. 

The ohmic value of the coil resistance (r,) is frequency 

dependent. It increases at higher frequency because of skin 

effects associated with current flowing in the coil conductor. 

IQO 200 400 

FREQUENCY IN KILQHERT Z 

FIGURE 5-1 2. - Unloaded Q versus frequency for a f 1 exi ble 

rectangular tubular Lexan antenna sing1 e rectangular coil 

(36" x 8" coil). 
FIGURE 5-13. - Unloaded Q versus frequency for a flexible 

rectangular planar Lexan antenna (1211 x 12" coil). 

When a single-coil loop antenna radiates at a high power 

level, high electric fields between turns can produce corona 

effects. This also causes the apparent r1 to increase. The 

resistance (rC) represents the dissipation loss associated with the tuning capacitance (Co). Depending upon 

thes type of tuning capacitor, the ioss resistance (rc) may 

decrease with increases in operating frequency and the level 

of current flowing through the capacitance. The transmitter 

to loop matching network may be realized by using two capacitors as shown in figure 5-14. 
RESONANT - NETWORK 

--- 

L 

TRANSMITTER 

--- 

FIGURE 5-14. - Typical matching network. 

The transmitter output impedance (ZT) IS transformed by the 

matching network and becomes part of the resonant network. 

The transformation ratio (n) is given by: 

Impedances are transformed by the second power of n and 

voltage by the first powe~ of n. The resistance Ro is the 

impedance of the resonant network at the antenna operating 

frequency (U ) and is given by: 0 

The circuit Q for the antenna structure is: 

where, BW is the 3 dB radian frequency bandwidth of the 

antenna. 

Because low dissipation loss porcelain capacitors are 

used in the structure, the ohmic value of rc (shown in 

figure 5-11) is much greater than Ro. Under this condition, 
the resistance (R~) represents only the parallel combination 

of the transformed transmitter out put impedance and the 

ohmic value of the coil loss given by: 

when Ro is large and I1 < I, then 

and 

The circuit capacitances are mathematically represented by: 

and 

when n is large, then Co = C1. Under this condition the 

upper capacitor primarily determines the resonance f requency of the antenna structure. When the loop is transmitting, 

the capacitor (c,) must withstand the RF voltage developed 

across the coil. In our case the transmitting loop antenna 

is also used for receiving (as a receiver input circuit). 

The matching to the receiver input, with the aid of the 

capacitive voltage divider of the ratio (n) must represent a 

good design compromise with the transmitting application. 

It must safeguard an acceptable value of receiving input 

bandwidth. The higher the unloaded Q value of the loop 
winding, the better will be the efficiency of signal power 

transition to the receiver input. 

The RF voltage (V) appearing across the inductances and 

capacitances in a transmitting antenna structure is mathematically represented by: 

When considering the antenna design problem, the question 

0% whether to use separate,antennas for transmitting and 

receiving or to combine the functions into a single structure arises. Receiving antennas are optimized by maximizing the number of turns (NR) in the loop4 A transmitting loop can have fewer turns, Regardless of how the 

antennas are used, the self-resonant frequency must be above 

the operating frequency range of the communication system. 

When considering the highest self-resonant frequency in the 

loop antenna design, the optimum loop shape is planar. This 

coil configuration minimizes the s e l f - c a p a c i t a n c e between 

turns. Capacitance can be reduced further by spacing adjacent turns, which reduces the electric field gradient between turns. It should be noted that the gradient can be 

partially canceled in a coil by crossover winding techniques. 

When the antenna area is constrained (as it is in this 

design problem), it has been shown (20) - that transmitting 

loop magnetic moment may be represented by: 
where P = the power dissipated in the loop, 

and k, = a constant that depends on area. 

Equation (33) is relevant in the antenna optimization problem. The magnetic moment depends on power and bandwidth 

and not on turns (N). Equation (16) shows that receiving 

sensitivity of an antenna increases with turns, thus the 

receiving function of a combination transmi ttinghecei ving 

loop can be optimized by increasing turns. As a practical 

matter, turns can be increased until the self-resonant frequency of the coil approaches the operating frequency band 

from above. Another limit on turns is implied in equation 

(32). Since L increases with the second power of turns, 

maximum turns are determined by the dielectric withstanding 

voltage of the capacitors and coils used in the structure. 

The principal objectives in the antenna design are: 

. To optimize the design of a combination transmitting 

and receiving loop antenna, and 

To optimize the available area in order to design a 

loop antenna which has the highest magnetic moment 

and lowest RF voltage across it. 

The last objective led to the solution involving twin parallel-coils forming the antenna in preference to an antenna 

using a single coil. The reduction in voltage between 

single and twin-coil antenna designs will be examined on the 

basis of equal power dissipation and loaded circuit Q. Both 

antenna designs will be assumed to have the same number of 
turns and occupy the same total surface area. The area of 

each twin-coil is one-half the area of a single coil. The 

inductance given by equation (21 ) of the twin-coil is as- 

%umed to be proportional to the square root of the applicable area. Besides the area difference, the fact that the 

twin-coils are mutally coupled must be taken into account. 

The power (P) dissipated in the single coil loop with an 

inductance (L) and ohmic loss resistance (r, ) is given by: 

In a twin-coil loop, the di%sipated =power iq mathematically 

represented by: 

where %, 1 is the inductance of a single coil in the twin- -T 

col.l struchure. The inductance includes the effects of the 

mutual coupling and smaller area of the coil. By equating 

equations (34) and (351, the relationship between current 

flowing in a single coil and in each coil of the twin-coil 

%tructut-e is given by: 

The RF voltages must be compared at resonance. The RF vol- ' 

tage(VS)appearing across a single coil and the voltage (VT ) 

apearing across a twin-coil structure are given by: 

VS = II~,LI (single coil) 
- vT - /IT uo~+l = 11u0 ILL'' I (twin coil) (38) 

2 

The voltage developed across the twin-coil is less than that 

developed across the single-coi 1. 

The currents and inductances in the mutual 1 y coupled twincoil loop are derived in the following coupled circuit 

analysis. The mutual coupling (M) in the twin-coil loop 

antenna is illustrated in figure 5-15. 

a.0) MUTUAL COUPLING b .) DEPENDENT SOURCES 

FIGURE 5-15. - Inductive coupling in the twin-coil loop 

antenna. 

The dependent voltage sources (shown in figure 5-1 5) have 

been substituted for mutual coupling. Assuming that the 

coils are identical, the branch impedances are: 
where S is the Laplace operator and (r , r ) represents 

the ohmic value of the dissipation loss associated with each 

coil as well as the equivalent load, Thi% is shown in 

figure 5-16, 

FIGURE 5-16. - Equivalent circuit of the twin-coil 

antenna. 

The voltage common to both loops" can be expressed as: 

and 

then 

from these equations 

v -- + VSM 

- I1 - 

- I1 ( SM) ' 

I1 z1Z2 z1z2 

The branch current becomes 
Since 

then 

where the "inductance (LTt) 13 given by: 

Equation (49) allows the twin-coil loop antenna structure to 

be considered as composed of two uncoupled coils of inductance LT9* The range of coil coupling is: 

The magnetic moments of sing1 e-coil (1~~1) and twin-coil 

-+ 

structures(IMTI) are: 

(twin-coil) . 

In the case of the planar twin-coil structure, the mutual 

coupling coefficient is negative and approximately k = -0.1. 

Then, the uncoupled inductance of each coil of the twin-coil 

follows from equation (51) as: 
and 

Since 

Then 

By substituting equation (53) into equation (38) and equation (521, a direct comparison of WF voltage and magnetic 

moment can be made between a single and twin-coil antenna 

+ 

and 18~1 z 0.88 lMsl (54) 

Single and twin-coil planar antennas were constructed and 

evaluated, The antennas were each driven From a vehicular 

transcei ver. The loop design parameters and measured results are shown in table D. 

During the test, the BC power applied to the vehicular 

transceiver power amplifies was 26.4 watts. When the transcei ver was terminated by a 50 ohm power meter, the power 

meter reading was 20 watts. The capacitances C1 and C2 were 

used to couple the transmitter (source) to the loop antenna 

coils. The transmitter output impedance (antenna source 

impedance ZS ) was known to be non-linear. The capacitance 

values were selected to produce an apparent circuit bandwidth OF 12 kHz centered about the operating frequency (400 

kHz) . When the loop antenna was used as a receiver, .it 

was terminated by vehicular transcei ver recei ver input impe- 
dance (50 ohms). The measured receiving bandwidth was approximately 12 kHz. 

TABLE D. - Comparison single and twin-coil vehicular antenna 

characteristics. 

PARAMETER SINGLE-COIL TWIN-COIL 

Frequency (kHz) 400 

Bandwidth (kHz) 12 

Turns (N) 11 (total) 

Inductance (pH) 149 

Area (m2) 0.161 

Unloaded Q (QU) 275 

Capacitance 

Cl (Pf) 1160 

C2 (Pf) 15950 

CO (Pf) 1081 

n = 1 + C2/C1 14.75 

uoL(ohrns) 374 

RF Voltage (Vpp) 2365 

V1 Voltage (Vpp) 222.5 

Loop Current (each coil) 

I (App) 6.32 

Moment (ATM peak) 5.6 

400 

12 

11 (per side) 

43.5 

0.168 

321 

The tabular results are noteworthy in several respects. 

First, even though the circuit Q of both antennas are identical, the unloaded Q of the individual coils are not. This 

follows from the fact that the smaller planar twin-coil 

exhibits a higher self-resonance frequency (2.8 as compared 

to 1.5 MHz in the single coil). Secondly, the RF voltage 

appearing across the twin-coil structure is less than the 

voltage appearing across a single coil structure. The relative ratio of 0.55 compares closely with the value predicted by equation (54). The magnetic moments were found to be 
almost identical in the single and twin-coil loop antennas. 

This was attributed to the non-linear behavior of the source 

impedance. In summary, the twin-coil loop antenna RF voltage for a given magnetic moment is less than in a single 

coil design. Because of the lower operating voltage, the 

dielectric withstanding voltage requirements are less severe. 

Design Optimization Process 

Coupling can be optimized by maximizing the (NIA) for the 

transmitting antenna and the (!+IRAR)- for the receiving antenna. Like all optimization problems, constraints placed upon 

the individual variables of the product significantly increase the design task. The optimization process to be 

described in this work examines each independent antenna 

A 

design variable (xi) and its upper bound (Bi) . The upper 

bound for each variable may be written as: 

where the subscripts i = 1,2,3,, . . identify the variables 

and constrained values. 

If separate antennas are used for receiving and transmitting functions, then the optimum feasible design will require different upper bounds for the antenna area and turns. 

If the functions are combined in a single antenna design, 

the same upper bounds apply. Antennas that operate in a 

hazardous atmosphere must have upper bounds on the transmit120 
ting antenna's magnetic moment. 

Physical reliability of the antenna structure imposes 

additional mechanical and electrical design constraints. 

The materials selected for use in construction of the antenna must allow it to survive extreme physical abuse. The 

electrical component reliability must not be adversely affected by 

. High circulating current in the loop, 

High localized electrical fields that can produce 

corona effects, and 

. High voltages that can break down dielectrics 

used in the construction of the loop components. 

Design For - Operation - In Hazardous Atmospheres 

The vest, and in some circumstances, the vehicular antennas must be designed to operate in hazardous atmospheres. 

As a result there are two types of design constraints that 

occur. The first is that the stored energy within the 

antenna inductances and capaci tances mus t be less than 

0.3 x 10-3 joules. The second is that localized electric 

fields developed in the structure can not be of sufficient 

strength to cause corona discharge effects or dielectric 

breakdowns. 

The first constraint has been examined in previous work 

(M)). The maximum intrinsic safe transmitting loop magnetic 
moment. may be represented as: h 

and 

MT = B3 (vest). h 

MT = B4 (vehicular) 

h 

For the vest-B3 = 2.5 ampere turns meter2 (ATM~). This 

1imi t follows from an analysis of Unde~wri ters Laboratory 

Document 91 3. In mines that have gaseous or explosive 

environments anywhere in the mining complex, the vest transceiver design (including the antenna) must be approved by 

the testing and certification center, Mine Safety and Health 

Administration (MSHA). In fre3h air and in non-gassy M/NM 

mines, the antennas may have greater magnetic moment. 

In emergency situations, when ventilation is lost in a 

gassy mine, the use of non-approved radio equipment is 

prohibited. The design precautions related to the avoidance 

of excessive local electric fields causing corona effects or 

discharges through or on the surface of dielectrics must be 

very rigorously observed in order to obtain MSWA approval. 

In preliminary experiments, corona discharges were observed 

and damage to the surface of the dielectric used in structural components occurped before a safe design was realized. 

Relatively high RF voltages and high RF circulating currents 

may occur in the antenna loops; approximately 10 Amps and 

1800 V peak-to-peak may be cited as upper-bound limits for 

these two situations, It is important to realize that high 

RF voltage per se does not cause corona effects, nor dielectric strain, The electric field, associated with RF voltage 

causes these effects. It is possible, by proper design, to 
limit these fields for a given value of RF voltage by avoiding sharp points and edges and, in general, low radii of 

curvature in construction. Printed circuit boards must be, 

coated by dielectric coating, capable of wi thstanding high 

values of electric stress. Circulating RF currents must be 

limited in order to reduce the stored energy in the tuned 

antenna circuit, the safe value being less than 0.3 x 10-3 

Joules, as before mentioned. The rationale for the above 

precautions is that ionization (corona) is caused by 

excessive electric field and sustained by the stored energy 

in the circuit. 

Vehicular Antenna - Area Constraint 

The vehicular antenna must be of a low profile mechanical 

design with the antenna coil situated at least 6 inches from 

'any metal mounting surface to avoid the effect of detuning. 

The antenna surface finish must not reflect light, must 

provide for vertical and horizontal mountings, and must be 

sealed to prevent ground water drip from contacting the 

internal loop components. 

In field tests, flexible rectangular tubular Lexan and 

flexible rectangular planar Lexan antennas were evaluated on 

various types of underground vehicles. High impacts in the 

mine environment caused rigid metal antenna structures to 

deform and change area; however, f lexi ble rectangular tubular Lexan structures proved effective in the environment. 
Tubular structures are difficult to produce because mu1 tiple turns (actually mu1 tiple conductors) must be forced 

around bends in the rectangular tubular structure. Because 

the turns are confined in a circular cross section pattern, 

the distributed capacitance between turns is greater than if 

the turns were in a plane. 

On the other hand, flexible planar antennas exhibited the 

highest unloaded Q (9,) and self-resonant frequency of all 

the antenna types investigated. Lexan laminated planar 

structures were found to withstand high impact abuse in mine 

usage, so this antenna structure was adopted for use in the 

program. 

Personnel Antenna Area Constraint 

Significant improvements in the design of the personnel 

antenna were accompl ished. Any personnel antenna must fit 

the miner's body and be lightweight and convenient to wear. 

Any change in area of the antenna which results from a 

miner's physical body size or movement during work will 

change the antenna's operational performance. Any change in 

inductance will detune the antenna resulting in lower transmi tting loop magnetic moment. 

Rigid oval tubular, flexible strap (band01 ier) and sewn 

garment antennas for roving miners were examined. The rigid 

oval tubular antenna (5 = 0.25m2) used on an earlier prototype MF transceiver (FM) design was found to b e o n e r o u s to 
roving miners because the area did not change with the 

miner's body size or movement. This resulted in encumbered 

movement. ' 

The strap-type band01 ier antenna used in the South African 

transcei ver (SSB) design satisfied many human .factor problems. Although roving miners were not encumbered when wearing the antenna, the antenna's detuning affects, due to 

changing area, prevent it from being used in a high performance communications system. 

Another earlier prototype transcei ver (SSB) employed a 

h 

sewn garment circular loop antenna (B1 = 0.1 5 m2) which was 

positioned over the miner's shoulder (10). - Like the bandolief antenna, the area depended upon the body size of the 

mi ner . 

To avoid these and other problems a sewn garment antenna 

capable of being placed upon the front or back of the miner's work clothing was developed. The sewn garment antenna 

was found to maintain a constant area. For convenience of 

use, the antenna was sewn onto the back of a vest garment. 

The upper practical bound for the area of the vest loop 

h 

antenna was found to be Bl = 0.15 m2. 

RF LINE COUPLER DESIGN CONS1 DERATIONS 

The RF line coupler is a tuned multiple turn toroidal 

(doughnut shaped) air-core current transformer. The coupler 

design follows many of the design considerations which are 
important in designing loop antennas. 

The RF line coupler equivalent circuit is shown in figure 

5-1 7. 

0.b- 7 RF line , , . 

coupler I +3 !I, 

I 

Vo 1~nl 1_ 7 F= 

- Vc3 

'b 

EQUIVALENT 

NETWORK ACTUAL NET WORK 

FIGURE 5-17. - WF line coupler. 

The wireplant conductor becomes a single turn secondary of 

the transformer. The transfer impedance of the coupler is 

gi ven by the following equation: 

where V1 = the coupler output voltage, 

IL = the line current flowing in any wireplant 

conductor. 

The coupler's efficiency can be characterized by its impedance because the output voltage applied to the receiver 

increases with a higher transfer impedance (ZT). 
Figure 5-18 illustrates how wireplant signals are coupled 

to the toroidal coupler. 

I *p__ WIREPLANT 

CONDUCTORS 

FIGURE 5-18. - Wireplant signal coupling to the toroidal 

coupler (cylindrical coordinates r, Z,$). 

The time dependent line current which flows through the 

center of t h e t o r o i d a l coil along the Z axis is given by: 

From Amperes Law, the magnetic field at a radial distance 

(r) from the line current is given by: 

+ IL (59) 

130 = a0 (ampere/meter) 

2 .rrr 

where, a4 is the azimuthal unit vector in the cylindrical 

coordinate system. The magnetic flux density in free space 

is: 
The magnetic flux (B ) passing through the area enclosid by 4 

each turn of the toroidal coil is: 

The voltage induced in an N turn is given by Faradayls law: 

Substituting equation (61) into (62) and differentiating 

with respect to time,, the induced voltage fs mathematically 

given by: 

emf = poNILfW [Ln(b/a)] 

When a capacitance matching network is used in the coupler 

design (see figure 5-38), the transfer impedance of the coupling structure may be mathematical1 y represented by: 

The transfer impedance increases with the first power of 

turns (N), the current level (I), the coupler's operating 

frequency (f) and length (W), and the logrithm of the coils 

inside to outside diameter ratio. It decreases as the patio 

63 /62 increases. 

Measurements were taken of the transfer impedance of an RF 

line coupler that could be clamped around a one inch cable. 

The results are shown in table E. 
TABLE E. - RF line coupler transfer impedance 

(1 inch) 

Test Frequency Transfer Impedance (ohms) 

350 kHz 

520 kHz 

Because the coupling efficiency and signal attenuation 

rate increase with frequency, the operating range between 

transceivers along a transmission line pair (which supports 

the bifilar mode) exhibit an optimum value with respect to 

frequency . 

The induced current flow in the wireplant by a transmitting coupler is dependent upon the resistance (impedance) of 

the conductors. Measurements were made to determine the 

induced current dependence on line resistance with the test 

setup shown in figure 5-19. 

i 

SINGER 

FS M 

NM-25T 

9 

FIGURE 5-19. - Test setup for measuring induced current 

t 

as a function of line resistance (RL) . 

I 

TRANSCEIVER 

. 20W 

b I 

, 

RF 

- LINE CURRENT ( 

PROBE 7 

COUPLER 
A vehicular transceiver was used to drive an RF line coupler. The line was a closed loop of 118 wire, approximately 

30 ft in length. A decade resistance box was connected in 

the loop as shown. The Singer Field Strength Meter (FSM) 

(Model NM-25T) was used with a calibrated Fairchild Current 

Probe (Model PCL-25) to measure current in the line. The 

induced line current dependence on line resistance is shown 

in table F. 

TABLE F. - Dependence and induced line current on line 

resistance. 

LINE CURRENT (MA) FREQUENCY 

RL (ohms) 350 kHz 400 kHz 

Tests were also made to determine the induced current in 

multiple conductors. Up to 10 conductors were placed in the 

RF line coupler as illustrated in figure 5-20. 

The circles symbolize the location where the standard current probe was used to measure induced line current. When 

conductors were removed from the RF line coupler, the line 

current in the remaining conductor did not change. 
TRANSCEIVER 

RF LINE 

COUPLER . 

FIGURE 5-20. - Over the earth multiple wire coupling. 

At 520 kHz, the induced current was measured to be approximately 3.9 mA in each wire. To a rough approximation, 

the data presented in table F indicates that the apparent 

circuit line and earth resistance is greater than 2000 ohms. 

In mine tunnels, the apparent resistance of telephone cable 

is also approximately 2000 ohms. 

CONS1 DERATIONS OF ACOUSTICAL DEVICE SELECTION 

It is necessary to address the criterion for selecting 

microphones and speakers that must be used with any mine 

communication system because the improper selection of t.hese 

devices can seriously degrade the performance of an otherwise good communication system. Within a mine, measured 

acoustical noise ranges from the threshold of hearing 
(0.0002 Dynes/~mz) to more than 104 dBA (158,489 time3 the 

threshold of hearing). 

Noise levels of more than 104 dBA have been measured in 

the reverberant surroundings of the longwall and noise levels of 108 dBA have been measured near mining equipment. 

Table G shows the typical noise levels measured in the near 

vicinity of mining equipment. For comparison purposes figure 

5-21 shows t-he noise spectra measured near an operatng 

longwall and inside the M-68A military tank. 

TABLE G. - Measured sound pressure bevel of mining 

machf nes . 

Diesel Powered Locomotive 

Joy 1 ORU Cutti wg Machine 

Wagner HMTT-410 Ram Hauler 

Jay Loader 

Exhaust Fan 

Wagner Loader 

Wagner MTT-168 Truck 

Sound Level In dBA 

300 500 1000 2000 3000 4000 

u THIRD-OCTAVE BAND CENTER FREQ., HZ 

FIGURE 5-21. - Acoustical noise comparisons. 
Comparison of the M-60A tank and longwall acoustical noise 

levels is noteworthy. A great deal of military research (21) - 

has been focused on the problem of communicating from high 

acoustical noise environments. Much of the military work 

considers the problem of improving communications from 

inside a moving tank. This work provides a basis for understanding how acoustical noise interf ers with speech intell igibility scores and how to improve the scores in the underground mine environment. Military research indicates that 

intelligibility scores of voice modulated radio signals from 

inside a moving tank is below the minimum acceptable limit 

(11). This is caused by the ambient acoustical noise mutilation of the speech pressure wave on the microphone diaphragm. By the same mechanism, voice radio communications 

near operating mine equipment are expected to be below the 

minimum intelligibility score. 

Microphones 

During the early years, microphones were designed using 

resistance responding and control 1 ed re1 uctance techniques. 

The carbon microphone is an example of the-resistance responding mi crophone. Control 1 ed re1 uctance microphones were 

designed with either a coil or magnetic armature mounted on 

a diaphragm; an example of this type of microphone is a 

telephone headset recei ver. 
Microphones have also been constructed with piezoelectric 

materials such as quartz, rochell salt crystals, and 

PZT/BaTiO ceramic materials. In recent years, the piezoelectric (PZT) properties of fluorocarbon polymers films 

have been investigated for use in the design of "thinw 

microphones. 

In general, microphones are broadly classified by their 

order. A zero (8) order microphone reproduces speech without inherent noise cancelling properties. That is, both 

speech and background noise cause the microphone to produce 

an output signal, Microphones that respond to the pressure 

"differencen across the diaphragm or to noise and speech 

waves that impinge on both sides of the diaphragm are called 

first order pressure gradient microphones. When first order 

gradient microphones are connected in arrays to improve 

noise suppression, they are cabled second order gradient 

microphones, 

There are two factors that make the first and second 

order gradient microphones effective in suppressing background noise. These factors are: 

(1) The "Proximity effectt1 whereby spherical speech pressure waves produce a greater pressure gradient 

across the microphone diaphragm than do planar wave 

noise signals which are generated at distant points. 
(2) The cosine or directional response of the microphone 

due to acoustical access to both sides of the diaphragm and the diffraction effects of the microphone 

in relation to the wavelength. 

First and second order gradient noise cancellation occurs 

because the phase .and amplitudes of the far field noise 

signal sum to zero on the PZT diaphragm. 

Several methods are available to obtain noise reduction 

in the design of a microphone. They are summarized as: 

( 1 ) Direction zero (0) order mi'crophone, 

( 2) Contact microphones, and 

( 3) Gradient microphones. 

In principal, a directional microphone will provide noise 

reduction; yet in many applications when ambient noise is a 

problem, these microphones have not been effective. 

Contact microphones are sometimes called l'Bone1l or llSkullll 

microphones. They offer little, if any, improvement in 

noise reduction since the skull itself becomes a noise 

conductor and theref ore does not produce noise cancelling. 

Other problems with contact microphones are that they provide considerable discomfort to .the users and moving vehicular vibrations may be conducted through the body causing the 

contact microphone to respond to unwanted noise signals. 

The discomfort of contact microphones was recently supported 

in development work done by the U.S. Marine Corp. The 

microphone was being developed for use in amphibian vehicles 

(landing craft), but the work was terminated when the marine 
radio operator complained about the microphone compresslon 

discomfort and resulting h'eadaches. First and second order 

gradient air path microphones offer theoretically predlctable noise cancelling possi bill ties. From a circuit theory 

point of view, a high pass filter can be used as an analogy 

for the gradient microphone. Using the high pass filter 

analogy, the noise discrimination capability is shown in 

figure 5-22. 

0 '=ic Fc Fe - - 

4 2 FREQUENCY 

FIGURE 5-22. - Normalized noise frequency response of 

a gradient microphone, a) voice signal, b) far field noise 

first order gradient microphone, and c) second order gradient microphone, 

The ordinate or y axis is labeled attenuation in dB. The 

attenuation represents the noise cancelling capability of 

the microphone relative to a ref erence-curve "af1. 

The abscissa or X axis is wscaledn or normalized in terms 

of the cutoff frequency (Fc) also called "cross over1' frequency by microphone technologists. 
The cutoff frequency of the noise cancelling microphone 

represents the noise frequency below which cancellation 

occurs. Curve "am represents the theoretical output ~i-~nal 

produced by a noise cancelling microphone to spherical wavefront speech signals. In practice, the microphone response 

will not be flat (constant) and will exhibit considerable 

frequency dependence. In a first order gradient microphone, 

the noise cancelling microphone attenuation increases by 6 

dB each time the noise frequency is reduced by one-half. 

For a second order microphone, the attenuation increases by 

12 dB for each 50 percent reduction of the noise frequency. 

In general, the asymptotic rate of attenuation is given by: 

dB = n6 per octave reduction 

where n = the order of the gradient microphone. 

With this background, the limitation of noise attenuation 

can be readily assessed. If the amplitude spectrum of the 

noise source has prinicipal components below the cutoff 

frequency (fc ) noise cancelling occurs. Also, the higher 

the cutoff frequency, the better the noise cancelling attenuation for a particular noise source (same amplitude spectrum). As it turns out, the cutoff frequency is dependent 

on the "thicknessn of the microphone diaphragm, so thinner 

diaphragms have higher cutoff frequencies and are better 

noise cancelling microphones. From the above information, 

it could be reasonably concluded that higher order microphones are important in this application. Although a second 

order microphone may offer much greater attenuation, this 
conclusion may not be correct. The reason being that changes in air path length occur during utterances and the 

apparent source position changes with lip movement (even if 

the microphone is stationary). Changes in air path length 

cause "phasen distortion to occur. 

In phase distortion the instantaneous pressure for a 

spherical wave is given by: 

where k = w/c is the free space propagation constant, 

c = the speed -of sound in air, 

w = the radiant speech frequency, 

r = the air path length , 

p a the air density, 

and P = ewA 

Equation (65) may be rewritten as: 

P 

p , 0 sin (at - kr) 

r 

The argument of the sinusoidal term includes the propagation 

constant (k) and the path length (r). Changes in path 

length (r) cause changes in phase (kr) or phase modulation. 

In a filter network, the non-linear phase response of the 

filter transfer function causes distortion to occur in the 

speech signal as it n%lows" through the network. The degree 

of distortion is dependent upon the type of transfer function and the asymptotic behavior of the attenuation characteristic. That is, the greater the rate of change of asymp- 
totic attenuation in the stop band, the greater 1s the 

speech distortion. Increasing the attenuation characteristic of a microphone can be expected to increase "phase 

distortion" problems. 

Since it can be expected that small changes in air path 

length will wmodula'tevv the speech signal in higher order 

noi se cancelling microphones, noise cancelling mi crophones 

should be positioned to the side of the mouth to reduce the 

"puffn sounds. 

The Advanced Narrowband Digi tal Voi ce Terminal (ANDVT) 

microphone and audio system study (21) - is the most recent 

and extensive research into intelligibility scores achieved 

by various acoustical devices in a wide range of background 

acoustical noise levels. 

Inside the M-60A tank, the military M-138 noise cancelling 

microphone is used with circum-aural earphones to achieve an 

intelligibility score of over 87 percent (72 percent is the 

minimum acceptable limit). To attain this score, a high 

pass filter with cutoff frequency of 300 Hz is designed into 

the audio proces.sing path. This discriminates against the 

lower octave noise shown in Figure 5-22. 

After evaluating various microphones for the MF communication system, the following conclusions have been reached: 

(1) Bone and skull microphones should not be considered 

for use in the underground mining situation. 

( 2) Thin diaphragm noise cance1lin.g microphones should 

be used in high acoustical noise areas. 
(3) A noise cancelling airpath microphone must be positioned on the radial line (to the side of the 

mouth) within a distance of 1/2 inch or less from 

the lip. This requires the use of a flexible 

mi crophone. 

(4) A noise cancelling microphone cannot be built into 

the control head enclosure because the ''apparentn 

microphone diaphragm would be thick, thereby yielding a low cut off frequency (Fe). The enclosure 

always needs to be placed within I/% inch of the lip. 

Noise Cancelling Microphones 

Acoustical noise and its effects on voice transmission 

quality were studied in the four mines used For field tests; 

however, a more extensive series of tests was conducted in 

the York Canyon Mine. The purposeof the tests was to 

evaluate various types of noise cancelling microphones and 

earphones in the underground mine environment. The poly 

(vinylhdene flouride) (PVF2) polymer film microphone, a 

military M-138 noise cancelling microphone, and a standard 

dynamic non-noise cancelling microphone were evaluated by 

voice recordings near an operating longwall. The voice 

recordings were made using ANSI phonetically balanced word 

lists. The results are shown in table H. 

Test averages in quiet environments show the microphone 

intelligibility to be very similar, yet when subjected to 
higher background noise there is a distinct difference. 

TABLE H. - Measured articulation index. 

Microphone Intelligibility Score 

Quiet Area 

Standard 

M-138 

PVF2 

Longwall 

Standard 

M-138 

PVF2 

In the vicinity of an operating longwall, the 'lstandard" 

non-noise cancelling microphone scored below the minimum 

acceptable level of 72 percent. Although the M-1 38 microphone performance was acceptable, it degraded from the .quiet 

value. The lvthin1l PVF2 microphone scored well above the M138 microphone intelligf bility score. The score did not 

. change from the quiet area score. 

- Vest Microphones 

A prerequisite for a vest microphone is that it be convenient to use and not onerous to the miner. A hand held 

microphone was found to be unacceptable because the microphone cord could accidentally become entangled in mining 

equipment and cause injury to the miner. 

A PVF2 boom microphone and circum-aural ear cup speakers 

(fig. 5-23) mounted on a miner's cap were found to be better 

for achieving an acceptable intelligibility score when the 
structed of tough plastic and survived mine testing. Evaluation indicated that acoustical transducers are required 

when the noise level is greater than 86 dBA, Working 

miners preferred the control head microphone (shown in figure 3-6) over the PVF2 boom microphone. 

The control head ceramic mi crophone achieved an acceptable 

score when the noise level was less than 86 dBA. However, 

in fresh air entries, the ventilation air flowing across 

the control head produced an unacceptable level of "windM 

noise. The "windw noise problem was solved by mounting the 

ceramic microphone element in acoustical wind screen foam 

material. 
VI. MF SYSTEM DESIGN SPECIFICATIONS 

INTRODUCTION 

The design specifications for the MF communications system are described in this chapter. The specifications define the overall system characteristics, module level specifications and equipment specifications. The equipment 

specifications are for: 

. A standard vehicular transceiver. 

A base station comprised of a standard vehicular 

transceiver and RF line coupler. 

. Remote control console for controlling base stations 

from remote locations via wireline control. 

. Vehicular transceiver and audio head. 

. Vest transceiver for roving personnel. 

. Global repeaters for increased operating range. 

. Cellular repeaters for wide area coverage in working 

sections. 

. RF line couplers. 

. Tuned loop antennas. 

Basic transceivers and repeaters are designed around a 

standard set of circuit modules, thus it is the module level 

specifications that determine the electrical performance 

characteristics of the equipment. To insure interface compatabil i ty between modules, s tandardi zed interconnect ing 
loads and levels have been established as part ofthe general system specif lcations. 

GENERAL SYSTEM SPECIFP CATbQNS 

The general system specifications are given in table I. 

These specifications should be fol l~wed in order to assure 

the best radio coverage and equipment reliability in the 

wine environment. Strict adherence to the equipment interconnection (interface) specifications, which are given in 

the table, will ensure compatabblity of MF equipment to 

other mine radio equipment. 

TABLE I. - General system specifications. 

1. SIGNAL EMISSIONS: 

-Type 

.Emission 

.Frequency Range 

.Tuning 

.Peak Deviation 

.Frequency Separation 

2. MQDULATPQN: 

.Processing (voice) 

.Frequency Range 

3. SQUELCH: 

Type 

Narrow band FM 

1 OF3 

60 kHz to 1000 kHz 

5 kHz increments 

2.5 kHz 

50 kHz (minimum) for 

repeaters 

Enhanced average 

voice signal processing 

200 to 2500 Hz (3 

dB BW) 

Two alternate types 

of squelch control: 

No1 se Operat-ed 

Tone Qperated 
Table I. - General system specifications -- Con. 

4. SIGNALLING TONE 

FREQUENCY: 100 Hz for subaudible 

tone & 1000 Hz for 

inband signalling 

5. TYPICAL OPERATING 

I 

RANGE : 

6. ENVIRONMENTAL: 

.Operating Temperature 

Range 

.Shock (drop) 

.Dust 

.Special Condition 

7. CONSTRUCTION : 

200 to 400 meters (in 

coal quasi-free conductor area) 

20,000 ft, 7 ft from 

wireplant conductors 

-20 to 60 degrees C 

4 feet onto soft pine 

board 2Iv thick 

Dust resistant 

Withstand corrosive 

ground water drip 

0'.045 inches P-P 

amplitude vibration 

10 to 55 Hz 

Sealed stainless steel 

enclosures 

Activated desiccant 

per Mil-D-3464D 

Interior protected with 

vapor corrosive 

inhi bi tors 

Humiseal coated printed 

circuit boards 

8. STANDARDIZED INTERCONNECTION 

LOADS AND LEVELS: 

.RF Signal 

Load Impedance 

.Remote Control Cable 

Load Impedance 

.Audio Interface 

Load Impedance 

Level for rated 

out put 

.Squelch Flag 

Unsquelched 

Squelched 

50 ohm load plane 

300 ohms (Twin lead 

cable) 

600 ohms 

-- Sq ~lagl 

Less than 0.7 V DC 

More than 7 V DC 
Table I. - General system specifications -- Con. 

.Key Transmitter 

Command (PTT) 

.Tuning Command 

.RCC Unit 

DC Command 

Tone 

9. PERMISSIBILITY : 

10. CABLING & CONNECTORS: 

.Wemote Control (RC) 

Cable 

.RF Signal (RS) 

Cable 

,Wemote Control Head 

(RCH) Cable 

.I2 V DC Power Cable 

3 3. VEHICULAR TRANSCEIVER 

HAZARD PREVENTION: 

.Vehicular Transcever 

Power Supply 

-- Trans Key 

Less than 0.7 V DC., 

F1 -t F2 Trans ---- 

Less than 0.7 V DC 

DC Command 

Mope than 7 V DC 

into 300 ohms 

More than 4 V rm 

into 300 ohms 

Vest per UL 913 and 

MSHA CFR, Title 30, 

Bart 23 

2 Conductors 

3 Conductors, Type 

SO with connector 

7 Conductors, Type 

SO with connector 

2 Conductors, Type 

SO with connector 

Every metal transcei ver 

must be grounded 

to vehicular frame 

ground. 

Installation of a 

transceiver on vehicles with floating primary battery 

sources and DC motors 

requires mandatory 

use of isolated DC 

to DC converter to 

supply power to the 

transceiver 

'underlined Command Sysbol indicates that signal mus t go 

low (sink 300 MA of current). 
STANDARD MODULE SPECIFICATIONS 

The standard module specifications are given in table J. 

The modules used in the radio system were provided as proprietary products by the Mine Safety Appliances Company 

(MSA). The modules are of the plug in type; this allows them 

to be easily serviced and interchanged. 

-TABLE J. - Standard module specifications. 

1. RECEIVER SPECIFICATIONS: 

.PC Board Symbol 

.MSA Part Number 

.Frequency Range 

.Sensitivity 

.Spurious Response 

Rejection 

Image 

IF 

Audio Output 

Transceivers 

. Squelch 

Threshold 

.Audio Output Power 

.Hum and Noise 

Squelched 

Unsquelched 

2. SYNTHESIZER SPECIFICATIONS : 

.PC Board Symbol 

.MSA Part Number 

.Frequency Range 

.Tuning Increment 

.Peak Deviation 

A 1 

D468179 

60 to 1000 kHz 

1 microvolt 12 dB 

Sinad 

Better than 90 dB 

Better than 60 dB 

No other responses 

0 d~k into 600 ohms 

for 2.5 kHz peak 

deviation signal 

(Fm = 1 kHz) 

1 microvolt 

0 dBm 

45 dB below rated 

audio output power 

40 dB 

A 2 

D468180 

60 to 1000 k~z 

5 kHz 

+ 2.5 kHz for 0 dBM 

audio input (A2) 

signal 
Table J. - Standard module specifications -- Con. 

3. EXCITER: 

.PC Board Symbol 

.MSA Part Number 

.Output Power 

.Power Flatness 

4. RF POWER AMPLIFIER: 

.PC Board Symbol 

.MSA Past Number 

.Output Power 

.Power Flatness 

.Load VSWR 

Operati ng 

Momentary 

5. AUDIO POWER AMPLIFIER 

.PC Board Symbol 

.MSA Part Number 

.Output Power 

6.. BATTERY PACK SPECIFICATIONS: 

Transceiver StandbvA 3 

D4681'98 

200 mW (adjustable) 

k 112 dB 

60 to 1000 kHz 

A 4 

D468183 

20 Watts (adjustabbe) 

1 dB 

60 to 1000 kHz 

3.1 

Open or short ckt. 

A 5 

D468184 

5 Watts 

TY pe 

.Voltage 

. Rating 

.Usage 

Sealed lead acid 

12 V BC 

5 Ampere-hour 

8 hours (R/T - 10/90) 

BASE STATION AND VEHICULAR TRANSCEIVER SPECIFICATIONS 

Both the fixed location base station and mobile vehicular 

transceiver use the standard transceiver specifications in 

table K. 

The remote audio head permits a standard transceiver to be 

installed on any desired vehicle. It uses a remote control 
head cable to interface with the transceiver. The head is 

mounted on the vehicle in a convenient location for the 

operator. 

The standard transceiver operates as a base station with 

the use of a remote control terminal. This terminal is 

mounted on the rib next to the transceiver and uses the 

remote control head cable to interface with the transceiver. 

Both the remote control head and remote control terminal 

can be plugged into the transceiver without making any 

adjustments. 

Standard Transcei ver 

The s tandard transceiver specifications are given in table 

K. The block diagram is illustrated in figure 6-1. The 

control board (All) is the mother board for the printed 

circuit modules - receiver (A1 ) , synthesizer (A21 , exciter 

(A31 and RF power amplifier (Ah). This board determines the 

operational characteristics of the transceiver. 11 

Recovered audio signals (audio out-A1 ) from the receiver 

(A1 ) are applied to the noise operated squelch network on 

the Control Board (A1 1 ) . Audio signals (audio input-A21 

from the microphone are connected through printed circuit 

wiring on the A1 1 board to the input terminal of the signal 

"A similar board (A1 1R) is used to construct a repeater 

transceiver. 
processor network on the synthesize board (A2). After processing, the audio signal modulates the digital synthesizer 

local oscillator (LO) output signal frequency. 

The power connector is used to supply DC power to the I 

transceiver. The microphone push to talk (PTT) switch generates a standard transmitter key command. 

The Trans KeY I 

command turns on the transmitter by switching on the DC 

power to the RF power amplifier (A4) and turns off the 

receives (All by switching off the receiver DC power. 

The standard transmit frequency select command --- F1 += F2 

(Trans) causes the A1 1 board to generate a digital binary 

frequency code to tune the synthesizer to the frequency F2. 

In the absence of --- F1 + F2 (Trans) command, the synthesizer 

is always tuned to the frequency F1. 

The transmit frequency select command --- F1 -+ F2 (Trans) also 

appears at the RF connector and is used to tune either the 

antenna or coupler to the Frequency F2. 

The noise operated squelch network generates the standard 

squelch flag (SQ -- Flag) signal. 

The A1 1 board uses relay switching to direct RF signals to 

the receiver in the receiving mode and to send RF power 

amplifier signals to the RF signal connector. Connectors 

fos the remote control head (RCH), 12 V DC power, and RF 

signal are located on the transceiver case. 
TABLE K. - Standard transceiver specifications. 

USE: 

2. OUTPUT POWER: 

3. RF OUTPUT LOAD: 

4. POWER REQUIREMENTS: 

.Operating Voltage Range 

.Demand Current 

Receiving Standby 

Receiving Audio 

Transmit 

5. ELECTRICAL SPECIFICATIONS: 

. Recei ver 

. Synt hesi zer 

. Exciter 

.Audio Power Amplifier 

Uni versa1 transceiver 

for use as either a 

base station or 

vehicular transcei ver 

1 to 20 Watts 

(adjustable) 

Optimized to drive loop 

antenna, 200 ohm 

transmission line 

or RF line coupler 

11 to 15 VDC 

35 mA (maximum) 

300 mA (maximum) 

3.5 Amperes 

A1 standard module 

specifications 

A2 standard module 

specifications 

A3 standard module 

specifications 

A4 standard module 

specifications 

A5 standard module 

specifications 

6. CONNECTORS: 

.RF Signal 

Pin No. 

RF Output 

Frame Ground 

F1 + F2 (Trans) 

.12 v DC Power192 

(base station application) 

12 V DC 

Ground 
Table k. - Standard transceiver specifications -- Con. 

.Remote Control Head 

- SQ Flag 

--- F1 -+ F2 (Trans) 

Audio Output (AI) 

Audio Input (A2) 

Trans Key 

12 V DC 

Frame dround 

1 

Enclosure hardware grounded to Pin 2. 

2 

12 V DC power supplied through remote control head connector in base station application. 

Fi 4 F2 (TRANS.) 

SQ. FLAG 

AUDIO OUTPUT (All 

TRANS. KEY 

AUDIO INPUT (A21 

12 VDC 

FRAME GND. 

DC POWER' 

CONNECTOR 

12 VDC 

FRAME GND. 

RF CONNECTOR 

FI 

- 

-F2 (TRANS.) - - -- - -- -- -- 

I2 VDC 

RF SIGNAL 

FRAME GND. 

CONTROL Al l RECEIVER A1 

SYNTHESIZER A2 

RF OUT 

r * 1 4 I RF POWER AMPLIFIER A4 I 

FIGURE 6-1. - Base station and vehicular transceiver block 

diagram. 
Remote Control Terminal (RCT) Unit 

The remote control terminal specifications are given in 

table L. The block diagram is illustrated in figure 6-2. 

The RCT unit includes the AC to DC converter and standby 

battery for the base station. The unit is used when the 

standard transceiver is operated a.3 a base station. The 

remote control cable is connected to the RCT unit line 

matching transformer terminals. The transformers cancel 

unwanted common mode signals and pass legitimate differential mode signals. The legitimate signals are: audio baseband (voice) signals, DC command signals, and subaudi ble 

control signals. 

Any remote control cable DC command signal is detected by 

the DC decoder. The decoder generates the Trans - Key command, keying base station transmissions at the frequency 

(F1). Any transceiver squelch flag command (SQ Flag) causes 

the DC encoder to generate a DC command signal (DC Comd) and 

apply it to the remote control cable. This allows the base 

station to key any other base station (or repeater) in the 

network. It also enables the control centers remote control 

console units to receive audio base band messages recovered 

in the base station receiver. Any subaudible tone on the 

remote control cable is decoded generating the --- F1 -. F2 

(Trans ) command. Thig permi ts the remote control console 

unit to remotely switch the base station to the. transmit 

frequency (F2). In this mode of operation, the base station 
can acquire any global repeater. Any remote control console 

unit can be used to talk through a stand alone global repeater to an out of ranze mobile transceiver. 

TABLE L. - Remote control terminal (RCT) specifications. 

1. - USE: 

2. STANDARD LOADS: 

. RC Cable 

.Transceiver Audio 

3. AC POWER SOURCE: 

. AC 

4. DC OUTPUT: 

.Operating Voltage 

.Regulation 

. Over Shoot 

5. DEMAND CURRENT: 

. Recei ving Standby 

.Transmit 

.Standby Battery 

6. CONNECTORS : 

.RC Cable 

.Wemote Control Head 

-- SQ Flag 

F1 + F2 (Trans) -- - 

Audio Output (A1 ) 

Audio Input (A2) 

Trans Key 

12 v DC 

Used with standard 

transceivers and 

repeaters to provide 

semote control (wireline) capabilities 

from the remote 

control terminal 

uni t 

300 ohms 

600 ohms 

12 to 14 V DC 

2 percent 

Max voltage 16 V DC 

40 mA (minimum) 

4 amperes 

Lead acid, 5 AH capacity 

Pin No. 

Frame Ground 
Table L. - Remote control terminal (RCT) specifications - 

Con. 

7. ENCLOSURE : 

8. SIGNALLING: 

.Remote Control Socket 

- SQ- Flag 

.O dB Audio Output (Al) 

Trans - Key 

.F1 F2 (Trans) 

.Audio Input (A1 ) 

0 dBM 

RC CABLE 

1 LINE MATCHING 1 

R TRANSFORMER 

Sealed enclosure 

-- RC cable 

DC comd 

3 V AC (RMS) 

DC Comd (3 V DC 

mini mum ) 

100 Hz signal 

112 V AC rms (minimum) 

1 V AC rms 

[AYDIO, I /TONE 1 / D;~OD,ER / 

DECODER ENCODER 

REMOTE CONTROL 

I I I I I SOCKET 

TRANS. KEY 

SO. FLAG 

I 1 1% FI - F2 (TRANS.) 

l -I-

AUDIO INPUT (A21 

AUDIO OUTPUT(AI) 

STAND-BY AC TO DC 

BATTERY CONVERTER . Z 117 VAC 

I 

FIGURE 6-2. - Remote control terminal (RCT) block diagram. 
Remote Audio Control -- Head (RCH) 

The remote audio control head specif i-tions are given in 

table Me The block diagram is illustrated in figure 6-3. 

The remote audio control head is enclosed in a sealed box, 

obtaining b ts DC power from the transceiver. The Audio 

Power Amplifier (AS) is controlled by the SQ Flag command. 

The amplifier and message lamp turn on when the transceiver 

is unsquelched by a received radio signal. 

Microphone audio input signals are transferred directly to 

the remote control socket. The push to talk (PTT) switch 

signals become Trans - Key commands, 

The FWF2 switch position F2 generates the -- F1 -+ - F2 (Trans) 

command. 

TABLE M. - Remote control head (RCH) specifications. 

1. - USE: 

2. STANDARD LOADS: 

. TransceJ ver Audio 

3. DC SUPPLY: 

. Operating Voltage 

.Demand Current 

Standby 

Full Audio 

4. CONTROLS: 

.Audio Output Level 

.Push to Talk 

. F1 -+ F2 (Trans ) 

Used with standard 

transceiver to allow 

convenient transcei ver 

operation by motorman 

600 ohms 

5 mA (minimum) 

250 mA (maximum) 

Volume control 

On microphone 

SPDT switch 
Table M. - Remote control head (RCH) specifications -- 

Con. 

5. CONNECTORS : 

Pins 

Microphone 

Ground 

PTT Switch 

6. REMOTE CONTROL SOCKET: 

.SQ -- Flag 

.F1 -+ F2 (Trans 

.Audio Output (Al) 

.Audio Input (A21 

.Trans Key 

.IZ v DC 

.Frame Ground 

7. SIGNALLING: 

.Remote Control Socket 

- SQ Flas 

Trans Kev 

F1 -+ F2 (Trans) * --- 

-- SQ Flag 

8. ENCLOSURE : 

RCH unit 

Enabl es audi o 

Depress PTT 

F1/F2 to F2 

Message lamp on 

Sealed case 

REMOTE CONTROL CONSOLE (RCC) 

The remote control console specifications are given in 

table N. The block diagram is illustrated in figure 6-4. 

Depressing the PTT switch transfers the standard DC command (DC Comd) onto the remote control cable. Microphone 

voice messages are simultaneously applied to the cable. 

Depressing the F2 switch generates a 100 Hz subaudible 

tone. The tone is applied to the radio frequency (RE) cable 
to command the base station to the transmit frequency (F2). 

This allows the base to acquire the global repeater for F1 

transmissions to an otherwise out of range mobile transceiver. 

REMOTE CONTROL 

SOCKET 

POWER 

AMPLIFIER 

AS 

I VOLUME 

CONTROL 

FIdF2 TRANS. 

SQ FLAG 

TRANS. KEY 

AUDIO OUTPUT (Al) 

AUDIO INPUT (A2) 

I I MICROPHONE SOCKET 

1 AUDIO CONTROL 

INPUT 

I2 VDC 

FIGURE 6-3. - Remote control head (RCH) block diagram. 

12 VDC > 

GNO > , 

* b 

< PTT SWITCH 

< 12 VDC 

< FI - F2 (TRANS.) 

SWITCH 
Table N. - Remote control console (SCC) specifications. 

1. - USE: 

2. STANDARD LOAD: 

. RC Cable 

AC POWER: 3. - 

. AC 

4. STANDBY BATTERY: 

5. CONNECTOR : 

.Microphone 

.RC Cable 

6. SIGNALLING: 

.Depressing F2 

Switch 

.Depressing PAGE 

Switch 

.Depressing PTT 

Switch 

7. ENCLOSURE : 

Used in the mining complex 

control centers to 

generate remote control signalling 

300 ohms 

117 V AC 

12 V lantern battery 

Type Mil 

Type 

Generates 100 Hz 

220 Hz 

Generates 1000 Hz 

2100 Hz tone and DC 

Cornd 

DC Cornd 

Sealed enclosure 

VEST TRANSCEIVER SPECIFICATIONS 

The vest transceiver electrical specifications are given 

in table 0. The block diagram is illustrated in figure 6-5. 

These specifications are similar to the standard transceiver 

specifications with the following exceptions: 

. The RF output power is limited to 4 Watts, preventing 

the transceiver magnetic moment (M) from exceeding the 
safe limit of 2.5 ampere turn meter (8) (10). The 

design includes tone signalling capabilities of 100 

and 1000 Hz. 

RC CABLE 

CONTROL 

FIGURE 6-4. - Remote control console (RCC) block diagram. 
. The transceiver modules are designed for use in 

hazardous atmospheres per Underwriters Laboratory 

Document UL 91 3 (and MSHA requirements ) . 

The receiyer , synthesizer , transmitter, antenna, and battery pack plug into a wiring harness that includes the 

speakers and control head. Maintenance requires troubleshooting to the module level. 

The tuned loop antenna module includes an F1 /F2 switch. 

Depressing the switch generates the F1 + F2 (~rans ) command. 

This tunes the synthesizer to the frequency (F2) and keys 

the transmitter module (A3). 

The transmitter is keyed to the frequency (Fl) by depressing either the PTT or SIGNAL switch on the control head. 

The receiver is always tuned to the frequency (Fl). 

Depressing the PAGE switch keys the transmitter to the 

frequency (F2). This is used in cellular repeater switching 

(see chapter I). 

The receiver (A1 ) module incorporates a noise operated 

squelch network. The recovered audio out (~1) signal is 

used to drive the dual epaulet speakers and the earphone 

jack. The receiver (At) module plugs into the interconnecting harness. 

The transmitter (A31 DC module incorporates the power and 

RF switching relay function. 
TABLE 0. - Vest transceiver specifications. 

1. - USE: 

.Model 

2. RF OUTPUT LOAD: 

3. POWER REQUIREMENTS: 

.Operating Voltage 

.Demand Current 

Wecefving Standby 

Recef vf ng Audio 

Transmit 

4. ENCLOSURE : 

5, GARMENT: 

6. WIRE HARNESS' : 

7. ANTENNA MODULE: 

.Type 

.Magnetic Moment 

8. CONTROL HEAD FUNCTIONS: 

.Volume Control 

.Squelch Control 

.Page Switch 

.Sf gnal Switch 

.PTT Switch 

Transceiver for use 

by roving personnel 

ART-29 Transceiver 

Optimized to drive 

tuned loop antenna 

20 mA (maximum) 

260 mA (maximum) 

808 mA (maximum) 

ABS plastic 2 1/2 x 

5 9 /2 x 1 inches 

(WLD) with D- 

'subminiature 

connectors 

Nylon mesh with 

pockets for modules 

Vest wiring harness 

including control head 

epaulet speakers, 

. tuned loop cpnnect or, 

speaker jack and 

transceiver module 

connect or8 

Tuned loop with sealed 

tuning unit 

Switch for F1 or F2 

(Trans ) command 

2.1 ampere turn 

meter2 

Adjust speaker volume 

Set threshold squelch 

Subaudible tone 

100 Hz tone 

Key transmitter 
Table 0. - Vest transceiver specifications -- Con. 

9. RECEIVER MODULE: 

. Enclosure Symbol 

.Frequency Range 

.Sensitivity 

.Selectivity 

3 dB BW 

70 dB BW 

.Two Signal Selectivity 

.Spurious Response 

Rej ector 

Image 

IF 

.Audio Output 

.Squelch 

Thresold 

Tight 

10. SYNTHESIZER MODULE: 

. Enclosure Symbol 

.Frequency Range 

.Tuning Increment 

.Peak Deviation 

11. TRANSMITTER MODULE: 

. Enclosure Symbol 

.Output Power 

. Power Flatness 

1 2. VEST ANTENNA MODULE : 

.Magnetic Moment 

. Area 

.Switching 

. Enclosure Symbol 

Intrinsically safe A1 

receiver ART-29 

P/N 301C193 

60 to 1000 kHz 

1.0 microvolt 12 dB 

sinad 

90 kHz (minimum) 

22 kHz 

65 dB (minimum) 

Better than 90 dB 

Better than 60 dB 

0.4 Watt into 8 ohms 

3.0 microvolt 

90.0 microvolt 

Intrinsically safe A2 

synthesi zer ART-29 

P/N 301C192 

60 to 1000 kHz 

5: kHz 

2 2.5 kHz 

Intrinsically safe A3 

transmitter ART-29 

P/N 301C194 

4 Watts (adjustable) 

9 dB 

2.1 ampere turn meter2 

0.13 meter2 

F1 /F2 switching 

Intrinsically safe APP-2 

Nicad battery ART-29 

Must be recharged in 

fresh air 
Table 0. - Vest transceiver specifications -- Con. 

.Output Voltage 

.Capaci ty 

.Maximum Instantaneous 

Peak Current 

.Fold back 

Nickel-cadmium battery 

with redundant 

current limiting 

circuits 

ll07 V BC 

500 mAH 

4.0 amperes (maximum) 

2.5 amperes 

. Enclosure Symbol 

.Type 

.Output Voltage 

.C.apaci ty 

. Fuse 

APP-1 Nicad Battery 

ART-29 

Nicad battery 

91.7 V DC 

588 mAH 

2A 

l~ire harness for approved vest will not accept power 

from standard battery pack, 

'use only in non-gassy mines 

The vest can only be used inby with an MSWA approved 

bat-tery pack. 
SIGNAL 

SQU 

FI/F2 SWITCH 

FI --+ F2 (TRANS.) LEFT 

RIGHT 

TRANSMITTER 

I-< EARPHONE 

JACK 

1 

A2 

--C~~~~~

KEY AUDIO IN (A21 

PTT 

PAGE 

ELc* 1- 

A -(voLuME MIC 

Fl BATTERY 

FIGURE 6-5. - Vest transceiver block diagram. 

Permissi ble Battery - Pack 

Figure 6-6 illustrates the block diagram of the permissible 

battery pack (APP-2) for the vest transceiver. The battery 

current limiting circuitry meets the Intrinsically Safe 

Electrical Circuits and Application requirements for use in 

hazardous locations per Underwri tiers Laboratory Document UL 

91 3. 
-1 LIMITER I 

CURRENT 

LIMITER 

I SENSING I 

I < CHARGE 

Figure 6-6. - Permissible battery pack block diagram. 

The battery pack at 11.7 V BC is limited to supplying less 

than 5 amperes instantaneously or continuously at any accessible terminal. The pack consists of an 11 .7 V BC, 500 mAH 

rechargeable nickel-cadmium "battery with double redundant 

current limiting circuits. The pack is potted and sealed in 

an ABS plastic case. 

The cireui t consists of three independent current limiting 

circuits in series but sharing one common sensing resistor. 

When the pack is in overload condition, fold-back condition 

occurs essentially disconnecting the battery from the load. 
FIGURE 6-7. - Vest transceiver. 

The transceiver modules are enclosed in special pockets in 

the vest and interconnected by a wire harness. A pair of 

epaulet speakers are enclosed in vest pockets located on the 

shoulder. The speakers are oriented to direct sound waves 

toward the miner's ears. The vest is designed with Velcro 

fasteners to enable the wire harness and all modules to be 

easily removed so the vest can be cleaned. 
FIGURE 6-8. - Internal tuned loop antenna sewn into the 

back of the vest in such a way that it is not exposed. 

REPEATER TRANSCEIVER SPECIFICATIONS 

The electrical specifications of the global and cellular 

repeaters are identi'cal. The specifications are also similar to the standard transceiver specifications because they 

use the same radio circuit modules with the exception of the 

A1 1R control board. Specifications for the repeater transceiver are given in table K (the standard transceiver specification table). Figure 6-9 illustrates the repeater block 

diagram. 
I00 HZ 

DECODER 

I RF POWER AMPLIFIER I 

REMOTE CONTROL 

CONNECTOR 

FIGURE 6-9. - Repeater transceiver block diagram. 

The A11R is the mother board for the Al, A2, A3 and A4 

modules . The A1 1R control board connects the RF power 

amplifier (A4) output directly to the RF output connector. 

In signal, the RF is connected directly to the receiver (A1 ) 

input circuits. 

FI ---c F2 (TRANS) > Al IR RF IN 

AUDIO 

SQ FLAG > 

A l 

CONTROL OUT (AI) 

AUDIO OUTPUT (All> 

TRANS KEY > 

AUDIO INPUT (A21 > 

12 V> 

GROUND > 

DC POWER 

12 vDC > 

GROUND > 

RF SIGNAL 

CONNECTORS 

RECEIVER 

I 

LO 

F2 RF IN 

FI RF OUT 

SET 

FREQ' c 

~RF 

i 

* 

1 

I 

A2 

SYNTHESIZER 

F2 . 

LO 

J 

SET w 

FREQ. A2 

SWITCHED 

DC * 

INPUT (A21, 

A3 

EXCITER 

* 

SYNTHESIZER 

F 1 

, I 
The repeater incorporates two synthesl zer (A2) PC boards 

to enable a simplex/half duplex mode of operation. F2 RF 

input signals are received and demodulated by the receiver 

(A1 ). Simu%taneously, the recovered audio output (A1 ) signal is applied through the A11R board to the synthesizer 

(A2, F1). This signal (audio input (~2)) frequency modulates the synthesizer low signal. The exciter (~3) and RF 

power amplifier (A4) increase the modulated signal power to 

20 watts and transmit PI signals. 

To conserve standby power, the A19R includes a noise 

operated squelch network. This network keys the exciter and 

RF transmitter on, when an RF signal is applied to the 

receiver input terminals, 

The A1 1 R board also includes a 900 Hz tone decoder. Unlike the All board, the tone decoder generates the - SQ - Flag 

command signal. 

The remote co-ntrol terminal unit can be directly interconnected with the repeater by a standard remote control head 

cable. 

The repeater receives its DC power from the standard 

system AC to DC converter which also includes a standby 

battery. 

RF LINE CQUPLER AND ANTENNA SPECIFICATIQNS 

RF line coupler specifications are glven in table P. The 

RF line coupler is used with the base station and repeater 
transceiver. The global repeater uses two RF line couplers 

to couple MF signals to the wireplant. 

Antenna specifications are given in table 8. Tuned loop 

antennas are used with vehicular transceivers and cellular 

repeaters. 

TABLE P. - RF line coupler specifications. 

Type 

Part Number 

4" inside diameter 

1" inside diameter 

Transfer Impedance (2) 

ACU-10-1 

350 kHz 

520 kHz 

820 kHz 

ACU-10-4 

520 kHz 

Dielectric withstanding 

voltage (cable to coupler 

frame and inductor winding) 

Clamp type toroidal design 

10.0 ohms 

11.2 ohms 

17.8 ohms 

4.0 ohms 

15,000 V (minimum) 

Insulated 

TABLE Q. - Antenna specifications. 

Type 

Enclosure 

Finish 

Magnetic Moment 

Physical size 

Twin-loop design 

Le xan 

Non-ref lecting black 

4.0 ATM2 (minimum) 

14 inches high 

28 inches long 
I. FIELD EVALUATIONS 

INTRQDUCTIQN 

MF equipment was i.nstalled and evaluated in the following four underground mines: 

* Star Point Mine 

Plateau Mining Company 

Price, Utah 

Mining Method: 

Continuous Mining and 

development for longwall with 

conveyor belt haulage of 

approximat e%y 1 MTPY, 

* San Manuel Mine (MAGMA) 

Magma Copper Company 

Subsidiary of Newmont Mining Corporation 

San Manuel Division 

San Manuel, Arizona 

Mining Method: 

Mu1 tiple level blockcave mining with 

sail haulage of 67,QQO tons per day. 

* Escalawte Silver Mine 

Redco Silver Mining Company 

A subsidiary of 

" Rancher Exploration and Development Company 

Enterprise, Utah 

Mining Method: 

Vertical crater retreat mining using diesei 

truck haulage up a decline of 500 tpd. 

* York Canyon Mine 

Kaiser Steel Company 

Raton, New Mexico 

Mining Method: 

Continuo'us mining-and development for 

longwall with conveyer belt haulage of 1MTPY. 
The location of the mines are shown in figure 7-1. 

Escolonte Mine (silver) \ f LEGEND 

S tor Point Mine (coal) ' v-7 

Yor k Canyon Mine (coal) Y N 

ARIZONA I ,= 

Magma Mine (copper) 

I "EW I 

COOPERAT/NG M/W 

FIGURE 7-1. - Field test locations. 

The installed MF communication systems were evaluated over 

a period of 6 to 12 months. During the installation and 

evaluation periods, extensive tests were conducted. This 

chapter describes the results of these tests. 

SAN MANUEL COPPER MINE (MAGMA) 

This mine is located north of Tucson near San Manuel, 

Arizona. The mining complex includes smelter, surface ore 

loading and rail transportation facilities, and an underground blockcaving mine. The San Manuel ore body is a large 

chalcopyrite mineralization which has been deposited as an 

elliptical, hollow cylindrical shell. The interior of the 

cylindrical ore shell is a marginally mineralized zone of 
FIGURE 7-2. - Isometric diagram of the blockcave mining 

method. 
Blockcave mining simultaneously occurs in three blocks 

in the ore body. The majority of the production comes from 

the mining block associated with 231 5 grizzly and 2375 

haulage levels. Mining is being completed on the 2015 

grizzly and 2075 haulage levels. The 261 5 grizzly and 2675 

levels are in the developmental stages. 

During development, undercut drifts are driven into the 

ore body as illustrated in figure 7-2. From the undercut 

drifts, the ore body is drilled producing a radial drilling 

pattern that is perpendicular to the heading of the drift. 

The pattern is replicated on 4 ft centers along the 

drift. The drill holes are then loaded with explosive charges. Detonation of the charges rubblizes a local zone of 

the ore block. The ore (muck) flows by gravity down draw 

raises to the grizzly level drifts. Large boulders in the 

muck are fractured with sledge hammers and forced to flow 

through steel rail grates at each grizzly level draw point. 

From the draw point muck flows down the transfer raises to 

the haulage drifts on the production level. The flow process 

continually grinds and reduces the size of the material. 

The transfer raises are designed to be holding bins for the 

muck. The bottom of each raise is closed by an air powered 

gate. Figure 7-3, illustrates the location of the gate and 

the train loading process. Eight or more transfer raises 

intersect each train loading panel. Mu1 tiple panels lie 

under the ore body. 
TRANSFER 

RAISE 

LOCOMOTIVE 

FIGURE 7-3. - Train loading. 

A trolley-powered locomotive is used to maneuver train 

cars below the transfer raise gate. T~ain loading personnel 

(loaders) stand in the pony set (the space above the loading 

gate) and use the gate to control muck flow into the 

train cars. The loaders signal the motorman to move the 

train forward after each car is loaded. The signalling is 

done with incandescent lamps in the panel, After all cars in 

the train are loaded, the train leaves the panel and enters 

the main haulage track. Figure 7-4 illustrates the planevlew 

of track installed on the 2375 haulage level. Similar rail 

systems are installed on the other two haulage levels. The 

loaded train proceeds along the north haulage track to the 

unloading area called the pocket. While moving through the 

pocket area, the trains are automatically unloaded into muck 

storage bins directly below the track. The unloaded trains 
CAR DUMP AND HOIST 

HAULAGE LEVEL 

LEGEND 

- - -DEDICATED CARR lER WlR

- a - r -AUDIO LINES 

eJ 

JUNCTION BOX -CONNECT

TO TRO

FIGURE 7-4. - Planeview of track installed on 2375 haulage level
proceed along the south haulageway to an assfgned production 

panel. As many as 15 trains are used in the haulage system. 

In addition to these trains battery powered locomotives are 

used as service vehicles in the mine. 

Seven hoist systems are used in support of the underground 

mining operation. The hoist head frames are illustrated in 

Figure 7-5. 

The hoists used in the shaft are capable of manual or 

automatic operation. The systems transport rock from 'the 

development headings and muck From the pockets to the surface. 

MAGMA9 Pni tia% Communi cations System 

Haulage traffic is controlled by dispatchers in the 

surface control center who use trolley radios to communicate with train motormen and an audio system to communicate 

with supervisors on the grizzly and production levels. A 

hoist carrier current radio system provides communications in the shaft. Figure 7-5 illustrates the communication system. 

The dash-x-line indicates the routing of the audio intercom system cabling in the mine. Remote audio heads (speaker 

symbol 1 are located at strategic locations in the grizzly 

and haulageway drifts. Each head includes a microphone, 

speaker, and push-to-talk switch. A four conductor cable 
GRIZZLY OR I FT 

ROOM WIRE PANEL 

& CONTROL CENTER * 

FIGURE 7-5. - MAGMAfs communications system. 

COLLAR AX 

connects each remote, audio head to a control audio ampli1 

fier located in the audio room. Unlike a typical pager 

telephone system, audio signal amplification occurs in a 

central amplifier instead of the remote audio head. The 

intercom design features improve the audio system's reliability. Maintenance cost is reduced because batteries are 

not required in the remote heads. Roving supervisors on the 

grizzly and production levels use the audio system to coordinate muck haulage with the dispatcher in the surface 

control center. The mucking supervisors on the grizzly 

level use the system to communicate with the dispatcher 

and identify which transfer raises are being filled. 
The dashed-dot-line indicates the routing of a dedicated 

wire pair that is used to distribute trolley signals in the 

main haulageway drifts. Trolley radio coverage is not acceptable along some incremental lengths of' the haulageway. 

Poor radio coverage is caused by several mechanisms. The 

locations of poor radio coverage and their causes are: 

. At locations where track bonding has degraded, 

a high impedance path for carrier current signals 

exf sts. This causes excessive attenuation of the 

-signals. 

. Near rectifiers where carrier current signals 

are shunted to ground. This prevents the signal 

from reaching the base station. 

At the far end of the mine where attenuation 

on long path lengths reduces the amplitude of 

the carrier signal below the threshold of the 

receiver , 

Signal propagation effects on the mine wiring 

that causes nulls in the carrier current signal 

along the haulageway conductors. The nulls are 

caused by standing waves on the wireplant. 

The present trolley radf o system uses dedicated cable as 

the means of signal distribution in the haulage drifts 

(along the dot-dash-line in figure 4-4). Series tuned filter 

networks (junction boxes in figure 4-4) are directly connected between the dedicated cable and the trolley bus at each 
location of poor radio coverage. In spite of this, poor 

radio c0verag.e areas still exist along the haulageway. 

The dispatcher is a keyperson in haulage operations. Every 

tracked vehicle's location and status must be known at all 

times. The required information includes: 

e which transfer raises are full. 

Identification of the train which is being 

loaded in the panel. 

. Identification and location of loaded trains on 

their way to the pocket. 

. Identification and location of unloaded trains. 

. Identification of trains out of service. 

. Location of battery powered service vehicles. 

. Changes in the orderly mining process. 

To aid in the safe and efficient dispatching of track 

vehicles, miniature replicas of all vehicles are manually 

positioned on a scale model of the ha-ulage system so the 

dispatcher can identify and track the location of each train 

and battery powered service vehicle on the haulage level. 

Figure 7-6 illustrates the scale model of the haulage system. 

The dispatcher moves the replica of the vehicles in accordance with location information received from each motorman. 

To insure safety and minimize accidents on the haulageway, 

the dispatcher coordinates the trains1 and service vehicles1 

movements while ensuring a minimum spacing is maintained 

between the vehicles. Production is improved when the 
trains are optimally dispatched to panels with enough muck 

to fill all cars and when the muck is evenly drawn from the 

caved block. This requires information on the number of 

cars loaded from each transfer raise. 

The hoist communications system has been difficult to maintain and improvements in its design are required. A. tuned 

loop antenna at the sheave wheel is used to radiate the 

hoist wire rope. The wire rope is the communications signal 

path to a carrier current transceiver'in the cage. The 

sheave wheel location of the antenna exposes maintenance 

personnel to risk of injury since they must climb the head 

frame to inspect and repair the antenna. On the cage, the 
transceiver is electrically coupled to the wire rope by a 

direct electrical tap on the rope just above the cage. 

Coupling via the tap, the wire rope, and the surface antenna 

enables' the communications link to be established between 

the hoistmen and miners in the cage. 

Installation -- of MF Equi pment 

The MF. radio system offered the potential for reducing the 

risk of injury and lowering mining cost. The system was 

expected to improve communications in the following areas: 

. Provide a reliable communications link between 

battery powered vehicles and the dispatcher while 

vehicles were moving on the rail system. 

. Eliminate poor communication areas along the rail 

haulage system. 

. Provide radio coverage areas for roving supervisors 

on the grizzly and production levels. 

Improve communications in the Hoist system. 

Provide a radio communications link between the 

train loader and the motorman. 

To improve the safety and efficiency of the haulage 

system, a communications link is needed between the dispatcher and motormen on battery powered vehicles. Although 

these locomot i ves have carrier current transceivers , the 

transceivers can only be used with a "stingerv (a pole with 

a capacitive coupling network that must be touched to the 
trolley conductor to communicate). 

An MF base station was. installed on the haulage level and 

a vehicular antenna and transceiver were installed on battery powered vehicles to permit communication wi th the 

dispatcher while the train was in motion. The vehicular 

antennas and transcei vers were evaluated in areas where 

communication was poor. The inductive coupling via tuned 

loop antennas induced signal currents on all nearby conductors. The conductors provided alternative signal paths 

instead of relying on current flow in high resistance paths 

(through bad fail bonds). Vest transceivers were also evaluated. 

A production base station was installed on the haulage 

level so the dispatcher could be in constant contact 

with the vest users when they were near conductors. 

Furthermore, a repeater was installed on the grizzly level 

to extend the talk-back range to the haulage level base 

station. 

MF was also tried in the shaft. The highly efficient MF 

antenna was installed to radiate the wire rope at the collar. This reduces risk because maintenance personnel do not 

have to climb the 200 ft headframe to service the antenna. 

An RF line coupler was installed above the cage to permit 

communications between cage and surface via the hoist rope. 

The vest was evaluated for shaft inspection use to provide a 

communications link to the hoistmen. 
- MF System Tests - and Ev2luation 

With the advice and assistance of the mine communications 

group, a series of tests were carried out in the mine. Each 

test focused on improving a specific aspect of the mine's 

communication problems. In the test MF equipment was installed on the 2375 level of the mine and evaluated over a 

period of time. For example, vehicular transceivers were 

left in service for more than 12 months. The objective and 

results of each test follows. 

Test No. 1 - Communications from Battery Powered Locomotives 

The object of the test was to provide a reliable communications link from the dispatcher to motormen on moving 

vehicles. A base station was installed near the No. 5 shaft 

on the 2375 haulage level. The installation was made by 

coupling the base station to a dedicated carrier wire via a 

1 inch RF line coupler. A vehicular antenna and transceiver 

were installed on a battery powered locomotive. For comparison purposes the motormen also wore a vest transceiver to 

communicate with the base station. All tests were made at 

400 kHz. 

The mine-generated noise and transmitted vehicular signal 

levels were measured at the base station location as the 

locomotive moved on the track. An RF line coupler was used 

as the coupling device between the dedicated wire and the 

field strength meter. 
The recovered audio quality was measured. The tests were 

subjective and pegged to a listening scale of 0 to 10 as 

follows: 

0 = no squelch break 

1 = squelch barely broken - not audible 

3 = speech approximately 80 percent audible 

with heavy noise 

5 = speech audible with medium noise 

8 = speech clear and audible with low noise 

10 a gpeech clear - no noise 

The test results are given in table R. 

Tabbe R. - Vest and vehicular communications to 5 shaft 

base station. 

NOISE 

LEVEL 

dB 

1680 VEHICULAR VEST 

TEST 

NO. 

S IG S/N Audio 

LEVEL dB CJ 

SIC S/N AUDIO 

LOCATION LEVEL dB r) NOTES 

36 departure 

3D departure 

Between 38 dep 

& 17 South 

17 S Crossover 

418 North - - 0 No carrier 

line 

8A South 

5 North 

2 North 

20 North 

30 Crossover 

5 Shaft 

Turnout 
Summary : 

The objectives of the test were met. Reliable communication was achieved from both trolley and battery powered 

locomotives anywhere in the haulage system. The carrier to 

noise ratio and communications signal quality were excellent 

except in areas which lacked a common line. This demonstrated that a dedicated cable extends radio coverage in a mine. 

The vehicular radio audio quality was acceptable in 12 of 13 

test locations. The audio quality of the vest was not as 

good because of its lower magnetic moment; however, a repeater could improve the quality of vest communications in the 

mine. 

Test No. 2 - Hoist Communications Test/Base Station in Hoist 

Control Center 

A vehicular antenna was mounted in the hoist control room 

at shaft 3C with the antenna about 3 ft from the wire rope 

and about 8 ft from the wire rope drum. Figure 7-7 illustrates the hoist communications system. 

The vehicular antenna was used with a field strength meter 

to measure noise and carrier signal at different locations 

of the cage in the shaft. A base station was located in the 

same area with a vehicular antenna mounted just ahead of the 

large antenna 3 ft from the rope. Another transceiver 

(battery powered) was mounted in the cage using a 4'' coupler 

on the rope just above the bonnet. The test results are 

given in table S. 
TEST 

ETER 

FIGURE 7-7. - Hoist communicatisns system. 

TABLE S. - Hofst eornmunicat%ona signal level cage to base 

at the drum. 

TEST 

NO. 

b 

2 

/ 

3 

4 

5 

6 

7 

8 

CAGE 

LOCATION 

Collar 

400' below collar 

1000 ' going down 

full speed 

1500 ' going down 

full speed 

2075' stopped 

2675' going down 

2750' pocket 

2000' going up 

NOISE AUDIO 

LEVEE SIG SIN QUALITY COMMENTS 

dB LEVEL dB 
~umiary: 

Good to excellent signals were obtained with no dead 

spots. A slight noise level was noted when the hoist was 

runni ng. 

Test No. 3 - Hoist Communication Test/Base Station at the 

Collar. 

A vehicular antenna was mounted on the headframe at the 

collar about 3 ft from the cage rope. This ,antenna was 

used with the field strength meter to measure noise and 

carrier signal levels at different locations (elevations) in 

the shaft. A vehicular antenna was mounted on the headframe 

opposite the other antenna and was connected to the base 

station. The cage radio and coupler remained as in test 2. 

The results of the test are given in table T. 

TABLE T. - Hoist communication signal levels cage to base 

at the collar. 

TEST CAGE NOISE 

NO LOCATION LEVEL SIGNAL S/N AUDIO 

dB LEVEL dB QUALITY NOTES 

1 Collar Clear E Loud 

2 400' moving down -27 +82 109 11 II 10 

3 1000 ' moving 

down -2 7 +80 10 7 10 II 18 

4 2015 -27 +81 108 10 11 I I 

5 2675 moving down -27 +86 113 II I I 10 

6 2675 -2 7 +8 5 112 II II 10 
Summary: 

This test showed the value of MF communications in the 

hoist area. The measured carrier signal to noise level 

ratios were large at all elevations in the shaft with the 

cage moving or stopped. This provided excelle'nt C/N margins 

in the hoist communications system. At the hoist house and 

the collar locations, the signal propogation bosses over the 

"sheaven wheel were found to range from 50 to 60 dB. The 

shaft contained various conductors (besides the hoist #cable) 

which included power lines, pager lines, audio lines, and 

shaft signal lines. These lines were bashed in the corners 

of the shaft and contributed to the efficiency of the radio 

signal propagation. 

Test No, 4 - Vest CommunEcations to Audio Room Base Station 

from Grizzly and Production Levels 

A base station was installed in the 23'75 audio room. A 1 

inch RF line coupler was clamped around the East or West 

audio cables emerging from the audio amplifier. A field 

strength meter was connected to a 4'' RF line coupler clamped 

around the cable for measurement purposes. Signal levels 

were measured at the base as a vest transceiver transmitted 

from various locations in the mine. It should be noted that 

a trolley rectifier was also located in the audio room. The 

results from this testing are given in table U. 
Summary: 

This test demonstrates the potential performance and usef ulness of the vest radio. Communications were acceptable 

to very good at 30 of the 33 locations tested. In general, 

good quality communications resulted when the roving miner 

was- within 3 to 6 ft of the audio lines, and acceptable 

quality communication was obtained up to 50 feet from the 

audio lines. The vest received clear signals from the base 

even in areas where it could not transmit. The low quality 

signals received by the vest in tests 16, 17, and 18 occurred because nearby audio lines were not coupled to the lines 

at the base. Further, noise level variations and poor signal 

to noise tests resulted because the trolley rectifier was 

located in the audio room. 

It was determined that with the installation of dedicated 

lines to fill the gaps1( nearly 100 percent communication 

coverage is possible; and with the use of a repeater, the 

talk-back range from the vest can be increased. 

Conclusion 

The tests at MAGMA show that MF equipment performs very 

well in a large multiple level mine. 

Wherever local conductors exist, good coverage resulted, 

including up and down the hoist shaft. To some extent, the 

quality of communications depends on the continuity of the 

electrical conductors. If the continuity is good, good 
communication can be achieved at distances up to 20 ft from 

the conductors in the mine. If the continuity is poor or 

non-existant, good communication can be achieved only within 

7 ft of the electrical conductors. 

TABLE U, - Vest transceiver communications to audio room 

base station (via 2315 and 2375 level audio cables) 

TEST VEST NOISE 

NO. LOCATION LEVEL 

- - dB 

SIGNAL 

LEVLL 

- dB 

S/N AUDIO 

dB QUALITY NOTES 

3. 2375 28 3 6 

Crossover Couplers on We st audio 

cables routed on 2375 6 past 

rectif ie~ 

2 N Haulage 2 36 

3 20 North 3 6 

5 Raise 3 6 

6 Raise 58" 3 6 

7 Top Manway 21 

2315 Couplers, on 

East audio 

I$ha - 2315' 

away from 

rect ifier 

8 NFD T.O. 2% 

9 Panel 2Q 25, 

10 Panel 18 21 

11 Panel 16 T.O. 20 

12 Pane1 14 

Nort h 20 

13 Panel12 

N Lunchroom 18 

fQ Panel 18 N 18 

17 Panel 2 N 

T. 0. 18 

18 2 Panel 6 

Line 1 a 

ESCALANTE SILVER MINE 

The Redco Silver (Escalante Silver) Mine is owned and 

operated by Ranchers Exploration and Development Corp. The 

mine is located in Iron County (42 miles west of Cedar City) 
Utah. The surface support facilities include a shop/warehouse building and ore handling and milling building. 

The host rock are water-lain tuffaceous sediments of midtertiary age. The beds are composed of volcanic material 

with the composition of latite to rhyolite. The material is 

fine grained to conglomeratic, rounded to sub-rounded, and 

occurs in beds ranging from a few inches to 4 ft in thickness that dip 90 to 20 degrees to the southwest. A cross 

section of the ore body is shown in figure 7-8. 

I 

FIGURE 7-8. - Cross section of the ore body. 

The vein strikes N25E and dips 70 to 75 degrees west. 

Northwest tending fracture zones intersect the vein in two 

or more areas. The ore reserve portion of the vein averages 

20 ft in thickness. 

Entry into the ore body is via an adit. Decline 

haulage drifts are developed in the country rock adjacent to 

the ore body. Entries are developed into drill sublevel and 
undercut areas. A modified vertical crater retreat (VCR) 

mining method developed in Canada is used to recover the ore 

from the vein. A aide view of the VCR drill sub-level is 

illustrated in figure 7-9. 

FIGURE 7-9. - Side view of the vertical crater retreat 

mining method. 

From the upper drill sublevel down-hole drilling devePops a drill hole pattern that intersects the ore body and 

the lower undercut muck draw point. Explosive charges are 

placed in the drill holes above the undercut levels. Deto- 
nation of the charges rubblizes the ore body directly 

above the draw point. Repetition of blasting creates a 

vertical retreat sequence for the recovery of ore in the 

vein. After each detonation, muck is drawn from the draw 

point with an articulated load-haul-dump (LHD) diesel 

v.ehicle. The vehicles load the LHD transport muck up the 

decline to the surface mill. Four diesel powered haulage 

vehicles are self dispatched in the mine. Up decline vehicles have the right-of-way to the surface dump area. Round 

trip time on each vehicle is approximately 21 minutes. The 

shift muck rate is approximately 500 tons per haulage vehicle. 

Escalantels Initial Communication System 

The Escalante communications sys tern consists of a pager 

telephone system. A two wire pager line connects the shop 

to the the 5091 drill sublevel. A block light signalling 

system is used in the haulage decline to control diesel 

traffic. The signalling system has traffic lights installed 

at strategic locations along the decline. Vehicle operators 

pull a switch rope to change the block traffic light. 

Communication Needs 

Radio communications were needed between the superintendent, foremen, and haulage vehicle dri vers . Radio coverage 

in the decline, drill sublevels, and the undercut areas of 
the mine were also needed so teamwork could be improved. 

Communication links to vehicles would improve haulage eff iciency because vehicle operators could be more efficient in 

self dispatching trucks to MFulllf undercut area%. 

Installation --- of the MF Radio System 

A base station was installed at the adit. An RF line 

coupler was clamped around the single pair telephone cable. 

The base station was controlled by a remote control console 

(WCC) In the superintendentts office. The office was approximately 700 ft from the base station. A buried cable was 

extended from the pager telephone cable to the dump in order 

to facilitate communf cations to haulage vehicles in the 

dump area. Vehicular antennas and transceivers were installed on four haulage trucks. Four vest transceivers were 

given to the shift foremen. 

For test purposes a second RF line coupler was installed 

on the telephone cable at the base station location. The 

coupler was connected to a field strength meter so that both 

carrier signal and noise levels could be accurately measured 

in the mine. 

The telephone cable only extended to the 5091 drill sublevel (location shown on Figure 7-10). Beyond that location, high pressure air pipe and AC power cable were the 

signal distribution conductors in the mine. 
DISTANCE IN FEET 

SOUTH SECTION -tFIGURE 7-10. - Telephone cable in the Redco silver minehaulage d
- MF System Tests - and Evaluation 

The carrier and noise levels were measured in the mine. 

Analysis of the measurements determined the attenuation rate 

for tunnel mode signals propagating on the wire plant. The 

measured level of carrier signal from both vest ,and haulage 

vehicles are shown in figure 7-11. 

I 2 3 

MANWAY DISTANCE FROM BASE (THOUSAND FT.) 

FIGURE 7-1 1 . - Measured level of carrier signal from vest 

and haulage vehicles. 

Analysis of carrier signal levels arriving at the base 

station from various locations of the test vehicle indicate 

that the attenuation rate was approximately 2.4 dB/ 1000 ft 
along the telephone cable. The carrier to noise ratio of 

received base station signals exceeded 32 di3 for mobile 

transmissions along the air pipe. Test measurements indicated that signal coupling loss between the air pipe and the 

telephone cable in the same drift .was approximately 30 dB. 

The minimum separation between the cable and pipe in the decline (outby the 5091 drill sublevel) was approximately 3 

f t. 

The quality of vehicular voice signals received at the 

base was excellent. The vest and vehicular talk-back range 

included. the decline outby the 5091 drill sublevel. Further, base station signals could be received everywhere 

electrical conductors existed in the mine. This includ'ed the 

drill and undercut levels in the vein. Vehicular talk-back 

range included the entire decline haulageway. Inby the 5091 

drill sublevel, the vest talk-back range was restricted to 

within 3 ft of the air pipe. 

The acoustical noise level of the di.ese1 engine exhaust 

system in the reverberant haulageway drift was high. Communications from moving diesel powered vehicles required the 

use of circum-aural ear cups and noise cancelling microphones. 

Radio Frequency Interference (RFI) generated in the diesel 

haulage vehicle's electrical system caused the transceiver 

squelch to break. The break occurred at the threshold setting of the squelch sensitivity adjustment. By setting the 

I 

squelch control to the tight position, nuisance squelch 
breaks were eliminated. The setting reduced the threshold 

sensitivity of the receiver and hence the operation range of 

the system. A study of the problem indicated that the 

source of RFI was the alternator. The RFI was radiated from 

the vehicle wiring. Filtering at the regulato~ terminals 

suppressed the RFI such that receiver gensicivity was only 

degraded by approximately 6 dB. 

The current induced in the blasting cables in %he drill 

sublevel were measured using the test set up shown in figure 

7-1 2. 

BLASTING 

e ORILL HOLES 

e 0 

ORILL SUB - LEVEL 

FIGURE 7-12. - Test set up for measuring current induced 

in blasting cable. 

The blasting cable extends from the sublevel drift in the 

ore body to the decline drift in the country rock, a distancg of approximately 500 ft. The current flowing in the 

blasting cable was measured with a calibrated current probe 
(circle symbol) and the field strength meter. When the base 

station at the adit was transmitting, the maximum current 

induced in the blasting cable was 0.34 microampere. When a 

transmitting vehicular antenna was within 1 ft of the blasting cable, less than 1 .2 milliamperes of current were 

induced into the cable. The CFR limit for induced current 

is 50 milliamperes. 

Concl us i on8 

Tests proved that the MF system could be easily and quickly installed in a metal mine. High voice quality signals 

are received when mobile transceivers are within 7 ft of the 

electrical conductors in the mine. The talk-back range from 

. the vehicular transceiver to the base was the length of the 

wire pair cable. The initial length of the cable was 

approximately 1,000 ft. The cable was later extended to 

approximately 3,000 ft. Radio coverage includes the entire 

width (14 ft) of the haulage drift. In drifts with only air 

pipes, the talk-back distance to the air pipe is limited to 

only 3 ft. A dedicated wire pair is required to extend the 

talk-back range to the entire decline. 

YORK CANYON COAL MINE 

The York Canyon Mine is owned and operated .by Kaiser Steel 

Corporation. It is located in Colfax County near Raton, New 

Mexico. 
The immediate area surrounding the mine is composed of 

flood plains, valley slopes and broad-ridge crests . Coal 

is produced from the Ysrk Canyon seam in a local zone of the 

Raton formation. The lenticular seam height ranges from 4 

to 12 f t. The formation includes beds of sandstone, siltstone, mudstone, and coal. Annual production exceeds 1 

MTPY. The mine uses continuous mining equf pment for longwall 

development. 

A simplified sketch of the York Canyon mining complex is 

shown in figure 7-13. 

SHOP 

OFFICES 

SUPPLIES 

I RETURN 

I AIR 

- -TELEPHONE 

AC PWR CABLE 

STATION 8ASESTATIBN WITH 

RF LlNE COUPLER 

FIGURE 7-13. - York Canyon mine. 

The four entry underground mining complex was developed on a 

south heading for a distance exceeding 10,000 f t. The coal 

seam height varies along the main entry; however, the average seam height is 7 ft. The entries are separated by 50 ft. 

The fresh air entry is used for vehicular transportation 
in the mine. Battery and diesel powered vehicles transport 

personnel, supplies and equipment. A cross section of the 

fresh air entry is shown in figure 7-19. 

/ 2T5ELP:EoNE 

CABLE 

FIGURE 7-14. - Cross section of entry (looking inby). 

A shielded 25 pair telephone cable is installed on the 

upper right hand corner. The belt haulageway is in neutral 

air and contains a Continental conveyor belt system, AC 

power cable and a fire detection system. The return air 

entry was originally used as part of the mine ventilation 

system. Ventilation shafts are now used to transfer the 

return air to the surface. 

Two entry longwall panels are developed to the left of 

the main entry. The entry next to the rib is used for the 

belt haulage system. Like the main haulageway, AC power 

and fire detection cables are installed in this entry. 

Telephone cable is installed in the adjacent fresh air 

entry. 

- York Canyon Mine's Initial Communications System 

The York Canyon Mine uses a private line dial pager telephone system. A 25 pair cable interconnects the underground 
phones to a surface PBX. The underground telephone cable 

enters several junction boxes along the fresh air entry. 

From these boxes, single pair cable branches out to each 

pager telephone. At distances of approximately 6,OQO ft the 

cable 1s terminated tn a junction box. A pair continues for 

an additional 3,000 ft and then enters the 9th left longwall entry. 

Comrnuni cati on Needs 

The longwall coordinator needs quick communi cations capability to reach the face boss, beltmen and maintenance men, 

and supply vehicles. In this mine the centers of activity 

are the face and the shop area. The longwall coordinator, 

underground production foreman, and maintenance foreman use 

battery and diesel vehicles in their work; therefore, vehicular communications are required in the mine. Vest communications are also required since these keypersonnel are 

often on foot. 

Installation -- of MF Equipment 

Vehicular transceivers and antennas were permanently installed on battery powered scooters and diesel powered 

j ee.ps . The remote control head (RCH) was mounted near the 

steering wheel. The antenna: was mounted in a vertical 

plane -on the vehicle's right near side. A base station 

was installed at the portal. The base was remotely control- 
led from the shop via a wire pair. The base station impressed MF signals in the 25 pair telephone line via a line 

coupler at the portal. 

Test Results - and Evaluations 

Duri.ng the equipmentfs installation, tests were conducted 

to determine MF performance in this mine. The carrier and 

noise levels were measured at the base station location with 

a second RF line coupler (the first RF line coupler was used 

to couple signals to the base station) and field strength 

meter. Subjective audio quality determination was made by 

listening to the recovered voice signals at the shop and on 

the vehicles. 

Figure 7-15 shows the measured line current as a *function 

of vehicular distance from the base station. The measurements were made at a frequency of 350 kHz. The measured 

mine noise was found to be approximately 8 dB above one 

microampere (8 dB re luA). At the end of the 25 pair telephone cable, the signal level was 52 dB above the noise. The 

figure illustrates the carrier level required for a 20 dB 

C/N ratio. The slope of the measured carrier level line 

indicates that the average attenuation rate is approximately 

1 dB/lOOO. Measurements were also made at 520 kHz (see 

figure 5-8). Where the attenuation rate is approximately 2.4 

dB/1000 ft. 
% 2 3. 4 5 6 7 8 9 18 IP 12~ 13 34 

MANWAY DISTANCE F RBM' BASE (THOUSAND FT.) 

FIGURE 19-15. - Measured line current as a function .of the 

vehicular distance from the base station. 

During the tests, the vehicle was in the center of' the 

entry and the vehicular antenna was approximately 7 ft from 

the telephone cable. This distance changed along the entry 

because of the seam height and the sag in the messenger 

cable between anchor points. The antenna was in a vertical 

plane (horizontal magnetic dipole). When the antenna is in a 

horizontal plane (vertical magnetic dipole), the induced 

current was reduced by 2 dB. The antenna was at least 6 

inches from any metal surface during this test. 

Interesting coupling effects were noted when the antenna 

was receiving a signal in an adjacent conductorless drift. 

The maximum received signal occurs when the plane of the 

antenna is directed toward the telephone cable in the fresh 
air entry. It is minimum, when the antenna is parallel to 

the cable. By way of contrast, the opposite results are 

obtained when the antenna is in near proximity to the cable. 

Tests were also made with the vest transceiver. As in the 

case of the vehicular equipment tests, the induced line 

current arriving at the base station was measured with the 

RF line coupler and field strength meter. The vest signal 

was approximately 8 dB less then the vehicular signals. 

This corresponds closely with differences in the vehicular 

and vest antenna magnetic moment, 2.0 and 5.2 ATM~, respectively. 

The radio coverage area included the fresh air entries. MF 

signals can be received in entries with electrical conductors. The talk-back range to the base depends upon two 

factors, the range of the mobile transceiver from the base 

station and the distance of the mobile antenna from the 

cable. The talk-back range and distance are always less than 

the receiving range. At a range of 1,000 ft the talk-back 

distance was approximately 80 ft. It decreased to 40 ft at 

a range of 6,000 ft. 

Within 4 ft of the cable, the vest to vest communication 

range exceeds 20,000 ft. It'is possible for the vest to 

receive base station signals all along the longwall face. 

The distance of the face from the base station exceeded 

15,000 ft. 

When a cellular repeater transceiver was temporarily installed outby a continuous mining section, the communica- 
tion range from the repeater to a vest transceiver exceeded 

500 ft. 

ConcP us i ons 

The objectives were met in the mine testing. Whenever the 

longwall coordinator was in a vehicle in a fresh air entry 

(within 7 ft from the cable), he could communicate with the 

foreman. The maintenance supervisor could communicate with 

crew members while riding on vehicular equipment and could 

communicate along the entire haulageway when using vests, 

The talk-back range from the vehicular transceiver to a base 

station was at beast 15,000 ft. (the range testing limit of 

this mine). 

The test demonstrated that the keyperson reach time was 

reduced from an average of 35 minutes (with the existingmine telephone system) to less than a second. 

The system was easily installed. The base station was 

installed and coupled to the existing mine wiring in less 

than 95 minutes and all vehicular equipment was installed 

in approximately three hours. Repeaters were not used in 

this mine because of the excellent test results. 

A number of equipment design problems were noted. One 

problem was related to the use of a floating ground battery 

supply in the vehicular equipment. In the initial installation the power source for the equipment was the vehicular 

battery. The vehicular transceiver ground was connected to 
the frame ground. The vehicular DC motor failed such that a 

positive ground of the battery supply was connected to the 

vehicular frame ground. This caused the transceiver wire 

harness to carry a short circuit current resulting in failure on the transceiver. To solve this problem a floating 

ground was designed for the radio transceiver. On one 

occassion a short to ground developed in the control head. 

Eventually the vehicular DC motor shorted with the* positive 

supply to ground. This caused damage to the radio wiring 

harness. Because of these instances, an isolated type of DC 

to DC converter must be used on every vehicular installation. This would enable a vehicle with a short-to-ground 

failure to proceed to the repair area and still allow the 

transceiver to remain in operation. 

To solve the short-to-ground problems, alternatives such 

as a fuse are not satisfactory since its failure would 

disable the radio under the vehicular short circuit si tuation. An isolated ground system in the radio is not satisfactory since it is possible for circuit to frame grounds to 

exist in the radio itself. It is recommended that a ground 

fault detector be included on every floating type of vehicular power system. 

STAR POINT COAL MINE 

The Star Point Coal Mine is owned and operated by Cetty 

Oil Company. The mine is located in Carbon County near 
I Price, Utah. The coal sequence's are in the Lover Black Hawk 

formation. Mining takes place in the Wattis upper and 

middle seam. The seams are bounded above by siltstone, 

I mudatone and sandstone. Annual production exceeds 1 MTPY. 

I Mining is advancing in the south area (Wattis Seam) while in 

1 retreat in the mudwater (middle seam). Room - and pillar 

mining techniques are used in the mudwater area and continuous miners are used to develop ahead for a longwall in the 

I, south area. A conveyor belt haulage system is used and 

I rubber tired diesel tractors, mancarriers and pickups 

(ISUZld) are used to transport men and supplies in the mine. 

1 Star Point's Initial Communications System 

The mine uses a dial-pager system. Pager telephones are 

located at "key points in the surface off ice, warehouse and 

ahop. Underground phones are located in the belt haulage 

entry in other locations along the fresh air entry and in 

the face. 

Communication Needs 

Mining operations are conducted in two widely separated 

areas of the mine. Keypersonnel are frequently in route 

between the portal and the production areas. Mobile radio 

was needed to improve coordination between supervisors and 

crew men and to reduce haulage downtime. Section foremen 

wanted radio coverage that extended from the pager telephone 
system to the operating face and a mobile telephone that 

interfaced with the mine's pager telephone system. 

Installation -- of MF Equipment 

The entries to the working areas are illustrated in figure 

7-1 6. 

MUD-WATER 

/ 

REACTIVATED 0 FFlCE , DRIFT 3 1.5 MILES / 

I /PORTAL ; 

SOUTH WEST ENTRY MAIN ENTRY I / 

FACES - - . - .I. --.. - ** 

- 1.5 3" xi MILE h 

k- WAREHOUSE 

- I 

REMOTELY CONTROLLED 1 ABOVE GROUND 

FROM WAREHOUSE AND I OFFICE. I 

BASE AND LINE COUPLER 

A REMOTE CONTROL CONSOLE (RCCI 

a REPEATER 

-* a - SINGLE-PAIR (RC CABLE) 

FIGURE 7-16. - Simplified drawing of mains in the Star 

Point mine. 

Mains are developed with four and five entries. The conveyor 

belt haulage system transports coal from the mudwater and 

south areas to the portal of the mine. At a distance of 

approximately 1 mile from the portal, the mudwater conveyor 
belt intersects the west main belt. -4 single pair telephone 

cable is installed in the conveyor belt entry. The fresh 

air entries provide a roadway for vehicular traffic. Separate 7100 and 13,200 volt three phase AC power cable supply 

power to the mudwater and all south areas, respectiveby. The 

AC power cables are installed in and out of the roadway 

entry. Transformers along the AC power cable provide power 

to the conveyor belt drives in the belt entry and in the 

power cable. MF equipment was installed to provide radio 

coverage along the roadway. A base station was installed at 

the intersection of the west and mudwater mains. Coupling 

to the AC power and telephone cables was made via RF line 

couplers which were clamped around the mudwater 7100 volt 

and the 13,200 volt cable. The telephone cable was tnstabbed with high voltage insulation tubing and was also placed 

in tehe coupler. The couplers were connected to the base 

station via a trif ilar wound matching transformer . The 

transformer was made by winding 19 turns of 820 magnet wire 

on a type A ferrite material core with a 1 inch outside 

diameter. The insertion loss of the transformer was 0.2 dB. 

The base station operated on a frequency of 400 kHz and was 

remotely controlled from the warehouse and off ice. 

Four vehicular transceivers were installed on diesel powered tractors. Figure 9-1 5 shows that in the mine two dual 

frequency F1 /F2 global repeater sys tems were used. Frequency 

(F1) was 400 kHz and frequency (F2) was 520 kHz. The use of 

two frequencies allows access to global repeaters when 
transmitting on frequency (F2) . The global repeater receives signals at F2 and retransmits it F1. The first F1 /F2 

repeater was installed in the Mudwater entry approximately 

3,600 ft from the base station and 5,000 ft from the face. 

The second repeater was installed near the south area entrance, approximately 7,000 ft frop the base station and 

6,500 ft to the furthest South face. 

Test Results - and Evaluation 

The system's performance was evaluated by measuring carrier and noise levels of MF signals arriving from the mobile 

equipment in the roadway entries. The i.nduced line current 

measurements were made by use of a third RF line coupler and 

a field strength meter. The noise level was approximately 8 

dB re at 1 v in the mine (about the same level as in the 

York Canyon mine). Range testing indicated that each AC 

power transformer produced from 8 to 12 dB of insertion 

loss. The talk-back range was dependent upon the number of 

AC power transformers between the vehicular transceiver and 

the base station. 

A vehicular transceiver could reliably receive base station signals within 7 ft of the AC power cable within a 

range of approximately 7,000 ft from the base. Beyond 7,000 

ft of a base station, F2 transmission enables vehicular 

reception via repeater action. Radio coverage was not provided in roadway entries without AC power cable. The maximum reliable tal k-back range was approximately 5,000 f t 
from the base station or a repeater. The south area roadway 

entry was conductorless. A dedicated single pair telephone 

cable was installed in the entry. It threaded the RF line 

couplers at the repeater and was directly connected to the 

pager telephone cable at that location. 

The vest receiving range includes all belt entries within 

5,000 ft of the base station or main south repeater. The 

vest antenna distance from any conductor in the belt haulage system was often as great as 40 Pt. At a range of 

10,000 ft from the base station, the vest antenna distance 

was restricted to approximately 7 ft. The vest talk-back 

range to the base station along the telephone cable was more 

than 10,000 Pt (at a distance of 7 ft from the cable). 

The vest talk-back range along AC power cables was consfder abl y 1 ess . It was approximately 3,000 ft when the 

antenna was approximately 7 ft from the cable. 

Conclusion 

The AC power cable provides good radio coverage f n the 

mine; but because of AC line switches and transformers, 

signal distribution fs often limited. To provide good continuity of signals throughout the mine, twin-lead cable 

instead of AC power cables should be installed in manways 

for MF signal distribution. The twin-lead cable is advantageous because it provides radio coverage all along the 

manways. 
The value of the MF system was demonstrated when a continuous mining machine was buried by a rockfall at the Mudwater face. The system was used to instantly alert the 

underground superintendent of the problem, saving valuable 

time in the recovery effort. 

Addi tional value of the sys tem wsas demonstrated along the 

conveyor belt line. In this case, the beltmen report that 

use of the vest saves them valuable time because they are 

always in contact with the bas? station and do not need to 

go to telephones to communicate. 

The face boss believed that the use of the cellular repeater and vest in an operating mine setting would be an important application of the system. Since an intrinsically 

safe vest was not available during the evaluation period, 

its effect on mine productivity could not be determined. 
VIII. RECOMMENDATIONS 

Because MF communication systems are expected to emerge as 

principal communications systems in mining, a need exists 

to improve and enhance the systems by the adoption of 

several basic addi tions . These include: 

A. REDUCE LOSS ACROSS AC POWER TRANSFORMERS AND SWITCHES 

MF signal propagation on mine AC power grids can be improved by developing efficient coupLeks for use around high 

loss transformers and switches. 

Be RFI SUPPRESSION 

R'FI suppression filtering techniques must be developed for 

use on various types of alternators encounte~ed on underground mine vehicles. These f flters should have the capability to suppress noise generated by contactor arcs on 

trolley powered equipment. 

C- FACE RADIO COVERAGE AREA 

The long term benefits of providing vest radio coverage 

inby an operating section must be evaluated in further field 

testing. Although the vest design is intrinsically safe and 

can operate inby, the cellular repeater must be operated 

outby. Its connection to the pager telephone system must 

be through an approved barrier so It won't affect the 

intrinsically safe approval of the telephone system. In the 
event of a mine ventilation failure, an MSHA approved procedure must be developed to turn off the repeater. 

D. VEHICULAR DC-DC CONVERTER ISOLATION 

Vehicular transceivers which derive their .power from 

floating ground electrical power sources must use an isolated DC to DC converter. Further, it is recommended that a 

ground fault detector device be included on floating-ground 

electrical power systems. 

E. RF LINE TO LINE COUPLERS 

RF line to line coupling techniques should be developed in 

order to simplify the installation of single pair cable in 

conductorless entries. A bidirectional amplifier should be 

developed for this purpose along with passive techniques. 

. F. MOIST SYSTEM COMMUNICATIONS 

Hoist radio communications can be dramatically improved 

with the use of MF antennas and couplers developed in this 

work. The RF line couplers mechanical design and the transceiver enclosure must be sealed. The transceiver enclosure 

should be filled with dry nitrogen to prevent build-up of 

water. The transceiver design should include a tone signalling module to enable miners to reliably signal the hoistman. Further , the signalling should include an emergency 

stop as well as the status of the slack rope detector. 
C. PACER FUNCTIONS 

The extensive receive coverage area demonstrated In mine 

testing shows that a pager receiver would operate extremely 

well in an underground mine. A small pager transceiver 

should be developed for the system, 

H. IDENTIFICATION OF RADI 0 COVERAGE AREAS 

Areas of radio coverage must be marked by a visable sign 

so a communicator knows from where he can reach the base 

station. 

Unlike surf ace radio coverage, underground coverage exhfbits many fringe areas of radio reception. In the fringe 

areas receive messages are often noisy and unintelligable so 

miners must be instructed to step nearer an electrical conductor to communicate. 

TRAINING 

Training materials must be developed to enable mine maintenance personnel to service radio equipment to the board 
LIST OF REFERENCES ' 

1. Pugh, D.S. Organization Theory. Penguin Modern Management Reading, 1971. 

2. Mayo, E. Hawthorne and the Western Electric Companythe Socia-1 Problems of Industrial Civili zation. Routhledge, 

1949, 60-76 pp. 

3. Bavelas, A. A Mathematical Model for Group Structure. 

Applied Anthropology, v. 7, 1948, pp 16-30. 

4. Leavit, H.J. Some Effects of Certain Communication 

Patterns on Group Performance. Journal of Abnormal and Social Psychology, No. 48, 1951, pp 38-50. 

5. Lohman, G.M. A Closed Network Queue Model of Underground Coal Mining, production, Failure, and Repair. Jet 

Propulsion Laboratory, California Institute of Technology, 

Pasadena, CA, Pub. 78-72, Aug. 1978. 

6. Marrus, L.D., etal. Logistic Performance of Continuous 

Miners. (contract S0144057). BuMines No. 4185, Jan. 1976. 

7. Dr. John Smoker. Medical Officer for Kaiser Steel York 

Canyon Mine, Raton, New Mexico. 

8. Sacks, H.K. and Chufo, R.L. Medium Frequency Propagation in Coal Mines. Paper in Preprints of Papers Presented 

at the 4th West Virginia University Conference on Coal Mines 

Electrotechnology, Aug. 1978. 
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Propagation and Coupling in Coal Mines. BuMines H0155008. 

Proceedings, Electromagnet is Guided Waves in Mine Environments Workshop, Boulder, C0, March 1978. 

10.' Lagace, R.L. Task Leader, Transmit Antennas for Portable VLF to MF Wireless Mine Communications. Arthur D. 

Little, Cambridge, Mass. BuMines H0346645, Task C. Task 

order 1 , May 1977. 

11. Hill, D.A. and Wait, J.R. Exci tatiow of Monof ilar 

and. bifilar Modes on a TransmiS3ion Line in a Circu%ar 

Tunnel. Journal of Applied Physics, v. 45, No. 8, Aug. 1974. 

12. Scott, et al. Electromagnetic Noise in McElroy Mine. 

Institute for Basic Standards, Electromagnetic Division, 

NBS, Boulder, CO. 74-389. 

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Mine. Electromagnetics Division, Institute for Basic Standards, NBS, Boulder, CO, June 1974. 

14. Lagace, R. La Emsile, A.C. and Grossman, M.A. Modelling and Data Analysis of 50 to 5,000 kHz Radio Wave Propagation in Coal Minea. (contract H0346045) BuMines OFR 83-80 

1980, 109 pp. Arthur D. Little, Inc. PB 80-209455. 
15. Cory, T.S. Propagation of EM Signal in Underground 

Mines. (contract H0366028, Collins Radio Group). BuMines 

OFR 136078, 1977, 158 pp. ~0289757/~S. 

16. Wait, - J.R. Electomagnetic Waves in Stratif ield Media 

Pergamon, 1970. 

17, Emsile, A.G. and Lagace, R.L. Propagation of Low 

and Medium Frequency Radio Waves in a Coal Seam. Radio 

Science, v. 11, No. 4, 253-261 pp, April 1976. 

18. Cory, T.S. Summary Data Report No 2 - Mine Wireless Propagation Test Program - Ireland Mine Test Data at 

Medium Frequency. Spectra Associates, Inc., Cedar Rapids, 

Iowa. 

19. Moser, J.R. and Spencer, R.F. Predicting the Magnetic Fields from a Twisted-Pair Cable. IEEE Trans. Eng., 

EMC-10, 324-239 pp, 1968. 

20. Private communications with Dr. Julius Hupert . 

21. Andvt Microphone and Audio System Study. BuMines 

N00039-77-C-0365. Naval Electronic Sys tem Command, Washington, DC, Aug. 1978.