MEDIUM FREOUENCY VEHICULAR CONTROL AND COMMUNICATION SYSTEMS 

FOR UNDERGROUND MINES 

Larry G. Stolarczyk 

A.R.F. Products, Inc. 

Raton, New Mexico 

Harry Dobroski , Jr. 

Bureau of Mines 

Pi ttsburgh Research Center 

Pittsburgh, Pennsylvania 

ABSTRACT 

Theoretical and experimental research sponsored by the U.S. Bureau of Mines shows that 

medium frequency (MF) electromagnetic (EM) signals propagate great distances in an underground 

environment such as a tunnel or mine. This propagation is enhanced by different mechanisms 

associated with the geology, and with the existence of metallic conductors in the entryways. 

In stratified geologies, a transverse 

electromagnetic (TEM) mode of signal propagation 

is possible if a low conducting layer is bounded 

above and below by higher conducting layers. In 

general, the difference in conductivities must 

be at least several orders of magnitude. Such 

geological waveguide conditions often exist in 

underground mi nes . Propagation vi a the waveguide 

effect is known as the "seam mode" of propagation. 

Another mode of propagation, the "tunnel 

mode," exists in underground entries with elec- - trical conductors such as power cables, metal 

pipes , and phone 1 ines. A monofi lar mode of 

propagation exists when signals are impressed 

upon conductors and return via the surrounding 

rock. A bifilar mode of propagation exists when 

all signals exist only on local conductors. In 

a given location there is a combination of monofilar and bifilar modes that make possible an 

interaction between the conductors and a transmitting device in a tunnel or entryway. Signals 

can be impressed on or received from local conductors via magnetic dipole antennas (loops) or 

line couplers. 

OBJECTIVE 

This paper describes an MP narrowband cornmunication system conshting of base stations, 

repeaters, mobi 1e transceivers, and personnel transceivers and their evaluation in both mining 

and nonmining applications. The base stations 

and repeaters induce signals into existing conductors via line couplers. Because of this, 

special distribution cables are not required. 

The existing conductors distribute the signals 

that can be received by mobile or personnel transceivers in the vicinity of these conductors. The 

vehicular transceivers are used on any kind of 

vehicle that requires direct, two-way corrununications. The personnel transceivers are of a unique 

vest design, suitable for use in tight quarters. 

They have been approved for use in methane 

atmospheres. 

The MF system paves the way for further 

advances in mining technology. It provides a 

reliable data link for the remote control and 

monitoring of vehicles, it can be used in wireless 

environmental monitoring, and it can increase the 

effectiveness of search and rescue teams. The 

system can be used in special applications such 

as inside high-density concrete and steel structures found in nuclear and thermo-electric power 

generating stations, large office buildings, 

prisons, and sub-surface military facilities. 

INTRODUCTION 1 

Almost everyone has experienced poor radio 

reception in underground parking lots and tunnels 

associated with the national highway system. 

Poor comnunications in these circumstances result 

in minor inconveniences in most situations, but 

the inability to reliably communicate becomes a 

serious matter for people working in high density 

concrete and steel structures and tunnels below 

the surface of the earth. Poor communications 

within the Three Mile Island facilities have been 

frequently cited as a contributing factor in the 

nuclear accident. Excel lent radio coverage i s 

essential in mining for increased safety and 

productivity. 

Ever since the emergence of radio for prac- tical surface communicatians, attempts have been 

made to apply it to underground mining. The 

Bureau of Mines was one of the early researchers 

in this area. As early as 1922, experiments 

showed that signal propagation was possible but 

not practiqal. Equipment was bulky and insensitive and only very large antennas could be used. 

CH1951-318410000-0316 $1.00 O 1984 IEEE 
[~l~portant contri buti ons to underground comuni catior~s were made by The Chamber of Mines of South 

~frica who conducted much research and evaluated 

Ill,\ny prototype radi o systems i n underground coal .III~ gold mines (1) (2). Both Bureau and South 

,If ri can researchers came to a simi 1 ar conclusion 

ttl~t radio propagation in the MF band (300 kHz3 MHz ) showed good promise. 

Parallel research was also conducted in the 

U.S. and Europe (3)(4) that showed that frequencies above the MF band (VHF, 30-300 MHz/ 

IJtfF, 300 MHz-3 GHz) were also useful, but only 

in unique situations. In general, VHF/UHF is 

qrcatly affected by tunnel geometry and obstacles, 

,~nd is useful only in less than line-of-sight corlditions. In addition, a very elaborate system 

of leaky feeder cables and repeaters was needed 

to be practical. The time was clearly right to 

take a serious, in-depth second look at, MF. 

MF PROPAGATION RESEARCH 

The Bureau initiated a series of programs to 

theoretically study the mechanisms of underground 

FIF propagation in mining areas with and 'without 

conductors (5). The theoretical studies were 

supplemented by exhaustive in-mine measurements 

to compare theoretical and measured results (6) 

(7). Modes of propagation in tunnels with electrical conductors were studied in a series of 

theoretical investigations and tests. Radiating 

loop antennas were used to induce signals into 

nedrby conductors which behaved like open 

multiple-conductor transmission lines. Radiation 

from the lines complete the comnunications path 

to the receiver. Research work indicated that 

bi fi lar and monofi lar modes of propagation 

existed. 

In the bifilar mode, all signals are confined to local existing conductors. Although 

sillall leakage fields exist, the main EM energy 

is confined between the conductors. This condition makes it more difficult to couple energy 

i n to such conductors, but once done, propagation losses are low (3 dB/1,000 ft) because of the 

111ctall ic propagation path. 

In the monofilar mode, any signals induced 

i n to 1 ocal conductors return via the surrounding rock, coal, etc., which results in higher 

propagation losses (23 dB/1,000 ft ) . However, 

it is easy to couple energy into the conductors 

.because of the nature of the fields. In practice 

there is a complex assortment of bifilar and 

r~~onofilar modes in a typical mine entry because 

of variations in line spacings, changes in entry 

cross sections, etc. 

Both bifilar and n~onofilar coupling to a 

conductor increase with frequency, but so do 

pr'opagation losses. In addition, mine-generated 

c lectronidgneti c interference (EMI) decreases with 

fr-cquency. These combined frequency effects resul t in a favorable window of operation that 

extcnds from 300 to 800 kHz with due consideration 

to voiding conflict with othcr services occupyirly this part of the spectruln. 

In certain stratified geological formations, 

natural waveguides exist. Coal, trona, and potash 

seams that are surrounded by more conductive rock 

are examples of this. This mode of propagation 

is known as a "seam mode." For this mode to 

exist, the seam conductivity (ac) must be several 

order of magnitude less than that of rock (ar). 

A loop antenna that is at least partially vertically oriented produces a vertical electric field 

(Ez) and a horizontal magnetic field (Hq). In 

the rock, the field diminishes exponentially in 

the 2-direction. In the seam, the fields diminish 

exponentially at a rate determined by the attenuation constant (a), which in turn depends upon the 

electrical properties of the seam. An inverse 

square-root factor also exists because of spreading. The effect is that the wave propagates 

between the hi ghl y conducti ng rock 1 ayers boundi ng the lower conductivity seam. The fact that the 

seam may have entries and crosscuts is of minor 

consequence. 

The electrical parameters of natural material s depend upon frequency. As a general rule, 

the conductivity increases and the dielectric 

constant decreases with frequency. Both increase 

with water content. They also depend on pressure. 

Excluding rock with high concentration of ferromagnetic metals, the permeability is close to 

that of a vacuum. Measurements show that operating range decreases for frequencies below 300 kHz 

and above 800 kHz. 

The tunnel mode and seam mode propagation 

mechanisms permit good comnunicating range in 

the lower MF band in underground environments. 

SYSTEM DESIGN AND CONFIGURATION 

An MF system was designed and evaluated that 

could operate either in the tunnel mode (use of 

existing conductors) or seam mode (waveguide in 

the strata). The system consists of base stations, 

repeaters and line couplers, vehicular transceivers 

and antennas, and a personal transceiver configured into a vest design. Fig. 1 illustrates a tunnel 

mode configuration. Existing mine conductors 

serve not only as the transmission lines but as 

the distribution antennas as well. Unit to unit 

comnunications are conducted on frequency fl. 

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 

station and repeater use toroidal radio frequency 

(RF) line couplers to efficiently couple signals into and receive signals from the existing conductors. The couplers are impedance matched and 

tuned, and can be quickly clamped to most conductors. Transfer impedance is approximately 

10 ohms. 

An important human factor problem was solved 

by the design of a vest transceiver (fig. 2). By placinq the radio circuit modules in pockets in 

the vest, the weight and bulk of the transceiver 

has been evenly distributed on the miner's body. The loop antenna is sewn into the back of the 

vest. Since the area of the loop does not change 

with rnove~~lent of thc miner, it rema ins tuned at 
a tic. Ttic control hcdd has been designed 

to nlatch thc rldtural rr~ovement of the hand. The 

thumb car1 cornfortably operate a1 1 control 

functions. The vest design a1 lows the miner to 

maneuver in tight quarters and perform normal 

mining tasks without the danger of entangling 

the transceiver in mining equipment. The vest 

transceiver design has been approved by the Mine 

Safety and Health Administration (MSHA) for use 

in a methane atmosphere. 

The repeater uses loop antennas to radiate 

and receive seam mode signals (fig. 3). This 

permits radio comunications wi th miners working 

in conductor-free areas that are the focal 

points of most mining activity. Since the 

repeater services a "cell" (local working area) 

in the mine, it is called a cellular repeater. 

It may be connected to the local pager telephone 

system to enable comnunications with other distant miners and with the surface comnunications 

center. 

SYSTEM SPECIFICATIONS 

Emissions 

Type Narrowband FM 

Occupied BW 10 kHz 

RF frequency 

(internally selectable) 60 to 1,000 kHz 

Peak deviation +2.5 kHz - 

Mod frequency 200 to 2,500 Hz 

Receiver 

Type 

Sensitivity 

IF BW 3 dB 

70 dB 

Squelch 

Transmitter 

Type 

Output Power 

Vest 

Vehicular/base 

Superheterodyne 

1.0 pV (12 dB Sinad) 

12 kHz (min) 

22 kHz (max) 

Noise operated 

Push Pull Class B 

Antenna 

Magnetic moment 

Vest 2.1 ATM~ 2 

(Amp-Tqrn-meters ) 

Vehicular 6.3 ATM 

COMMUNICATION SYSTEM PERFORMANCE 

To ascertain the actual performance in 

underground mines, it was necessary to install 

the system and measure the perfo~-rll~ncc ovcbr 

long period of time. Equipment w~s instc~llcJ 

in the following mines: (a) York Canyon Minc 

(Kaiser Steel Corp. ), coal ; (b) Escalantc Mi11c 

(Ranchers Exploration and Develop~nent COI-p. ) , silver; (c) Magma (Magma Copper Co. ), copper; 

and (d) Star Point Mine (Plateau Mining Co. ), 

coal . 

The comuni cation system performance was 

similar in every mine. Base station radio signals could be received wherever electrical conductors existed in the entryways. The talkback 

range depended upon the distance of the radia!ing 

antenna from the conductors. It a1 so dependcd 

upon the type of conductors in the entryway. 

Far a radiating antenna-to-line separation of 

approximately 7 ft , the vehi cul ar-to-base communication range exceeded 30,000 ft along unshielded single-pair telephone 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 a-c power cable exceeded 10,000 ft. At each point where the primary path cable connected to sub-stations, the 

cable path loss increased from 8 to 12 dB. At 

400 kHz, the attenuation rate was approximately 

4 dB/1,000 ft. 

In coal mines, the talkback distance to a 

base from a vest or vehicular transceiver depends 

upon the distance from the cable and the distance 

of the base along the cable. At a range of 1,000 

ft, the talkback distance was approximately 80 ft 

from the cable. It decreased to 40 ft at a range 

of 8,000 ft. In a conductor-free area the cellular repeater talkback range exceeded 500 ft. 

In-mine tests of the system demonstrated 

that the time to reach key personnel could be 

reduced from an average of 35 minutes (with the 

existing mine pager telephone system) to less 

than a second. A base station could be installed 

and coup1 ed to the existing mine wiring in less 

than one hour. The ease of installation is 

important since the mine management can assess 

system performance before they buy equipment. 

Besides being able to hear messages almost 

anywhere conductors exist in the mine, the talkback range was measured in miles when the mobile 

transceiver was within a few feet of the niine 

conductors. For ranges up to a mile, the received base station carrier/noi se (C/N) ratio 

exceeded 60 dB. This suggested that the systerii 

could be used as a high quality data link. 

LOCOMOTIVE REMOTE CONTROL USING MF 

These MF techniques were successfully applied 

to the remote control of locomotives in a loading 

operation in Union Oil's Molycorp Mine near 

Questa, New Mexico. 

This mine uses multiple level block cavirlg 

methods to mine the ore. The upper ore body is 
f ~..li t~l.,~i by bl asti ng and ore f 1 ows by,gravi ty to 

t)lr' I,I.I :IY level where miners fi 11 transfer 

r,li.~13. 111 the loading process a trolleyposcr.ctI loco~liotive is used to maneuver cars below 

the ~..~isc. Powered gates control the flow of ore 

into ttw cctr. Multiple panels of eight or more 

,.,~i.;cs ,lrc located directly below the ore body. 

Aftcr. loc~dinq, the train leaves the panel and 

entr'rss the ~lrain haulage track where it continues 

to ttlc CILIIII~. A conveyor be1 t is used to transport ttlc ore out of the mine. Fig. 4 shows the 

loadi rly) oyerati on. 

E~ch loco~notive used in this operation has 

been cquipped with an MF transceiver modified for 

two- f rcquerrcy operati on and interfaced wi th FSK 

decoders and control circuits. When used on fl, 

the locomotives can communicate with other locomotives on the main haulage tracks to coordinate 

movcr~~crit to the dump. When used on f2, the 

loco~l~otives are on a frequency that permits reniote control of the loading process. 

In the loading process, the motorman takes 

the tr'lin to a transfer raise location and 

switches to f2. He then leaves the locomotive 

and goes to the gate. At the gate there is a 

coritrol box connected to an MF base control station. The base control station frequency is modulatcd by a FSK digital comnand signal (mark 2070 

Hz. space 2500 Hz) which is generated in a remote 

control ~~~odulator (RCM) unit. Depressing the 

forw,lrd/ bra ke re1 ease or reverse/brake re1 ease 

swi tches on, the control box (one at each loading point) lnl tlatlng digital encoding of the comnand 

signal. For safety purposes, each locomotive is 

assigned a unique key. Insertion of this key in 

the KCM uniquely codes the comnand signal with 

the train identification. The FSK signal modu- - lates the base station which induces an MF signal 

current on the control cable via a line coupler. Thc train transceiver receives the FM signal from 

the cable and recovers the FSK digital comnand 

signal which is applied to a remote control dewodul~ tor (RCD) uni t. The RCD pi lot re1 ay 

colltrols the required contactors in the locomotive control circuit. The RCD unit is designed 

wi tti double redundant fai 1 -safe ci rcui try. The 

digi tL1l code includes self-checking features to 

prevclit fa1 se operation of the train. 

OTHER APPLICATIONS 

Thc MF system performance has been evaluated 

inside a high-densi ty concrete and steel nuclear 

rc~ctor containment vessel, a large thermo-electric 

powcr plant, and a fi re-f ighting training structur-e. A large-diameter loop antenna placed on the 

gnligwdy inside of the containment vessel was excited by a base station via an RF line coupler. 

In nothe her series of tests, loops were placed 

at-mind the outside of the electric power generator station and the fire fighting training 

str.r~cturcs. As in the containment vessel test, 

the 1 oops wcrc exci ted by RF 1 i ne coup1 ers . In 

thcsc tcs t.; , co~~rnercial ly avai lable handheld UHF 

tr.L~n5cci vcrs were compared wi th the MF vests. 

Although the UHF transceivers performed well on 

all line-of-sight paths, communication problems 

existed in many areas such as lower level work 

areas, elevators , and behind steel reinforced 

walls. Tn contrast, MF coverage included these 

areas. When an MF vest was within 4 ft of the 

loop, the received base C/N was greater than 40 dB. 

SUMMARY 

A whole mine radio system has been developed 

that exploits the low loss tunnel and seam modes 

of MF wave propagation in sub-surface tunnels. 

This Bureau of Mines program was initiated because 

of the need for more effective connnunications for 

improved safety and productivity. The system reduced the average time to reach key personnel from 

35 minutes (pager phones) to less than a second 

(MF system). The system permits underground per- sonnel to maintain comnunications almost anywhere in the mine and with the surface comnunications 

center. As a consequence, communications are 

possible to the point of equipment breakdown, a 

fact of significance in reducing mining costs. 

Inductive coupling of signals into the existing mine conductors insures extensive coverage in 

the underground complex. No special comnunication 

cable usually needs to be installed. Base stations and repeaters can be installed in minutes in 

locations of prime mine power. Backup batteries 

are included for additional re1 iabi 1 i ty. 

Both vehicular and personnel transceivers are 

available. The vehicular antennas utilize a new 

antenna design of exceptional efficiency and durabi 1 i ty. The personnel transceiver uti 1 i zes a 

unique vest concept that enables miners to maneuver 

in tight quarters and perform normal tasks without 

the danger of entangling the transceiver in mining 

equi pment . 

Extensive tests in numerous underground coal 

and metal mines have demonstrated the reliability 

and ease of installation. The equipment survives 

and performs well in the hostile mine environment. 

Since the same equipment can be used for both voice 

and data transmissions, it forms the basis for 

further advances in mining technolody. For the 

case of train haulage, the voice segment of the 

system enables coordination of rail traffic for 

reduced haulage costs and improved safety. During loading, remote control of the train frees at 

least one man to do other mining tasks. 

REFERENCES 

(1) Nelson, T. U. and B. A. Austin. "A Medium Frequency Radio Comnunication System in a 

Coal Mine." Research Report 42/80, Project CSlC10, 

Chamber of Mines of South Africa, Johannesburg, 

1980, 30 pp. 

(2) Austin, B. A. "Underground Radio Communication Techniques and Systems in South African 

Mines." PROC. Electromagnetic Guidcd Naves in 

Mine Environment Workshop, 1978, pp 87-102. 
Figure 2. - Medium frequency vest transceiver 

320 
Pager phone line (existing) 

I T T 

Transrni tter 'w 

Cellular Receiver 

repeater 

pry transceiver 

T/f, or f2 

Vehicular 

I transceiver ( 

Figure 3. - Cellular repeater concept 

/-A GRIZZLY LEVEL 

TRANSFER 

RAISE 

CONTROL CAeLE 

TROLLEY WIRE A/ 

REMOTE 

CONTROL - BASE 

~OOULATOR 

- 

RAIL 

Figure 4. - Train remote control system