ANALYSIS OF COMMUNICATION SYSTEMS 

IN COAL MINES 

Dr. M. D. Aldridge 

Associate Professor 

USBM GRANT FINAL REPORT 

Grant No. G 0101702 (MIN-39) 
USBM GRANT NO GO101702 (MIN-39) 

ANALYSIS OF COMMUNICATION SYSTEMS 

IN COAL MINES 

Dr. M. D. Aldridge a 

Associate Professor 

DEPARTMENT OF ELECTRICAL ENGINEERING 

WEST VIRGINIA UNIVERSITY 

MORGANTOWN, WEST VIRGINIA 

USBM GRANT FINAL REPORT (GRANT NO. GO101702 (MIN-39)) 

May 1, 1973 

DEPARTMENT OF THE INTERIOR 

BUREAU OF MINES 

WASHINGTON, D.C. 

The views and conclusions contained in the document are those of the 

authors and should not be interpreted as necessarily representing the 

official policies of the Interior Department's Bureau of Mines or the 

U. S. Government. 

QC 67 (.; 
FOREWORD 

This report was prepared by West Virginia University, Department of 

Electrical Engineering, Morgantown, West Virginia, under USBM Grant 

No. G 0101702 (MIN-39). The contract was initiated under the Coal 

Mine Health and Safety Research Program. It was administered under 

the technical direction of the Pittsburgh Mining and Safety Research 

Center with Mr. Howard Parkinson acting as the technical project 

officer. Mr. Joseph Herickes was the contract administrator for the 

Bureau of Mines. 

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

this contract during the period June 5, 1970 to June 29, 1972. 

This report was submitted by the authors on May 1, 1973. 

This technical report has been reviewed and approved. 
mine cmunication for improving mine safety and operating efficiency is shown. 

17. Key Words and Document Analysis. 170. Descrivors 

I Coal Hines 

Telecommunicatfons 

I 1% Identifiers/Open-Ended Terms 

I 

COlUl NTIS-30 110-70) I L UICOMU-DC ~OS~O-P~ 1 

. % X 

.- .- 1 . - .- - - - - - - (IIR 

-- 

17c. COSATI Fie ld/Group .. I 

21. 'No. of Pager 

138 

18. Availability Statement 

22. Price - 

19.. Security Class (This 

Report) 

UN- ~elease Unlimited 30. Securicy Class (This 

4- ,s Pa~~~~~
TABLE OF CONTENTS 

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

Present Communication Methods ........... 

Telephone ................... 

Carrier Phones ................ 

Outside Communications ............ 

Research Program Rationale . ..... .. . 

Through-The-Earth Communication .......... 

General Problem ................ 

Power Transfer Between Coplanar Loops ..... 

Radlation Resistance of a Small Loop 

Located in a Spherical Cavity ......... 

Maximum Transfer of Power Between 

Coplanar Loops ................ 

Power Transfer Between Coaxial Loops ...... 

Power Transfer Between Loops in a 

Half-Space .................. 

Outslde-to-Inside Transmission ........ 

Comparison of Experimental and 

Theoretical Data ............... 

Noise Measurements .............. 

System Optimization .............. 

Personal Paging System Design ........... 

Summary of the Pertinent Aspects of 

Autocorrelation ................ 

Auto-Correlation Detector Design ....... 

Page 

1 

1 

P 

4 

6 

7 

9 

9 

11 
Table of Contents (continued) Page 

Optimization of Pre-Filter Bandwidth and 

Sampling Data ................. 

Selection of Sampling Period and 

Signalling Frequency ...........a. 

Circuit Design ................ 

Test Results ................. 

Coding System Design ............. 

Prototype System ............... 

Typical System Design ............. 

System Optimization .............. 

Comments ................... 

The Telephone System ................ 

Manual Exchange ................ 

Automatic Exchange .............. 

In-Between Systems .............. 

Carrier Current Systems .............. 

Auxiliary Systems ................. 

Walkie-Talkie Systems ............. 

Usefulness .................. 

Outside Radio ................. 

Outside Telephone ............... 

Conclusions .................... 

References ..................... 

Appendix A ..................... 

Appendix B ..................... 
LIST OF FIGURES 

Page 

3 

Figure 

Paging Type Telephone .......... 

Use of Telephone Line to By-Pass 

Haulage Motor .............. 

Antenna Orientations Analyzed ...... 

Antenna Low Frequency Equivalent 

Circuits ................ 

Radiation Resistance of a Small Loop 

in a Spherical Cavity .......... 

Power Transfer, a = 0,l mholmeter .... 

Power Transfer, a = 0.01 mholmeter ... 

Power Transfer, a = 0.001 mholmeter ... 

Maximum Transfer of Power Between 

Coplanar Loops ............. 

fopt Versus Range, Coplanar Loops . . 

Comparison of Power Transfer for 

CP and CA Orientations ......... 

Secondary Field Dependence Upon 

zfzo .".............. 

Power Transfer Between Loops in a 

Lossy Half-Space, a = 0.1 mholmeter a . . 

Power Transfer Between Loops in a 

Lossy Half-Space, a = 0.01 mho/meter . . 

Power Transfer Between Loops fn a 

Lossy Half-Space, a = 0.001 mholmeter . 

Surface Versus Inside Transmitting 

Antennas ................ 

Block Diagram of Test Equipment a.e.. 

Comparison of Calculated to Measured 

Power Transfer at 2 Kilohertz ...... 

Measured Power Transfer, Test No, 1, 

Upshur Mine ............... 
Measured Power Transfer, Test No. 2, 

U.S.SteelN0~50 ........... 

Measured Power Transfer, Test No. 3, 

U. S. SteelNo. 50 . . . . . . . . . . . 

Measured Power Transfer, Test No. 4, 

Badger No. 14 . . . . . . . . . . . . . . 

Measured Power Transfer, Test No. 5, 

Badger No. 14 . . . . . . . . . . . . . . 

Measured Power Transfer, Test No. 6, 

U. S. Bureau of Mines . . . . . . . . . . 

Measured Power Transfer, Test No. 7, 

U. S. Bureau of Mines . . . . . . . . . . 

Composite Noise Data - Highest and 

Lowest Readings . . . . . . . . . . . . . 

Composite Noise Data - Highest and 

Lowest Readings . . . . . . . . . . . . . 

Block Diagram of Detector . . . . . . . . 

Illustration of the Sampling Procedure. . 

Autocorrelation Gain . . . . . . . . . . 

Optimizing Pre-Filter Bandwidth 

and Product Size . . . . . . . .,. . . . 

Pre-Filter SNR Density Versus 

Product Size . . . . . . . . . . . . 

Detector Noise Bandwidth Versus 

Integration Time . . . . . . . . . . . 

Appropriate Nulling of Troublesome 

Harmonic . . . , . . . . . . . . . . . . 

Schematic Diagram of Prototype . . . . . 

Analog Multiplier . . . . . . . . . . . . 

Correlator Transfer Characteristic . . . 

Correlator Transfer Characteristic . . . 

Sample-Hold Outputs . . . . . . . . . . . 

Detector Test Results with 

Reset Disabled . . . . . . . . . . . . 
Detector Input-Output Transfer 

Characteristic ............. 

Assembled Prototype .......... 

Maximum and Minimum Length Codes .... 

Encoder Block Diagram ......... 

Delay Register Schematic ........ 

Decoder Schematic ........... 

Decoder Waveforms ........... 

Sample Mine Outline .......... 

Sample Magnetic Field Noise Spectrum . . 

Antenna Placement Within the 

Mine ................. 

Typical Telephone System Layout .... 

Sectionalization of Telephone System . . 

General Telephone System Layout .... 
LIST OF TABLES 

Table 

I 

I1 

Page 

Comparison of Loop Orientations ...... 26 

Measured Conductivities at Mining 

Locations ................. 37 

Protytype Component Identification .... 73 

Decoder Component ............. 90 

Example Selection of Operating 

Frequency ................. 97 

Sectionalization of Phone System ..... 101 

vii 
INTRODUCTION 

This document reports the results of a two-year research effort 

aimed at improving communication systems used in coal mines. This 

work was conducted under a program whose original goal not only involved 

the development of practical and effective electronic systems for 

operational communications, but also for monitoring, at a central 

location, remote instruments at key positions in a mine. About half 

way through the originally proposed three year effort, the program 

was redirected to phase out the communication effort at the end of 

two years and to concentrate the program" resources on the investigation 

of new monitorfng system concepts developed by this research. Thus, 

the work reported here is incomplete relative to the originally proposed program. However, the work was carried to the point of providing 

sufficient evidence to aid in the comparison of certain new communication concepts. For the concepts chosen for this study, the work was 

stopped short of hardware development and field testing, 

Several reports in the form of Master's theses and a Doctoral 

Dissertation have resulted from this research. They each report in 

considerable detafl the results of several individual" contributions 

to this program. In preparing this report, an attempt has been made 

to provide a complete description of the significant results but without burdening the reader with the minute details. These more detailed 

reports are referenced heavily for the convenience of those interested. 

PRESENT COMMUNICATION METHODS 

Two types of communication systems are presently used In 

coal mines. They are: 

1, Various forms of telephone systems, and 

2. Carrier current radio systems using the trolley lfne as 

the interconnection circuit. 

Carrier current radio is used almost exclusively for control 

of rail trafflc by the dispatcher. Almost all other communication 

uses the telephone system, Telephone lines generally extend to areas 

where trolley lines are not present such as the outside offices and 

warehouse. Therefore the telephone system provides the primary means 

of voice communication. 

Telephones - Telephone systems employed in American coal mines 

fall into three categories. 

1. Paging type telephone systems - these systems have individual 

stations which use battery powered transistor audio amplifiers to 

boost signal levels for normal communication. When in the page mode, 
a higher powered amplifier is connected to a speaker at each station. 

This type of system generally has all telephones connected in parallel 

throughout the mine yielding a "party-line". In a few large mines, 

the telephone system is split into two to four areas which can be 

patched together when needed. 

A typical schematic for this type of telephone is shown in 

Figure 1. A push-to-talk button in the handset activates the auxiliary 

amplifier. Paging is accomplished by pressing a spring loaded switch 

which applies DC voltage across the telephone line. This activates 

a relay (or electronic switch) in all other stations which applies 

power to the paging amplifier. 

2. Crank ringer systems - for many years, the only communications in coal mines was supplied by the conventional telephone with 

carbon transmitter, dry cell batteries, and crank generator to supply 

AC ringing current. Theywere connected in a party line mode with 

coded long and short rings to identify the station being called. This 

type of system is still used in many coal mines. As coal mlnes grew 

larger, these systems suffered from inadequate signal level. Larger 

mines were forced to break the telephone system up into separate 

areas to malntain adequate signal-to-noise ratios. 

3. - Dial telephone systems - automatic dial systems are used at 

several American coal mines where they operate their own private 

automatic exchange for both inside and outside telephone service, Some 

systems feature the abilfty for key personnel, such as the mine superintendent, to override a busy signal, One system has the abflity to 

put all or any number of phones into a party line mode. These systems 

are maintained completely independent of the local commercial telephone 

system, and no attempt is made to interconnect them. 

The paging telephone system has wide spread use for two reasons. 

First, they permit persons and station areas to be paged by name and 

thus do not require the miners to learn ringing codes or telephone 

numbers. Secondly, use of booster amplifiers makes the system less 

affected by poor line splices and induced noise. It also yields a 

larger sound level at the receiver which is important when in the 

vicinity of large machinery, The primary disadvantage of paging telephone systems is that the telephone line must be used in a "party-line" 

arrangement, This prevents simultaneous conversations in the system and 

reduces its usefulness for "talking out" maintenance problems or other 

uses which can tfe up the system for long periods of time. 

Dial systems provide for many simultaneous conversations but 

do not possess the paging abilfty. Their need to use mufticonductor 

cable discourages use of the dial system because of obvious maintenance 

problems when cable breaks occur from roof falls or accidents with 

machinery. This problem has been minimized by using the multiconductor 

cables only in the main haulage way where few roof falls occur and by 

taking conventional two-wire cable up to each section. However, this 

means all telephones on a section must be on a "party-lfne". 

All the systems presently used in coal mines are inherently 

unreliable. That is, if a telephone line is broken or shorted by a 

roof fall, for example, all telephones beyond that point are severed 

from communication to the outside. If the line is shorted, communications in the entire system may be severely affected or inhibited 

completely. There apparently has been no attempt within the mining 

industry to develop a system employing alternate signal paths in the 

telephone system. 

Carrier Phones - Carrier current communication is used almost exclusively 

in mines with extensive rail haulage for coordinatfon of rail traffic, 

By placing carrier current communicators (called carrier phones) on 

moving vehicles plus units at key stationary points such as the dispatcher, 

dumper station, and outside . Aneffective mobile communfcation system 

can be achieved for distances up to 10 to 15 miles with presently 

available equipment. As long as haulage co-ordination is the main 

function of the carrier phone system, the present single channel arrangement is adequate if properly maintained. However, in mines where 

maintenance and management depend heavily on the carrier phone system 

for routine messages, it appears that a two-channel system would be 

desireable. 

Few people are aware of the potential usefulness of the carrier 

phone system in an emergency if the equipment is battery powered. 

There is a gross lack of understanding about trolley phone systems 

among both engineering and management as well as the rank and file 

miner. Proper care and maintenance of carrier phone equipment is a 

common problem. These same conclusions are true for all types of 

electronic equipment used in and around coal mines. 

Carrier current equipment possesses several operational difffculties. Faremost is the fact that a large haulage motor operating 

along a trolley lfne causes a short circuit to the carrier current signal. 

This effect can be minimized by utilizing the telephone wires as an 

alternate path for the signal to by-pass the short circuit as shown in 

Figure 2. Various configurations of coupling networks have been used and 

are available from manufacturers of carrier phone equipment, They all 

provide the same action of providing coupling of the carrier current 

signal to the telephone lfne from the trolley lfne. This solution works 

well when the coupling boxes are properly located and moved as the need 

arises, ioe7 as the extremities of the trolley lfne circuits change. 

Another problem with carrier current systems in many coal mines 

is the existence of "dead spots" which are caused by "destructive wave 

interference" in the system and can efther be elfminated, or at worst, 

they can be "moved" to places in the system which cause the least 

problem* This is accomplished by properly placing coupling boxes. 

The proper positions for cross coupling between the trolley and telephone 

lines must usually be found by trial and error, 

The manufacturers of paging telephones and carrier phones offer 

equipment to permit interconnection of the two systems. However, when 

talking from the trolley phone, the entire system operates in the page 

mode. This 1s not only annoying but it also causes the telephone 

batteries to discharge faster. Thus, the interconnection of the telephone and trolley phone systems is generally not employed except in 

special circumstances such as weekends when few people are in the mine. 

Carrier current equipment is also used for control and other 

voice communication applications in several coal mines, The main use 

is for control and voice communications from cages and elevators in 

vertical and slope shafts, In a few mines, the technique is also 

employed to provide communication to the battery powered, rubber-tired 

vehicles. In both cases, a "messenger wire" is used in place of a 

trolley line and the signal is transferred to and from the vehicle 

via a tuned loop antenna mounted on the vehicle. The need for this 

type of service appears to be rather lfmited, 

Outside Communications - Integral to the operation of an efficient mining 

operation is an effective communication system for activities outside 

the mine workings, Many mines have their own private dial exchange which 

may or may not be compatible to the local telephone sytem. Many mines 

now employ two-way VHF radio systems to provide mobile communications 

to the outsfde foreman, and maintenance crews, Very little fntegration of the outside and insfde systems has been attempted. Those mines 

which use inside dial phones also utilize the same exchange for outside phones and thus achieve a fully integrated systemo The general 

policy is to place paging telephones and trolley phones only at those 

outside points where someone is readily available to receive and relay 

the messages. Almost invariable this fs the lamphouse since someone 

is always on duty there, 
RESEARCH PROGRAM JUTIONALE 

As the meetings with industry progressed and the conclusions 

from our findings began to take shape, it became apparent that research 

should exist in four areas. 

1. Through-the-Earth Communfcations - From the beginning of 

this program it was apparent that the feasibility of some form of 

direct through-the-earth communication for routine uses must be 

investigated. Whereas the Mine Rescue and Survival System program 

(Ref, 1) emphasized communication with entrapped miners between inside 

and outside directly through the overburden, the emphasis of this 

work was placed onsignalling between two inside points. Some work was 

done on signalling from the outside to the inside for paging system 

applications, The unique aspect of this work was the fact that the 

theoretical and experimental analysis techniques were formulated in 

a way to be most beneficial to the communications system engineer. 

This also required that noise data be obtained. 

2. Paging System Design - Early in this program it was obvious 

that the weakest link in the mine communication system is its ability 

to alert men during an emergency in the face area and in other areas 

of the mine not sufficiently close to the telephone to know when they 

are being called. This is not only important to individual miner 

safety but also to the efficient operation of a mine. In view of the 

limited range which is inherent in any through-the-earth system, 

and numerous discussions with mine personnel it was concluded that rhe 

availability of some form of wireless personal paging system would 

provide the best solution to this problem. Ultilizing the early 

findings of the through-the-earth communication studies, a unique paging 

receiver design was investigated. Eventually the results of the 

through-the-earth studies were applied to design of an optfmum paging 

system. 

3. Improvement -- of Wired Telephone Systems - Even with the 

possibility of wireless communication methods, it is evident that 

wfred telephone systems will continue to be the primary system for 

routine mine communication. In view of the investment in communication 

equipment already in coal mines, a solution using the conversion of 

present equipment appeared the most viable. Thus manpower was devoted 

to the investigation of ways mine telephone systems can be ~mproved. 

4. Carrier Current Systems - Although the use of carrier 

current on trolley systems is generally adequate, additional studies 

were deemed necessary in two areas* First was the potential usefulness of carrier phones during emergencies by using through-theearth radfation? This was to be covered primarily by the through-theearth communication studfes utilizfng data taken in the carrier phone 

frequency range (100 KHz), Secondly was the study of using carrier 

current signalling over high voltage power cables. 
Consideration was given to other aspects of the communfcation 

system but did not involve the establishment of a research effort. 

Such were the use of "walkie-talkie" or hand held communication devices, 

the use of the two-way radio outside the mine, and incorporation of 

the local public telephone system into the mine system. 
THROUGH-THE-EARTH COMMUNICATION 

The possibility of signalling "through-the-earth" is not new 

to the scientific communfty. Theoretical analyses applfcable to the 

problem date to Sommerfeld in 1909 (Ref. 2) and experiments were conducted as early as 1928 (Ref 3) with communication applications in 

mind, Despite this early knowledge and a continuing experimental 

effort by the U. S. Bureau of Mines for about 20 years (Ref, 4), it 

was concluded that wireless communication in coal mines was not 

practicalo However, these conclusions were based on the technology 

of the date and they no longer hold because of the advent of solidstate electronic devices and more sophisticated filtering and signal 

detection techniques. 

Until recently, analytical and experimental programs involving 

"through-the-earth" propagation have been concerned primarily with the 

study of the geophysical properties of the earth and there was no 

attempt to conduct a systems analysis of a "through-the-earth" 

communication link, The work reported here was an attempt to bring 

together the necessary theory and experimental data to optimize the 

design of a practical "through-the-earth" communication system for 

use in an operating coal mine, The design of a paging system whfch 

evolved from this effort is described briefly. 

GENERAL PROBLEM 

Thls project was limited to the design of a communication system 

for use In an operating coal mine as well as having application during 

emergencies, It was not to be an "emergency only" communication 

system, With this constraint and recognizing the needs of the mining 

industry, the link configurations analyzed were limited to two sifuations: (1) between two points within a mine, and (2) between a 

surface transmitter and an underground receiver. Although, several 

different types of transmitting and receiving antennas can be used, 

lt was decided to limit this initial endeavor to the use of tuned loop 

antennas for two reasons, First, when portability IS desired che 

loop antenna is most practical, and second, the tuned loop is theoretlcally the best understood of the possible candidates. 

Two arrangements of transmitting and receiving antennas were 

also analyzed as shown in Figure 3. These are described by their axial 

orientations, iree horizontal magnetic dipole (coaxial) where both 

loops have colinear horizontal axes, and vertlcal magnetic dipole 

(coplanar) where both loops have vertical axes and the loops are coplanar, 

Since the important quantity in communfcation systems is the 

available power at the output of the receiving antenna in relation to 

the transmitter power, the various arrangements described above were 

analyzed both analytically and experimentally by determining the power 

transfer between two loop antennaso These results are developed in 

detail in Reference 5 and will only be summarized here. 

Since radio waves are attenuated rapidly in the earth, it 

appears reasonable to assume that for an inside-to-inside link in a 

deep mine, the transmission medium could be assumed to be of infinite 

extent, Moreover, although the conductivity of the earth in the 

vicinity of the mine may not be constant, calculations would be greatly 

simplified, if an average conductivity could be utilized and the 

medium treated as having a homogeneous conductivityc Thus the first 

part of the research program involved the study of power transfer 

between two tuned loops in an infinite homogeneous conductive medium 

in an attempt to determine if these assumptions would be valid and to 

study how system performance would depend on various parameters* 

Power Transfer Between Coplanar Loops 

At lower frequencies, where stray capacative reactance may 

be ignored, the loop antenna may be represented by an inductance in 

series with a resistance, as shown in Figure 4, 

L is the reactive component of the transmitting antenna, 1 which is inductive at low frequencies. R: is the antenna resistance 

which includes radiation resistance plus copper losses of the loop, 

The input impedance may be made resistive, and equal to RI, by correct 

selection of an external capacitance, Cl , placed in series with the 

antenna. 

The receiving antenna is represented by a voltage source, V2, 

in series with a resistance, R2 , and an inductance, L2. Similar to 

the transmitting antenna these impedances represent the resistive and 

reactive components of the receiving antenna. In order to obtain 

maximum transfer of received power to the receiver, the receiver impedance must be conjagate matched to that of the antenna, The receiver 

is represented by a capacitance, C2, in series wich the resistive 

The capacitive reactance of C2 negates the inductive react- :::: %&, and R is equal to R2, The maximum output power obtainable 

1s : 

L 

The transmftter source impedance will not be included in the 

power equations. Only the power delivered to the antenna will be 

considered. This exclusion will allow for a transmftter with a nonlinear output impedance to be used to drive the antenna, High 

transmitter efficiency may be obtained by tuning the antenna (varying 

C1) to the fundamental frequency of a square wave produced by a 

switching type of transmitter. 

The power delivered to the transmitting antenna is: 

Utilizing these equations and the relationship developed 

between V2 and Py from known theory, reference 5 shows this yields 

the power transfer equation for coplanar loops: 

where : 

w signalling frequency in radfan/second 

A antenna area 

N number of turns in antenna 

R total resistance of antenna 

r distance between antennas 

6 skin depth of earth at operating frequency 

Although this equation gives the power transferred between two 

loops in a lossy medium it is not applicable without knowledge of the 

antenna resistance. 
Radiation Resistance --- of a Small Loop Located -- in a Spherical Cavity - As 

mentioned previously, the reactive impedance of a small loop will appear 

inductive at the very low frequencies of interest. In addition to the 

reactive component as a result of the finite resistance to current 

in the loop (copper losses) plus the resistance associated with the 

radiated power (radiation resistance). 

The copper losses of a loop antenna is determined by the size 

and length of the wire used in its construction. The total resistance 

due to copper losses becomes 

where 

N = number of turns 

a = loop radius (meters) 

Rw = wire resistance in ohms per meter 

of length 

The equation 

is shown in Reference 5 to be an expression of the radiation resistance 

of a small loop located at the center of a spherical cavity of radius 

b within an infinite homogeneous lossy medium of skin depth 6. The 

only restriction on the equation is that the cavity radius must be 

small compared to a wavelength. The method used to develop this 

equation was to calculate the total power dissipated within the medium 

and equate that to radiation resistance. 

The radiation resistance of a ten turn loop of one square 

meter area located in a 1 meter radius spherical cavity within a lossy 

medium is shown in Figure 5. 

The total antenna resistance is the sum of R, and R, and 

equal to: 

Figure 5 shows that at the lower frequencies (less than 50 to 

100 Kilohertz) the copper losses are much greater than the loss due to 

radiation resistance. Thus it is concluded that the antenna resistance 

may be approximated by the copper losses. 

Maximum Transfer of Power Between Coplanar Loops 

This section is directed at finding the frequency at which the 

maximum transfer of power (minimum loss) occurs in the coupling between 

loops. Combining equations (3) and (6) and using the conclusion that 

the radiation resistance is negligible we find: 

This equation can be put into a form suitable for maximization 

by replacing the skin depth by its equivalent expression and grouping 

terms that include frequency to yield 

where 

Maximizing the expression within the brackets as a function of 

frequency will maximize the power transfer independent of the parameters not within the bracket. Substituting x2 = f and designating 
the expression within the brackets as g(x), the maximum power transfer 

occurs at 

d Taking the derivating, - . dx 

- 2 Yx 

g'(x) = 4 x3[- ~(Yx)~ + ~(YX)~ + ~(Yx)~ + 2(Yx) + l]e (101 

Setting equation (10) to zero, real solutions occur at 

x = 0, a, and when: 

The solution at x = 0 and a are readfly seen to be minimums 

while the solution to equation (11) is the maximum power transfer, 

The only real solution to this equation occurs at approximately 

Yx A 3.86. 

By designating the frequency at which quation (11) is satisfied 

as f and substituting x2 = f and Y = (~po)~~~ r yields the frequency opt at whnch maximum power transfer occurs for coplanar loops as 

Substituting p = 4~ x henrys/meter simplifies the 

expression 
Equation (13) shows that the optimum frequency of transmission 

between two coplanar loops is independent of any of the loop parameters 

(assuming Rc 2, Rr) and is dependent only upon the distance between 

loops and conductivity. Substitution of equation (13) into the skin 

depth equation yields the skin depth at the optimum frequency as 

By replacing 6 by tiopt and f by fopt in equation (7) the 

maximum transfer of power for coplanar antennas is found to be 

Maximum power transfer is dependent on the inverse of range 

to the tenth power. Doubling the range creates an additional 30 dB 

loss in power compared to the normal 6 dB loss in free space. The 

frequency at which maximum power is obtained is given in equation (13). 

The relationship in equation (13) can also be useful in 

determining the effective conductivity of the earth in the vicinity 

of the antennas by measuring the power transfer between two underground coplanar loops, observing the peak value and calculating the 

conductivity from equation (13) . 

Figures 6, 7, and 8 are power transfer curves derived from 

equation(7)forstandard antennas defined by: al = a2 = 1 meter, 

N1 = Np = 1, and Rw = R = 0.01 ohmlmeter (15-gauge copper wire). 1 9 

Figure 9 is a graph of the maximum power transfer versus range 

at the optimum frequency at each range. Figure 10 plots the optimum 

frequency versus range. 

The equations in this section are adequate for use in analyzing 

the transmission parameters for a communication system in which the 

transmitter or transmitters are located generally at equal depths and 

have loop antennas orientbd as vertical magnetic dipoles. 

- 

*During the preparation of this report ft was discovered that the 

solution to equation (11) is more accurately given by Yx = 3.85 which 

yields the numerator in equation (13) more correctly as 3.75 x lo6. 

However 3.78 x lo6 is used throughout this report. 





Power Transfer Between Coaxial Loops 

Beginning with the equivalent circuit used in the previous 

development for coplanar loops as shown in Figure 4, one can derive 

the relationship between V2 and I1 from known theory for coaxial loops. 

Reference 5 gives the details of this development and shows that the 

ratio of power received to the power transmitted is 

where it is again assumed that the radiation resistance is insignificant as compared to copper losses in the antennas as discussed in a 

previous section. 

Equation (16) can be analyzed in the same manner as equation (8) 

to yield the following coaxial loop optimum frequency properties: 

A comparison of the properties of coplanar and coaxial loops 

is shown in Figure 11 and Table I. From these it is seen that when 

operating at the optimum frequency for each antenna orientation, the 

coplanar orientation shows a 7.2 dB advantage over the coaxial orientation and the optimum frequency for the coplanar orientation is higher. 

Since noise usually decreases with increasing frequency, the coplanar , 

orientation holds a double advantage over the coaxial orientation when 

operating in the vicinity of their optimum frequencies. Coplanar 

loops possess the advantage of producing an omni-directional pattern 

in the plane of the antenna. Due to a11 these properties a practical 

system will be assumed to be designed with coplanar loops. 

TABLE I 

Comparison of Loop Orientations 

Power Transfer Between Loops -- in a Half-Space 

Communication between two points inside a mine will be in a 

half-space lossy medium rather than the space of infinite extent as 

assumed in the preceding sections. Although it appears reasonable 

that in a deep mine the infinite medium assumption would be accurate, 

it is important to understand the extent to which this assumption is 

acceptable. This may be particularly important for propagation behavior 

for shallow mine applications. 

In order to solve this problem, Layman adapted a procedure 

developed by Wait (Ref. 6) for analyzing the field radiated from a 

loop above a conducting half-space. Wait's procedure decomposed the 

total wave into two parts -- the primary wave or that wave which would 

be radiated in the absence of the conducting half-sp.ace; and the 

secondary wave which resulted from reflections from the half-space 

boundary. 

In Reference 5 Layman adapts this procedure by considering the 

situation shown in Figure 12, wherein the field within the conductive 

medium is decomposed into the primary and secondary components, He 

presents a complete derivation which shows that the magnetic Hertz 

potential inside the homogeneous, conductive half-space is given by 

where R = [(z - zo)2 + p2]1/2 

I = current in transmitting antenna 

A = area of transmitting antenna 

N = number of turns in transmitting antenna 

y1 = propagation constant of wave in conductive medium 

yo = propagation constant of wave in free space 

zo = depth of transmitting antenna 

z = depth of observation point 

p = distance between transmitting loop axis and observation 

point 

The first term in equation (20) describes the primary field 

while the remaining term describes the secondary. In order to 

study the significance of the secondary field, equation (20) is 

normalized by setting the N1A/4r term equal to unity. Maxwell's 

equations are then used to find the resulting magnetic field intensity 

which has only a z component due to the cylinderical symmetry. The 

primary field yields 

where 

e = arc cos f ; 20) 
Based on these two equations the power transfer between two 

loops buried at various depths in a homogeneous conductive half-space 

was calculated using digital programming to perform the integration 

in equation (22). These results are shown in Figures 13 - 15 for 

different conductivities for both the complete field at various depths 

and also for the primary ffeld only. As expected, as the antennas 

are placed deeper in the medium, the primary field approximation 

becomes more accurate. However, it is also shown that in the vicinity 

of the optimum operating frequency, as determined by the primary 

field approximation, the primary field is essentially equal to the 

exact solution even for very shallow depths, The large errors occur 

for frequencies much higher than fqpt where analysis of the equations 

show propagation is taking place via free space along the boundary. 

These analyses show that when antenna separatfon is less than approximately four times the depth, the primary field approximation is 

excellent. Even with larger separations the approximation is still 

good when operating at frequencies in the vicinity of f and below. opt 

The interesting point raised by this analysis is the possibility 

of an operating frequency more optimum than that found from the primary 

field approximation. This possibility is vividly demonstrated in 

Figure 15 in a low conductivity situation. Caution should be exercised 

fn interrupting these results at the higher frequencies because of the 

assumptions made early in the analysis which neglects radiation 

re.sistance and assumes that B .> WE= However, it appears reasonable 

that in some shallow mine situations where communication distances 

much greater than mine depth may cause the optimum operating frequency 

to be much larger than would otherwise be anticipated because of the 

outside propagation path. 

Outside Inside Transmission 

The results of the preceding station sheds some light on the 

problem of placing the transmitting antenna outside the mine rather 

than inside. Consider a comparison of the two situations shown In 

Figure 16. It is evident that the primary wave will always have a 

longer range for the surface antenna case, and thus one would expect 

poorer results. However, the secondary wave due to outside propaga- 
$4 

a 

J-J 

g 

--. 

0 

s 

€i 



tion may suffer less attention since it is launched outside the mine. 

Thus as the desired radial range is made larger and R1 and R become z nearly equal, it should not be surprising to find superior slgnal 

strengths arising from the outside transmitting antenna because of the 

larger secondary wave. 

From the data of the preceding section, the secondary wave 

was found to provide significant contributions only when the desired 

radial range was larger than about four times the mine depth. Therefore an outside transmitting antenna would probably provide larger 

field strengths than an inside antenna only in the cases of very 

shallow mines or in areas of extremely low earth conductivity. Of 

course other factors would enter into the choice of transmitting 

location. One practical problem is the fact that the earth consists of 

layers which may have widely varying conductivities. A band of high 

conductivity in a mine overburden might prevent outside to inside 

transmission whereas the earth in the vicinity of the coal seam may 

be rather low in conductivity and thus permit inside to inside communication. 

Even if one can assume the earth is homogeneous, the use of 

an outside transmitting antenna might offer some signal strength 

advantages only if the mine depth and earth conductivity were such 

that practical communication ranges were on the order of three to four 

times the mine depth. The above statement is made on the assumption 

that the inside and outside antennas and transmitters are effectively 

the same. If one permits the possibility of a larger antenna and 

transmitter outside, the outside transmitting system becomes attractive 

for a wider range of conditions. 

Com~arison of Ex~erimental and Theoretical Data 

In order to determine the actual transmission characteristics 

of the earth near coal seams, experiments were conducted in several 

coal mines in West Virginia and Pennsylvania. By comparing data 

acquired from these measurements and that obtained from the theoretical 

analysis, it was hoped to determine the extent to which the homogeneous 

approximation is legitLmate, the earth's conductivity in the vicinity 

of coal seams, and how variations in the earth's structure affect low 

frequency transmission over short distances. 

A block diagram of the test equipment used is given in Figure 

A brief description of the test equipment follows: 

Transmitter - variable frequency, crystal controlled, 

switching type, maximum power output of 

10 watts. 

Resonating Capacitance - capacitor bank, variable from 

1000 pic0 farads to 10 micro-farads. 

Transmitting Loop - 30 turn, loose wound, (1 turn per 

inch) 1 square meter area, 3 ohms d.c. 

resistance (Rw = 0.028 ohmlmeter), 

L = 22 millihenrys. 

Receiving Loop - 50 turn, (close wound) 1.3 square meter 

area, 20 ohms d.c. resistance (Rw = 0.1 

ohmlmeter) . 

Amplifier - 40 dB gain, 13 megohm input impedance, 

8 ohms output impedance. 

voltmeter - Philco Frequency Selective Voltmeter, 

Model 127 C, 300 Hz Bandwidth. 

The transmitter delivered a square wave to the resonating 

capacitor and the transmitting loop. This circuit was tuned to the 

fundamental frequency of the square wave where it exhibited an 

impedance of 3 ohms resistive. The power dissipated by the harmonics 

contained in the square wave was insiginficant considering their 

amplitude and the input impedance of the resonating capacitor in 

series with the loop at those frequencies. The open circuit voltage 

of the receiving antennacqwas measured and the available power calculated. 

The entire test setup was calibrated by the Standard Field 

Method (Reference 7) except that both transmitting and receiving loops 

were calibrated simultaneously. 

Surface conductivity measurements were made according to a 

method described by Williams and Benning (Reference 8) using two loop 

antennas at the surface. 

Table I1 lists the name and location of the mines that were 

tested, the measured surface conductivities and the conductivities 

obtained by observing the measured data for fopt and applying 

equation (12). 
Table I1 

licasured Conducti.~iz<cs at >!ir,i.ng Locations 

Xf r.e Depth Conductivities rn 

iest 3~2, Fl in e Lccss t ion (nc ter) yy .: ------------.. - -------------- Sur. {mha3/~ii) ~lee (

2 9.S. St2el, No. 50 ?ir:evil.le, I?. Va. L'+ Q Note .2 1.. 46 :c 

2 a U.S. Steel, Xc. SO Flnevlile, W. Va. 40 3skc a 1.09 x 

4 aa@ger No. 14 Phillfpi, W. Va. 135 z.,F; x 7 9-2 :),49 ?2

3 

d 336g3r No. 14 Philliyi, W. Bas 135 2.5 ;i 10'" ~~~~e 

7 U.S. Bur. of 12incs Eluef ield, Petln . 1.1--27 1.4 x I 9-2 2.4 x 

3 'I.over idge Fairvic~l;, W. Ve. 1.1 x 30-" Note 
Figure 18 shows a comparison of the calculated power transfer 

between standard loops at 2 KHz and those measured at the mines given 

in Table 11. This curve was obtained by evaluating equation (7). 

Figures 19 through 25 contain the measured power transfer 

points made at the mines outlined in Table 11. The solid line is the 

infinite homogeneous power transfer approximation. Each curve is 

offset by the value as shown in Figure 18 to allow for correction of 

system calibration errors. The conductivities within the mine were 

calculated by making use of the range, fopt, and equation (12). 

These calculated conductivities are shown in Table I1 along with the 

conductivities measured at the surface. 

One great irregularity appears in the data contained in 

Figure 23 taken at the Badger No. 14 mine. The peak power transfer 

point, fopt, did not occur within the range of frequencies that measurements were made. A second factor, as seen in Figure 18, is that the 

power transfer at 2 KHz is 22 dB greater than calculated. One possible 

explanation for this condition could be that coupling occurred from the 

transmitting antenna to a metal object such as the rails or power lines 

which carried the signal along its length and retransmitted it near the 

receiving antenna. However, there is evidence there may be some 

special characteristic of the overburden as will be discussed later. 

With the exception of the data presented in Figure 23, all 

other sets of data agree in general with the infinite homogeneous 

medium approximation curve shown on each graph. This data supports the 

hypothesis that the infinite homogeneous medium case is a good 

approximate model for the analysis of electromagnetic communication in 

most mines. The data points, in general, fall near the predicted 

curve on the graphs. Some isolated data points are off the curve a 

significant amount. Many factors could contribute to errors in the 

operation of the test equipment and in making the measurement readings. 

Although care was taken to determine the range, some latitude for error 

was present. Signal coupling onto wiring within the mines could have 

some effect, particularly at the higher frequencies, but care was 

taken in all cases to stay at least 100 feet away from wiring, 

pipes, etc. 

The conductivity data presented in Table 11 indicate that 

conductivity measured at the surface using the two loop method may 

not be an accurate indicatfon of the effective conductivity experienced 

within the mine. 

The experimental data presented thus far are only for the 

case of communication betdeen two points inside a mine. Some data were 

also taken by transmitting from outside to insid.e the mine. These 

data are tabulated in Appendix A along with the data used to establish 

the curves already presented, It should be noted that the outside to 

inside data behaved similarily to that expected for an infinite homo- 








geneous medium in that an optimum frequency existed. However, the 

use of Equation 12 tended to yield unusually large conductivities. 

Unfortunately sufficient time was not avaflable to fully analyze 

the outside to inside data relative to the theory developed herein. 

The outside to inside tests did yield some interesting results 

which should be documented. Of the mines where experiments were 

attempted, the mine whose roof was dripping with water (U. S. Steel, 

No. 50), yielded some of the best data. On the other hand, the mine 

which was completely dry with no drainage from the roof (Badger No. 14) 

absolutely no signal could be received over a dfstance no greater 

than those which were successful at other mines. Note that Badger No. 14 

also yielded unusual data for the inside to inside experiments. 

Noise Measurements 

The successful operation of communication systems depend on 

the existence of an adequate ratio of signal to noise level at the 

receiver. Thus, knowledge of the earth's transmfssion characteristics 

alone is not sufficient to determine the feasibility of through-theearth communication application, The magnitude of nofse which must 

be tolerated at the receiver must also be known. Since the noise 

generated by the first amplifying stages in a receiver may be made 

very small by proper design, the noise emitted by electrical circuits 

or from other sources outside the receiver becomes the noise which 

must be overcome by the signal. 

Since coal mines contain many electrical circuits of varfous 

forms it is to be expected that noise will exist inside an operating 

mine. Since the objective of this work was to study the feasibility 

of through-the-earth communication for normal operation applications, 

measurement of noise in operating coal mines was conducted, An 

attempt was made to take data in a varfety of situations with the 

objective of determining the order of magnitude of noise as a function 

of frequency. No attempt was made to determine the exact cause of 

source of the nofse measured. 

Data were taken with the receiver arrangement shown in Figure 17 

and described in that section. The tunable voltmeter had a bandwidth 

of 300 Hz. The antenna used was found to possess a self-resonance 

in the vicinity of 50 KHz. A gain correction factor was determined 

by calibrating the 50 turn loop against a one turn loop of similar 

dimensions. Attempts were made to correct all data according to 

this calibration information, but in some cases this yielded erratic 

data points which were neglected. 

Data were taken with the receiving loop axis vertical and horfzontal and in the vicinity of various types of electrical wiring and 

machinery at the following types of locations: 
Near AC and DC power centers 

Near HV and LV AC power cables 

Near DC trolley lines 

Places isolated from electrical cables and 

equipment 

Outside the mine 

The following general conclusions were reached from these 

tests: 

1. In general, the worst noise generator in an operating 

mine is the DC trolley line system. The trolley line 

acting with the grounded rail return acts as a large 

transmitting loop antenna. The current fed into this 

radiating system is rich in harmonics of 360 Hz on up 

to the 15 to 20 KHz region. On main line haulage 

circuits, current surges are common which drastically 

increase the noise. This type of "rectifier noise" 

can be detected anywhere in and around a mine which 

uses track haulage. The tests indicate that the 

magnitude of the noise decreases as the distance from 

the trolley line increases. 

Noise radiated by HV shielded AC cables attenuates 

rapidly to a very low level within about 10 to 15 

feet (i.e. 10 dB in the first foot, 5 dB in the 

second foot). Mines using all AC and DC cables 

(i.e. no trolley lines) were found to exhibit no 

measurable noise over most of the spectrum in the 

next entry adjacent power carrying cables. Noise 

from unshielded Low Voltage cables was about 10 dB 

higher than for the High Voltage cables. 

3. The orientation of the receiving loop can have a large 

affect on received noise. With the plane of the loop 

horizontal, the noise was 10 to 15 dB larger than when 

vertical. This was true in mines with trolley lines and 

mines with only power cables. However, in an AC mine 

away from power circuits, there was little difference 

in noise with loop orientation. 

Figures 26 and 27 show plots of the maximum and minimum 

measurements made at each frequency from all the tests taken inside 

operating coal mines. The noise data is also presented in tabular 


form in Appendix B for several test sites. The data taken within a 

few inches or feet of the cables and power center may not be accurate 

since a large loop was being employed. Thus in areas where the 

electric ffeld is not constant over the loop, voltage due to the 

electric field may have been induced. 

System Optimization 

With the establishment of the earth's transmission characteristics and typical noise spectra to be encountered in mines, sufficient 

information is available to proceed with the investigation of the 

optimum operating frequency for a through-the-earth communication 

system. Since it is apparent from the data presented in the previous 

sections that the optimum frequency will depend on several factors 

which can be determined only by the application involved, the purpose 

of this section is to discuss those parameters which must be determined and the procedure which can be used to find the optimum frequency. In a succeeding section on the paging system design this 

procedure will be applied to an example situation. 

Receiving Antenna Design -- it is well known that low frequency 

amplifiers can be built with noise figures less than 10 dB using 

designs that can be mass produced. Thus it is assumed that although 

amplifier noise may be present its effect can be made insignificant 

if the receiving antenna is designed with sufficient effective area. 

That is, due to the existence of noise fields from mining machinery 

and natural sources it is possible to make the noise voltage appearing 

at the antenna terminals much larger than the amplifier noise. Once 

this situation has been achieved, any further increase in receiver 

antenna gain will be of no advantage since the induced voltage due 

to signal and that due to noise would increase at the same rate 

yielding no further increase in the signal to noise ratio. 

The receiving antenna design then would be determined by the 

minimum amount of external noise to be experienced at the frequency 

of operation and the receiver noise figure. A good rule of thumb 

might be to require the receiving antenna effective area and number 

of turns to be that required to yield the induced noise voltage to 

be ten times the amplifier noise voltage referred to the input. 

Receiver Bandwidth -- as in the design of any communication 

receiver its bandwidth must be sufficiently wide to pass the information but yet be as narrow as possible to minimize the input noise. 

This optimization of receiver bandwidth depends on the type of 

modulation used. It should be pointed out that most classical forms 

of filter optimization in the literature are built around a "white" 

gaussian input noise whereas in coal mfning applications this assumption would be very incorrect. In particular the existence of 60 Hz 

harmonics in the spectral region below 25 KHz can be very significant 

and be the dominate system noise to be combated. Thus the optimization 

procedure may not be of the classical form. 
Threshold Level -- assuming the receiver must be capable of 

operating in a noise only environment (i.e. no received signal present) 

without producing erroneous outputs, some form of threshold or 

squelch circuit must be used. For example, in a signalling system a 

maximum allowable false alarm rate will be specified which would 

establish the required threshold level depending on the magnitude 

and statistical characteristics of the noise at the receiver output. 

The important point is that the threshold level must be set on the 

basis of the noise environment without consideration of the signal 

level to be present. 

Received Signal Level -- the desired probability of detection 

can now be used in conjunction with the threshold level to establfsh 

the required signal level. That is, the received signal must be 

sufficiently larger than the threshold level to produce the required 

probability of detection for the noise statistics involved, This 

level can be referred to the receiver input to determine the signal 

level required at the receiving antenna output for successful system 

operation. 

Frequency Optimization -- the design parameters that remain 

to be chosen are the transmitter power, the transmitting antenna 

desfgn, operating frequency, and the maximum possible range between 

the transmitter and receiver. Although these can be traded off it 

is assumed here that since range is severely limited, the transmitter power and antenna gain design have beeti chosen as large as 

practical. In this case, the problem is reduced to determining system 

operating frequency that will yield the maximum possible range and 

still permit reception of an adequate signal level. The following 

iterative procedure can be used: 

1. Arbitrarily select an operating frequency. 

2. Assuming this frequency is optimum and using desfgn 

parameters already established, solve equation (15) 

for range. 

3, From the known conductivity and the range found in 

step 2, use equation (13) to find fopt* 

4. Compare fopt found in step 3 to that used in step 2. 

If not equal, choose a new frequency and repeat 

steps 2 and 3. Continue until equality is established. 

5. Select the exact final frequency based on practical 

considerations. 

6. If the final frequency is near the optimum frequency, 

equation (15) can be used to yield the approximate 

maxfmum range that can be expected in the optimized 

system. 
Since the resulting maximum range may not be large enough to 

cover an entire mine, the use of antenna arrays and phase synchronized 

transmitters may have to be utilized. Work on this idea was terminated 

before an adequate study of this problem could be undertaken. Some 

preliminary results are presented in reference 5. 

The use of this optimization procedure in example situations 

had led to the following conclusions: 

1. Even with an optimized system using a small [relative 

to one wavelength) transmitting loop more than one transmitting antenna 

will be required to completely cover most coal mines. 

2. The use of other transmitting antenna configurations such 

as grounded rods should be studied for systems where the transmitter 

is located outside the mine. 

3. Signalling systems designed to operate in a designated 

area of the mine, such as one section, are practical with radial 

ranges in excess of 400 to 500 feet using transmitting loops. 
PERSONAL PAGING SYSTEM DESIGN 

The usefulness of personal paging receivers in various 

business and industrial situations has been proven through many 

different applications over the past few years. Various designs using 

VLF, VHF, and UHF frequencfes have been employed depending on the 

practicular operating environment. Very low frequencies have been 

used in and around metal buildings to take advantage of the propertles 

of magnetic induction, 

fnd 

of 

The coal mine application is similar in many ways to other 

ustrial uses in and around metal buildings. That is, the presence 

the earth attenuates the propagation of the electromagnetic field, 

However, as was shown in the preceding section of this report it is 

possible to achieve ranges of several hundred feed between two antennas 

placed underground if the operating frequency is properly chosen. 

In view of the potential application of the personal paging concept 

to extend the calling range of the telephone system into important 

areas such as the mining section, it was decided to study one way a 

personal paging system might be designed in order to achieve the 

maximum possible range and to operate in the noise environment normally 

encountered in and around operating mining machinery. 

It should be pointed out here that this approach is not 

necessarily considered to be optimum for all mine applications, It 

is hoped the knowledge and experience reported here will add to the 

overall program of research where other approaches may have been 

Two factors motivated the study of the autocorrelation detector 

for this application. First, due to the severe attenuation of signals 

by the earth it was desired to utflize a detector which could achieve 

extreme sensitivities. Secondly, due to the presence of large 

amplitude harmonics of 60 Hz in many operating mines a detector must 

be used which can attenuate or cancel their effect when operating at 

low frequencies. Since longer ranges have been shown to requfre 

a lower operating frequency for optimum results, it is expected that 

most practical applicatfons would requfre operating frequencies in the 

region where 60 Hz harmonics can be severe. 
This section summarizes the results of a study which is 

reported in considerable detail in references 9 and 10. The theory 

of operation, circuit design consideration, and results of laboratory 

testing of prototype circuits are summarized herein. 

Summary of the Pertinent Aspects - of Autocorrelation -- in binary 

communications, which can be used in a paging system, the receiver 

is required only to make a decision as to whether or not a signal is 

present in its particular channel. Since it is the presence and not 

the shape of the signal that is to be detected, receivers can merely 

rely on the differences in strengths between the signal and the noise 

to make this decision. To enhance this decision-making process conventional receivers use filtering techniques to improve the signalto-noise ratio. The minimum detectable signal (MDS) is then governed 

primarily by the economics and practicability of obtaining a very 

narrow noise bandwidth. 

Detection using autocorrelation techniques, however, relies 

on the differences between the statistics of signal and noise to aid 

its decision-making process. Signal-to-noise ratio improvements which 

are possible with autocorrelation lower the MDS to levels otherwise 

impractical to implement using conventional filtering techniques. 

The autocorrelation function (ACF) of the signal plus noise 

input to a paging receiver in a typical mine environment may be 

represented as: 

where 

RS+,(~) = ACF of composite signal plus noise input 

Rs(~) = ACF of signal 

R,(T) = ACF of random noise 

Rh(') = ACF of the interferring 60 Hz harmonics 

The second and third terms on the right hand side are undesired and 

constitute the noise to be combated. The Rs(~) and Rh(.r) terms both 

originate from periodic sources and thus are likewise periodic while 

the R,(T) term is caused by the random noise. Signal-to-noise 

enhancement in this new domain may be realized by: 
1. Choosing r to be near an integral multiple of the slgnal 

period to increase the effect of R,(T) while 

2. Choosing r large enough so that the Rr(r) terms is quite 

small while 

3. Choosing r such that Rh(r) is negligible. 

It will be seen in this report that these three options can be 

simultaneously realized. 

Auto-Correlation Detector Design -- Figure 28 shows a block diagram 

of one approach to realizing an autocorrelation type detector, A 

crystal controlled oscillator and countdown chain provide a time 

base consisting of two complementary outputs spaced T seconds apart, 

These outputs are followed by one-shots which generate 0,5 psec 

sample command pulses for the analog gates. The input signal 

(a sinusoid) which may be contaminated with noise is preconditioned 

by a bandpass pre-filter and applied simultaneously to both analog 

gates. The input is then sampled alternately by each gate with a 

timing difference of r seconds. The sampled input from each gate is 

"held" by the hold amplifier until the next sample arrives 2r seconds 

later. The output from the hold amplifier then changes quickly to 

its new sampled value. The outputs from both hold amplifiers are 

applied to the two inputs of an analog multiplier which continuously 

forms the product of the two sampled values. The multplier is followed 

by an integrator which continually time averages the output, 

If the input consists of only noise and if T IS chosen 

long enough so that successively sampled noise values are independent, 

then the products coming from the multiplfer are as equally likely 

to occur positive as they are negative, and the average value is 

zero, The noise is truly uncorrelated only for T approaching infinity 

and so a small mean or dc component will be obtained from the integrator even without a signal input. Furthermore, since the averaging 

period is not infinite the integrator output also contains a variance 

term with a noise-only input which must too be considered. 

If a sinusoidal signal of proper frequency is applied to the 

input, the outputs from the multiplier will all be of like sign and 

the output from the integrator will continually build up. The tnme 

constant of the integrator is easily varied to give it sufficient 11 memory" to look at an input for several seconds duration 

The integrator is followed by a threshold detector which 

continually monitors the averaged output, If, at any time, the integrator output builds up to a pre-specified value a comparator will 

change state and reset the integrator. After an adjustable length 

of time the reset is removed and the integrator is once again able 

to build up (hopefully, only from an input signal), The reset-hold 

feature is necessary in order to keep the integrator from responding 

more than once to the same signal pulse in cases of high input signalto-noise ratios. 

As illustrated in Figure 29, one channel of the receiver 

samples the incoming signal plus noise at time xl and holds the sampled 

value until several cycles of signal later at x2 when the next sample 

is obtained. The sampled value obtafned from the second channel at y, 

is then multiplied with XI and the product entered into the integrator, 

The sampled value yl is similarly held until the first. The number 

of products averaged is dependent on the time constant of the integrator 

and the rate at which the products are being received. The approximate 

mathematical operation performed by the detector is then an estimation 

of the autocorrelation function for a particular r and is expressed as 

where N is the number of products averaged. 

Actually, equation (24) is the arithmetic mean of samples 

of the function 

where 

S(t) represents the input signal as a function of time 

N(t) represents the input noise as a function of tlme 

T, 1s the time delay between samples. 

The presence of 60 Hz harmonics is ignored for the moment. The transmitted sfgnal can be represented by 

S(t) = Em sin (wit 4- 8) 
Mate: sanplkuq rake not to scale, - - - 

Figure 29 

Tl.lustration of rhe Sanpling Proct =fixre ' 
with a normalized input signal power of 

The noise is assumed to be white Gaussian noise (WGN) with 

a zero mean and a variance or normalized average power of 

where the bar indicates a time average. 

Since the actual or true value of the ACF for an aperiodic signal is 

obtained only with an infinite sample size, the actual operation must 

be examined to note the effects of a finite sample size. Letting 

be the sample mean as a randcm variable which is the measured value 

of the autocorrelation function at T = rO; Mayhugh shows in reference 9 

that its variance is 

If the input had only the desired signal present, the autocorrelation 

function would be 

Assuming that wl.ro will not be an exact multiple of n/2 and a sufficiently large number of samples are taken over the complete cosine 

wave cycle, the variance of the correlator output becomes 

The effective detector output signal to noise ratio is the ratio of 

equations (30) and (28) yielding 
which describes the relationship between the output and input signal 

to noise ratios as a function of the number of autocorrelation function 

samples. Equation (31) is plotted in Figure 30 which shows the large 

increase in signal to noise ratio which can be obtained via autocorrelation. 

Optimization of Pre-Filter Bandwidth - and Sampling - Data -- The necessity 

of bandpass fztering in the early stages of the receiver means that 

a bandlimited noise spectrum will be applied to the correlator instead 

of the usually and conveniently assumed white input. As this "prefilter" bandwidth is decreased, the signal to noise ratio applied to 

the correlator increases. One might suppose that better performance 

would be obtained from the correlator with a more and more narrow 

pre-filter bandwidth. However, as adjacent samples become correlated 

with the decreasing bandwidth, the signal to noise ratio improvement 

due to sample averaging decreases. Therefore, as the pre-filter 

bandwidth is decreased two opposing effects occur - one which improves 

and one which degrades detector performance. Thus for a given detector 

integrationtime, there may exist an optimum choice of pre-filter 

bandwidth and sampling rate. The purpose of this section is to study 

this problem to show that an optimum choice of these quantities exists 

and to determine their approximate optimum values. 

The problem to be analyzed is depicted in Figure 31. It is 

desired to find the pre-filter bandwidth and sampling rate of the 

correlator which yields the most signal to noise improvement for the 

pre-filter and correlator combination. The relationship between the 

input and output signal to noise ratio for the correlator is given 

in Equation (31). The following equation describes the pre-filter 

act ion 

where 

noise density input to the pre-filter 

(wat ts/Hz) 

B noise equivalent bandwidth of the 

pre-filter 

r4 

~4 

? 

r?) 

$4 

Pi. 
One method of studying the optimization problem is to set a 

desired output signal to noise ratio and search for the optimum 

arrangement of the remaining variables. Thus, let so/N0 = 10 dB 

for the present analysis. 

Another factor which must be considered is the relationship 

between independence of noise samples and the bandwidth B of the 

pre-filter. It is well known that if the following relationship 

holds 

adjacent noise samples will be approximately independent. 

Combining equations (31) , (32), and (33) and solving for 

sin/q yields the result 

where T is the total length of the integration period and is equal 

to NT~. Equation (34) is plotted in Figure 32 as a function of N and 

T. The minimum clearly shows that a sample size exists where the 

least sin/-atio is required to produce the desired output SNR of 

10 dB. Fortunately for the range of T investigated the optfmum N 

is relatively constant near 55. 

It will be convenient in system calculatfons to know the noise 

equivalent bandwidth of the pre-filter and correlator combination. 

This can be easily found since 

--- Sin - So 

'IBN No 

where BN is the noise equivalent bandwidth desired. Since So/No = 10 

for Figure 32, BN can be found from the relation 

and is graphed in Figure 33. 


Selection of Sampling Period - and Signalling Frequency -- at the beginning 

of thPs section on the paging system design, the basic thesis of utflizfng 

autocorrelation was set forth. It was assumed that if a sampling rate 

could be established which would simultaneously yield (1) zero correlation of samples of 60 Hz harmonics, (2) zero correlation of random 

noise samples, and (3) large positive or negative correlation of signal 

samples, then a detector could be built which would permit the signalling 

frequency to exist at very low frequencies in between harmonfcs of 60 Hz. 

The problem of selecting a sampling period and a signalling frequency 

to achieve these ends are discussed in this section. A summary of the 

more pertinent results presented in reference 9 is given here. 

Obviously a sampling period can be found to yield zero correlation for any one 60 Hz harmonic. For example, the n-th harmonic has 

the autocorrelation function 

where Sn is the average power. If T is chosen such that 

then Rh(~) will be zero. The problem arises in the desired rejection 

of two adjacent harmonics of 60 Hz. What if it is also desired to 

reject the (n + 11th harmonic simultaneously with the n-th harmonic. 

Thus in addition to equation (38) it follows that 

must also be true and since T must be the same for both cases 

Since N must be larger than M and both must be odd, then 

where k must be even and positive. Substituting equation (41) into 

(40) yields 
which must be true if the autocorrelation function for both the n-th 

and (n + 1)th harmonics are to be zero. However, (42) can never be 

true since M must be odd while k must be even. Thus it is not possible 

to make both harmonics correlate to zero simultaneously. 

This result is not too discouraging since the strongest 

harmonics are multiples of 180 Hz due to the use of three phase AC 

power to operate mine rectifiers. Also, by choosing the operating 

frequency sufficiently high that the nearest harmonic has a large n, 

the correlator output for the (n + 1)th harmonic will still be small 

and can be made of opposite sign to that expected from the signal, 

Coupling these two properties with the fact that the pre-filter can 

be designed to offer some attenuation to nearby 60 Hz harmonics, it can 

be concluded that the combination of correlation and filtering can be 

used to reject strong harmonics of 60 Hz while providing long averaging 

times for extremely sensitive detection. 

The fact that the sampling period can be chosen to minimize 

the non-zero correlation harmonic can be seen in Figure 34. Ffrst it 

must be noted that since both harmonics are multiples of 60 Hz, the 

two autocorrelation functions will peak simultaneously every 1/60 seconds. 

This means that the least separation between the zero points will exist 

one quarter cycle on either side of the simultaneous peaking points. 

Therefore if one chooses 

where m is any positive integer then the n-th harmonic will correlate 

to zero while the (n + 1) or (n - 1) harmonic will exhibit the least 

possible correlation. Since it will normally be desirable to sample 

as rapidly as possible, commensurate with the pre-filter bandwidth, 

m will usually be one. This means the sampling rate will require a 

pre-filter bandwidth on the order of 60 Hz which is in line with the 

optimum sample size of N = 55 when a one second integratfon time is 

used. 

Using the early results of the through-the-earth communication 

studies, it appeared that an optimum operating frequency might lfe some- 

where between 2 and 3 KHz. Thus in order to proceed with the design 

and testing of this detection concept it was decided to pick an operating 

frequency between the 47th and 48th harmonic of 60 Hz, i.e. between 

2820 and 2880 Hz. The 2820 frequency was chosen to yield zero correlation. It would have been better in practice to have chosen the 2880 

frequency since it is a multiple of 180 Hz. TJtilizing equation (43) 

where m is one and n is 47 yields 

which is a sampling rate of 

fo = 1/ T,= 60.32085556 Hz 

Utilizing this value of r, in equation (37) for the two harmonics 

yields 

Rh (ro) = 0 for n = 47 

Rh (T,) = - .035 for n = 48 

thus producing the desired result. 

Using the sampling period given in equation (44) one can 

easily show that only one frequency between the 47th and 48th 60 Hz 

harmonics will yield a maximum positive correlation which fs 

Substitution of (44) and (47) into equation (29) wall yield a correlation of Sin. However, this frequency is not suitable to be used as 

the signalling frequency with the above sampling period, Since the 

correlator uses only samples of the incoming signal, and in this case 

would be taken exactly every 47 cycles of fl, the possibility exists 

that the first sample could be near the zero crossing of the incoming 

signal, In thfs case the correlator output would be zero even if no 

noise were present. There are several ways that can be devised to 

circumvent this problem. The one used here is to choose the signalling 

frequency slightly different than fl given in equatlon (47) such that 

two adjacent samples of the signal are nearly but not exactly equal, 

Because of the slight frequency difference sample pairs will be taken 
over the entire cycle of the signal sine wave. This operation is 

in agreement with the use of equation (30) in earlier derivations. 

Reference (9) discussed this problem in more depth and concludes that 

a frequency difference which yields a phase difference between adjacent 

samples of ten degrees over one cycle of the signalling frequency should 

be adequate. Arbitrarily choosing a frequency slightly less than fl 

given in equation (47) yields the desired signalling frequency 

The success of this approach obviously depends on the sampling 

frequency and signalling frequency closely maintaining their desired 

values. To analyze this problem assume the two frequencies can be 

maintained within + .005% of the designed value. Assuming worst case 

situations where txe two frequencies are at opposite extremes it can 

easily be shown the signal correlation term (i.e. output signal power) 

varies less than 2.5% of its maximum value. Thus it is seen frequency 

drift will have very little effect if crystal controlled sources 

are used. 

Circuit Design -- a prototype circuit was designed and the schematic 

diagram is shown in Figure 35 followed by a complete parts list in 

Table 111. This circuit was used in all tests described in subsequent 

sections. The sub-circuits are of conventional design except for the 

analog multiplier. The particular device employed is the key to the 

autocorrelation technique being suitable for actual use. Thus the 

discussion of circuit design presented here concerns the analog 

multiplier. 

The multiplication operation was the most difficult to implement. One common approach to the multiplication process is to A/D 

the input initially and perform the multiplication digitally, The 

analog approach was chosen because it is less complex and inexpensive. 

However, it was soon learned that the Lccuracy of most analog multiplication techniques falls far short of their digital. counterpart-. 

Multiplier accuracy is usually defined by the manufacturer 

as a per cent of full scale value. Nearly all of the present multipliers 

(hybrid, monolithic, or discrete) have a linearity of approximately 

- + 1% full scale. This means that if the maximum output product is 

10 volts, then the maximum error for all products is + 0,1 volt. The 

answer obtained is then .09V - +. lV, and the error is Targee 

K C5 

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Our first attempt at a multiplication scheme was to use the 

Motorola MCl595 monolithic four quandrant multiplier. This relatively 

inexpensive IC is specified with a linearity of 9 1% of full scale, an 

adjustable scale factor, and "excellent" temperaiure stability. Three 

samples were tested and found to be unacceptable for this particular 

application for the following reasons: 

1. Although tests showed that lfnearity was near that specified, 

the zero reference was not stable and varied as much as a 

few tenths of a volt over a several minute perf~d.~ 

2. The input impedance to the circuit is so high (35 Meg-ohms) 

that the output was particularly sensitive to stray 

capacitance as well as stray RF fields. (In fact, with a 

six inch lead connected to the X-Y terminals, a local radio 

station was received quite well). This would require that 

particular care would have to be paid to shielding and 

layout. 

3. Alignment and adjustment of offsets were particularly 

difficult and time consuming. Four controls are used to 

align the device, and they are so interactive that at 

least three iterations in the trimming procedures are 

requi red, 

4. The scale factor and therefore the output is directly 

dependent on the supply voltage. Tight voltage regulation 

would be required in the final receiver. 

It was decided that since all variable transconductance 

multipliers such as the MCl595 have a common inherent linearity problem 

that it might be wiser to investigate a discrete "quarter-square" 

method with diode shaping networks. Two versions were constructed 

using three operational amplifiers and two one-quadrant squaring 

circuits. Two of the op amps perform the summing and the diodes 

perform the absolute value operation. The squaring modules consisted 

of diode shapiug networks that contained six biased IN914 diodesc 

The circuit is inexpensive, relatively easy to align (compared to 

the MCl595), and linearities on the order of .4% were obtained. 

This approach was used temporarily in an early version of the 

prototype and performed quite well. It may be an attractive alternate 

in the final receiver since diode arrays on monolithic chips are 

available whose characteristics track with temperature, 

li his instability may have been peculiar only to the particular IC's 

tested and not generally indicative of the MCl595, Neil Wellenstein, 

a Motorola instrumentation application engineer in Phoenix, Arizona, 

advised that he had not previously noted such instability. 
It was then discovered that Motorola had available a new 

experimental product, the MC1594, which was hoped to overcome the 

many difficulties encountered with its forerunner. The MC1594 

is an improved variable transconductance multiplier with internal 

level-shift circuitry and an internal voltage regulator which biases 

the entire IC to make it essentially independent of supply variations. 

Scale factor, input offset, and output offset are easily adjusted 

with four potentiometers. This version boasts a typical linearity 

of + .5% in addition to being comparable in price with the MC1595. 

~i~cre 36 is a connection diagram for the multiplier as it was used 

in the detector prototype. 

- Test Results -- the purpose of this section is to report the results 

obtained from laboratory tests on the prototype. Three specifications 

are of particular significance: 1) the availability of the detector 

to reject 60 Hz harmonics, 2) the sensitivity obtained, and 3) linearity 

of the output as a function of signal input. 

Figures 37 and 38 are plots of the correlator transfer characteristic as a function of signal frequency for integration times of 

1 second and 6 seconds, respectively. Here the frequencies of key 

interest are 2.820 KHz and 2.880 KHz, the two troublesome to Hz 

harmonics. As can be seen from both plots there is negligible output 

from each. These plots were obtained with the input applied after 

the pre-filter which, in the completed detector, offers even more 

attenuation of the adjacent harmonics. 

The photographs of Figure 39 illustrate two examples of the 

output from the sample-hold circuits. The top photograph shows the 

sampled output with an applied sine wave at the design center frequency. The output is, of course, a replica of the input reconstructed 

at a much lower frequency while still retaining a zero mean. The 

bottom photograph was taken with a noise-only input, Inspection shows 

this output to be random with a zero mean also. 

Figure 40 shows photographs of test results illustrating the 

detection capability of the completed detector. Photo (a) shows both 

the signal and noise, for comparison, that is applied to the pre-filter 

input during the 2 second signal pulse. Here, the ms value of the 

input noise is about 14 times that of the signal. Photo (b) shows 

the output from the pre-filter during the signal pulse. The measured 

SNR from the pre-filter into the correlator during the signal pulse 

is 1.9 dB. Photo (c) shows the integrator output for the 2 second 

input pulse (reset disabled). 



be Output of X-Ecld amplifier with 

1 volt rms ncire from a 20 mz 

scurce at input to correlator. 

Ver t . = . ~V/C:II. 

Eoriz. = .5sec/cn:. 

Figure 39 Sample-Hold Outputs 
Vert. = 4~/cm. 

Boriz. = 2rnsec/cm. 

Output from correlator 

with 1 sec integrator for s 2 sec input 

pulse 

Vert. = .5v/cm, 

Horie. = 2sec/cm. 

Figure 40 Detector Test Result ,th Reset Disabled 
Figure 41 shows the detector output amplitude as a function of 

signal input amplitude into the pre-filter. As can be seen from this 

plot the output is essentially a linear function of the input for 

amplitudes ranging frog 0.8 Vrms to 11.5 dB. The non-linearity at 

the lower end of the curve is due to low-level distortion in the 

pre-filter and could be reduced with an improved design. The upper 

bound could be extended by reducing the multiplier scale factor K 

as already discussed. 

Figure 42 is a photograph of the complete breadboarded prototypeCoding System Design -- since the autocorrelation detector output 

indicates the presence or absence of an input signal by virtue of 

the integrator's output relative to a pre-set threshold, some form 

of coding must be employed to selectively call different receivers. 

Codes involving changes in amplitude, frequency, or duration of the 

code pulse were ruled out by virtue of the type of detector being used. 

Since the detector output is binary when one threshold is used, two 

types of codes were studied. They were pulse position coding and 

binary coding. The system requirements established for the coding 

scheme were: 

1. The ability to selectively signal any one of forty-nine 

receivers. 

2. Maximum calling cycle time of seventeen seconds. 

3. Low false call rate. 

4. Emergency page facility. 

After careful study of possible binary and pulse position 

codes that could be used, it was determined that a pulse position 

coding technique would be more practical to implement. Thus, the 

particular pulse position code chosen is described here. A more 

detailed description is presented in reference 10. 

The pulse position code consists of three pulses -- each one 

second long. The spacing between the pulses can be varied from one 

to seven seconds in increments of one second. The longest and shortest 

codewords are shown in Figure 43. The code address refers to the 

spacing between the end of a code pulse and the beginning of the 

succeeding pulse. The address of the longest codeword is thus 7,7 

and the address of the shortest is 1,l. For each of the seven time 

slots between the first and second pulses there are seven time slots 

between the second and third pulses, thus allowing forty-nine possible 

code words within the seventeen seconds calling time. 

Figure 42 

Assembled Prototype 
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The code pulses can be generated by digital circuitry with 

a master clock oscillator to generate the time base, The desired 

spacing between the pulses can be obtained by counting the clock 

pulses and generating the code pulse at the correct time. 

The decoder for this code is relatively simple and has the 

advantage of not requiring the presence of a clock signal in the 

receiver. The decoding action is initiated in each of the receivers 

by the first pulse from the detector. Each decoder opens a windbw 

around the expected time of arrival of the second pulse of its 

codeword. If the second pulse arrives while this window is open, a 

second window is opened. The arrival of the third pulse in this 

window trfggers the alarm. This entire sequence occurs for only 

one of the decoders. The action proceeds as far as opening the second 

window in six of the unaddressed decoders since for each time slot 

there are seven receivers with a pulse in this slot. The first window 

only will be opened in the remaining forty-two decoders. The decoding 

circuitry will return to the quiescent state in a time decerrnined 

by the address of the decoder, (The maximum time required is seven 

seconds). 

The code is repeated automatically until the called party 

answers the telephone. The operator then cancels the call manually. 

For this reason the encoder provides an eight second delay between 

the end of the last pulse of one code word and the beginning of the 

repeated code word. This prevents the first pulse of the repeated 

code word from being interpreted as a continuation of the previous 

code word by another receiver and thus causing a false call. 

The emergency page feature is implemented by merely bypassing 

the encoder and transmitting the carrier continuously. An explanation 

of this alarmwill be presented after a description of the detector. 

The principal advantage of this coding system is that the 

decoder does not require a clock oscillator to provide the timing, 

Prototype System -- hardware was breadboarded to demonstrate and test 

the pulse position coding scheme. Figure 44 shows a block diagram 

of the encoder and Figure 45 provides the schematic diagram of the 

delay register employed, Operation of the encoder is stranght-forward 

with the delay registers providing the proper timing pulses to gate 

the correct pulses to the output. Switches S1 - S7 establishes the 

delay for the second pulse while Sg - Sr4 is for the third pulse delay, 

The decoder was constructed according to the schematic shown 

in Figure 46 and the components listed fn Table IV. The decoder 

operation is easily understood by waveforms shown in Fngure 47 which 

shows the sequence of events for detecting the porper code. The 

encoder-decoder was tested in the laboratory and found to operate 

as designed. 



LJ-gilt-Enit t ii~g Diode 

10K, 10 tcrn Pot. 

Table IV. 
1 

TH. DET 1 1 

--- I I . ' -- - -----. 
Typical System Design -- in order to study the feasibility of implementing a paging system and to demonstrate the optimization procedure 

described earlier, this section presents the results of the first 

design attempt using the knowledge gained from this research project. 

A set of hypothetical system requirements were established 

based on what our experiedce showed might be required in an actual 

operating mine situation. 

System Definitions -- this system may consist of multiple 

transmitters located within the mine. The transmitters are to be 

sychronized in frequency and controlled from the mine's dispatch 

office. It is estimated that this system will be used thirty times 

daily and that an average of twenty men will carry a receiver during 

a period of eighty hours per week. 

The false message rate is to be less than one per one thousand 

messages sent. The msssed message rate is to be less than one missed 

message per one thousand messages. 

Mine parameters -- the desired range of coverage is outlined 

in Figure 48. The minimum depth of the mine is 375 feet. The conductivity was measured inside the mine and was found to be fairly 

consistant at 1.2 x mhos/meter. The darkened areas are locations 

at which coal will be removed during the next few years. The paging 

system must be designed such that no modifications will be required 

during this period. 

The noise spectrum was measured at various locations wnthin 

the mine. The maximum values of the z component of the magnetic 

field is plotted in Figure 49. The 60 Hertz harmonic noise is omitted. 

Transmitter -- the transmitters will each transmit ten watts. 

The transmitting antennas loops consist of one hundred turns of 15 

gauge copper wire (R, = 0.01 ohms/meter), having a radius of one 

meter. 

Receiver -- the receivers are to have an effective nonse 

bandwidth of t.hree Hertz,* a noise figure less than 8 dB, and antenna 

mismatch loss*, bias circuit losses, etc. of less than 6 dB. The 

receiver loop antenna will have an effective radiCs of 0.4 meters. 

*This bandwidth is about one-halq that actually achieved for a one 

second pulse width. Thus this design exercise gives a slightly large 

operating range. 


Required Detector Parameters -- as pointed out earlier the 

threshold is set on a false alarm rate basis. Each false alarm, however, 

does not constitute a false message. An example would be a single 

false alarm occuring during a time in which no message had been sent 

for a time period of 15 seconds before and after its occurrence. A 

receiver would not decode this single occurrence as a message. 

There are two basic types of false message. Type I may be 

described as a false message occuring as a result of three false 

alarms occuring in a single receiver each in a particular time slot 

so as to be decoded as a message. 

A Type 2 false message is aided by the normal transmission 

of address signals. An example of this type would be a receiver 

receiving the three signals of an address other than its own pulse 

having a false alarm occur, providing a combination in which it 

decodes as its own address. 

Layman shows in Reference 5 that in order to meet the false 

alarm requirements requires an average time between single pulse false 

alarms of 337 seconds due to Type I alarms while 9.500 seconds is 

required to meet the Type 2 alarms. The latter requirement obviously 

dominates and it is further shown this translates to a single pulse 

false alarm (at correlator threshold output) probability of no greater 

than 0.33 x which requires the threshold level to be set 13.1 dB 

larger than the noise level. 

The minimum acceptable signal is determined on the basis of 

the desired probability of detection as discussed earlier. Each 

message consists of three pulses. The probability of a missed 

message is three times the probability of missing a single pulse 

(assumgng the probability of missing any two or all three is neglfgible). 

For the requirement set forth, Layman shows in Reference 5 that the 

minimum acceptable signal to noise ratio is 15 dB. 

System Optimization -- using the data given and derived in 

this section, the bptimization procedure presented in the section 

on through-the-earth communication can be applied. 

Starting with an assumed frequency of 2100 Hz and continuing 

with four interations the resulting data is presented in Table V. 

These data show that the optimum frequency occurs between 800 and 

900 Hz. Thus the chosen operating fceqbency is 870 Hz which is midway 

between two 60 Hz harmonics where the maximum range is approximately 

600 meters. Using this range one can use an array of seven antennas 

to achieve coverage of the entire coal mine as shown in Figure 50, 

From the graph of mine noise and the assumed receiver noise 

properties Layman shows the receiving antenna would require 29 turns 

with an area of 0.4 square meter -- or 11.6 square meter-turns. 

1st Selection 

1st Iteration 

2nd Iteration 

3rd Iteration 

4th Iteration 

Frequency Hnoise Range 

(kilohertz) [~Bw/ (Hz) 1'2 ] (meters) 

2.1 -167 600 

1.7 -159 520 

1.2 -157 533 

0.9 -161 620 

0.6 -150 520 

£opt 

(kilohertz) 

Table V 

Example Selection of Operating Frequency 

Comments -- the design approach presented here should not be viewed 

as the only one which can yield a practical system. Other antenna 

arrangements should be studied, in particular the excitation of 

earth currents by grounded probes. These should be studied in a way 

their performance can be compared to loop antennas on both a theoretical and experimental basis. On the basis of theory and experiments of this project there seems to be no question but what throughthe-earth signalling can be used over a limited range of several 

hundred feet or more. However, much more study needs to be devoted 

to the subject toedetermine the best design for such a system where 

both operational and emergency operation are required. Because of 

the potential usefulness during emergencies, technology should be 

pushed to its limit to obtain maximum range of direct through-theearth operation. 
THE TELEPHONE SYSTEM 

Although new communication methods, such as through-the-earth 

signalling, are needed in the coal mining system, it appears that 

the backbone of the normal operating communication system should 

continue to be the hard wired telephone system. Not only does the 

hard wired approach appear to offer the best balance between economy 

and reliability (when properly installed), but the telephone wiring 

can be used for numerous other applications such as monitoring by 

frequency division multiplexing techniques. 

After careful consideration of the communication needs in 

coal mines, the following items have been established as the main 

characteristics desired in an underground telephone system: 

1. Multiple Paths to Outside -- the objective here is to 

prevent all telephones in-by a line break or short from 

becoming useless. Two possible paths appear adequate, 

2. Audible Emergency Signalling -- the telephone system 

provides the main means of alerting miners during 

emergencies. The telephones should include means to 

emit distinct audible signals for emergency signalling. 

Initiation of these signals should probably be controlled 

from a central outside point such as a monitoring system 

control room. 

3. Emergency Override -- provisions must be included to permit 

any conversation to be overridden with emergency commun8- 

cation. 

4. Selective Area Page -- as coal mines grow larger it is 

apparent that the entire telephone system paging mode 

need not be activated each time a call isinitfated. 

When individuals are being called in the general area of 

their location is usually known. This characteristic 

would possess the advantage of paging but reduce the 

paging activity at each telephone. 

5. Simultaneous Conversation Capability -- although the 

ultimate for this characteristic would be a private line 

for each telephone, this is not necessary to produce the 

desired effect. In general, each section does not produce 
much communication activity. Haulage and maintenance 

activities dominate telephone use. Since these activities 

tend to originate on the basis of mine "areas" it appears 

that praviding different areas of the mine with a separate 

telephone circuit could meet the simultaneous conversation 

need and maintain circuit simplicity. 

6. Manual or Automatic Connection Between Subsystems -- not 

only must provisions be made for connecting telephones 

within the telephone system, but also for connecting the 

telephone system into the other communication systems used 

at mine. 

7. Two-wire Transmission Line -- although not a mandatory 

requirement, two-wire transmission lines are much easier 

to install and maintain than multi-conductor types and thus 

should be used as much as practical in the telephone 

system. 

8. Remote Signalling -- the design of the telephone equipment 

and circuits should be compatible with frequency division 

multiplexed equipment so frequencies above 3,000 Hz can 

be used for signalling applications, 

Relative to the present telephone system design, radical 

changes will be required to effect the desired reliability, selective 

area page, and simultaneous conversation capabilities. The system 

must be battery powered at least to the extent necessary to permit 

adequate operation during situations when inside power is turned 

off (fan failure for example). A minimum of electronics should be 

used in the signal path consistent with high system reliability. 

Although several different configurations can be used to 

achieve the desired system properties, one configuration was chosen 

by this project because it can use existing paging telephones with 

a reasonable amount of modification. The desirability of selective 

area paging and simultaneous conversation capability along with the 

maximum possible use of two-wire transmission line makes the use of 

a zoning or sectionalization of the mine telephone system attractive. 

By locating zones or sections on the basis of power borehole and fan 

shaft positions, each telephone zone or section can be designed to 

have at least two independent paths of communication to the outside -- one by a cable routed inside the mine, and one by a cable routed 

outside via a borehole or fan shaft. The feasibility of this appraoch 

was investigated by considering a large mine with 16 operating sections. 

A schematic of the mine layout fndicating the location of all telephones and boreholes fs shown in Figure 51. Using the indicated 
FIGURE 51 Typical Telephone System Layout 
boreholes, the mine was sectioned into eight zones as shown in Figure 

52. This yielded the distributfon of telephones as shown in Table VI. 

This gives an average of about six phones per area with a maximum of 

three mining sections in any one area. 

In order to see how the telephone sections would be fnterconnected, consider the simplified four area system shown in Figure 53. 

Within each area, the paging telephones would operate normally (see 

earlier section on present communication systems). That is, all 

phones in each area would operate on a party line basis, and the page 

mode would be established via direct current, When contact with a 

phone outside the local area is desired, an additional button or 

buttons would be provided to effect the desired signalling. Connection 

to the area being called would be made at an outside central exchange. 

TABLE VI 

Sectionalization of Phone System 

AREA 

INSIDE TOTAL 

OUTSIDE TOTAL 

NUMBER OF PHONES 

MAIN-LINE COAL MINING 

WULAGE TRANSFER SECTIONS 

TOTAL 

TOTAL 

The system is made more reliable by having two different 

signal paths available between each area and the central exchange, 

fee. the outside line and the inside line. Consider the situation 

when the outside line from area No. 1 is borken. The inside line 

may be normally idle or used for telephones needed along the main 
FISCRO 52 Sectionalization of Telephone Syscem 

haulage way. If a continuous test signal is sent over the outside 

circuit, the broken line is immediately detected, and a signal 

would be activated over the inside circuit to connect area No, 1 

to the inside line. Area No. 1 circuits at the central exchange would 

also be connected to the inside lines and the exchange operator 

notified of the line break. Despite the line break the system could 

operate almost normally Qntil the outside line is repaired. 

If both outside lines for areas No. 1 and No. 2 were broken 

at the same time, the system would switch both areas cnto the inside 

line. The system would operate in this mode until the first outside 

line is repaired. 

The system described here has the major advantage of providing 

redundant paths from each area and simultaneous conversation capabilfty 

without the need of in-line electronics (other than that at each 

telephone). That is, a direct hard-wired connection exists between 

all telephones and the central exchange. This yields a more reliable 

and economic system than possible with other methods which require 

electronic processing between the telephones and central exchange. 

The outside central exchange can take on many different forms 

with various degrees of complexity. Dtscussed here are the two 

extremes -- the manually operated and the automatically operated. 

Manual Exchange -- The central exchange can be designed to 

require a human operator to effect the switching between the telephone 

areas. The simplest arrangement would utilize the paging telephones 

as now used with no alterations, When a caller desires to talk between 

telephone areas, he would page the central exchange operator 

by paging in the normal manner. A light could be activated by the 

paging signal to identify the calling circuit. The operator would 

answer and throw a switch to establish the connection between sections. 

One abvious difficulty with this arrangement is that the central 

exchange operator hears all paging in the sections. In a large mine 

this could be confusing to the exchange operator. Thfs problem could 

be alleviated by providing each phone with a special button for 

calling central exchange. The button would activate a tone or tone 

pair to signal the central exchange operator which circuit required 

service, By using the tone no other telephones in that area would 

be activated to page. Similarily the central exchange operator would 

not be bothered when calls within a section takes place. 

A problem with both of the above arrangements is that the 

operator must monitor a connecti-on between two sections until the 

conversation is completed so that the two circuits can be disconnected. 

If the central exchange operator is to be used for other jobs, the 

monftoring activity could occupy a large portion of his time. The 

problem may be cured by employing a voice operated relay at the central 

exchange which would automatically open the connection when voice 

signals on the two lines cease for some set time period (ten second$ 
for example). This would be a relatively inexpensive addition and 

would release the operator for other duties. 

Automatic Exchange -- In this arrangement the need for a human 

operator at the central exchange is eliminated and the switching 

between sections takes place automatically by signals sent from the 

calling telephone. A device capable of sending 16 unique ouch- one" 

signals over the normal audio line is avilable at a price of less 

than $40.00 and could be added to each paging type telephone. For 

example, if the caller wants section No. 6 he would push button No. 6 

which would cause him to be connected to that section. He could 

proceed to page for the person or station in that area in the normal 

manner. When the conversation is over, the connection between the 

two areas would be automatically disconnected after a time delay of 

several seconds. A special button could be provided for calling all 

sections simultaneously in case of emergencies. 

Various designs for the automatic exchange are possible. In 

all designs considered, the biggest problem encountered was the 

automatic disconnect between areas. The voice operated relay method 

discussed above may not prove practical in an operating system. 

The time delay for cutoff may have to be so long for normal use, that 

when a caller is waiting for a busy area, he may not wait sufficiently 

long for the previous connection to open. The present paging telephone circuit prohibits the use of DC current as in conventional dial 

telephones to supply this information. Because of this problem, 

a recommended central exchange design was not established by this project. It fs apparent that distinct advantages exist in using existing 

paging telephones at coal mines, but it is equally apparent that circuit 

additions and modifications will be required to make them compatible 

to the type of service described here. This problem should be considered further. 

No Exchange -- For relatively small mines where telephone 

activity% not large enough to merit need for simultaneous telephone 

conversations within the mine, the concept of zoning or sectionalization can still be utilized to yield a more reliable telephone 

system. For example, some coal mines have only one, two, or three 

operating sections which may be far removed from one another or may 

use very long underground haulageways. In these cases alternate 

outside telephone lines can still be utilized but with the entire 

system still tied together in a party line mode. Electronics to 

continually monitor the alternate paths should be used and provisions 

made to switch circuits in case of a line break. Thus the increased 

system reliability is achieved without the added cost of the central 

exchange or modification to the paging telephones, 
In-Between Systems -- One can visualize many variations of 

the above system designs. They were chosen to demonstrate the varfation in possibilities one has in designing a more reliable system. 

The three examples show that no one system can be expected to answer 

all possible situations. The zoning or sectionalization to achieve 

higher system reliability is the major point for safety. The choice 

of exchange should be traded off against the potential increase of 

mine operating efficiency. 
CARRIER CURRENT SYSTEMS 

Work conducted in this area included testing currently 

available carrier phones (197l), use of standard carrier phones for 

emergency through-the-earth communication, study of ways to alleviate 

present carrier system problems, and use of carrier current signalling 

on power cables. 

In 1991 there were three manufacturers of carrier phone 

equipment in the United States which served the coal mining market, 

They were Mine Safety Appliances Company, Femco, Inc., and Marshall 

Elevator Companyc New carrier phones were supplied to us by each 

manufacturer for testing and evaluation in our laboratory. The purpose 

of the tests was not to determine which was the "best equipment," 

but rather to identify ways in which carrier phone communication can 

be improved in coal minesG 

A complete series of identical tests were performed on the 

units as received from the manufacturers. Prior to the tests no 

adjustments were made on the carrier phones. From a study of the 

schematic diagrams provided and the results of the tests, the following 

conclusions were drawn: 

1. All units were of similar design, i.e. narrowband 

frequency modulated in the 100 KHz frequency band, 

utilizing about 15 watts of transmitter output 

power into a 25 ohm load and tuned frequnecy (TRF) 

receivers. 

2. Transmitters and receivers were not accurately tuned 

to the channel frequency when received from the 

factory" Receiver bandpass was not well tuned for 

maximum signal-to-noise ratio (3 to 5 KHz off channel 

frequency). 

3. Squelch behavior was vastly different, Two manufacturers achieve the desired characteristic of 

opening on the basis of signal-to-noise ratio over 

some region of noise input. One manufacturer's 

equipment had poor squelch behavior. 

4. Minimum input signal required for proper reception 

(i,e. full limiter action) ranged from 1.3 to 14 

mil$fvolts and thus presented an order of magnitude 

difference in receiver sensitivity between the 

different brandsc 
5. Only one brand had good audio bandwidth characteristics 

for suppressing high frequency noise. 

6. One brand showed very poor short and long term transmitter 

frequency drift. One brand showed exceptionally good 

frequency stability. 

7. Only two units were designed for operation on a 12 volt 

storage battery. The other unit would require extensive 

redesign for efficient battery operation. 

8. Widely different methods of construction are employed 

with differing degrees of ruggedness and ease of repair. 

There has been no attempt by the manufacturers to emphasize 

the potential usefulness of carrier current equipment in case of 

emergencies such as entrapment. That is, design features have not 

been employed which would make trolley phones useful through-theearth communicators even if power is cut off and all telephone and 

trolley lines have been severed. Indeed, the early work of the United 

States Bureau of Mines on carrier current techniques recognized that 

through-the-earth communication might be possible with carrier current 

equipment in the event of isolating rock falls. In present designs, 

the low receiver sensitivity generally prevents this possibility. 

For normal trolley line operation low sensitivity is desired because 

of the high noise levels present. With high sensitivity and large 

noise levels, undesirable effects normally occur. However, in 

emergency situations when inside mine power has been removed, the 

maximum receiver sensitivity is needed c$ue to highly attenuated 

signals. 

If adequate sensitivity and battery operation were employed, 

through-the-earth operation could be achieved by three possible 

methods : 

1. System left in place and hooked to trolley line 

remaining after emergency. Trolley line acts as 

antenna. 

2. A stored pre-tuned loop antenna could be deployed. 

Our through-the-earth experiments indicated ranges of 

at least 500 feet might be achievable by this 

method (inside to inside). 

3. Earth currents could be established via conductive rods 

on roof bolts. 

Audio tones for signalling could be easily achieved by adding 

additional electronics. This method offers the most range but would 

be most difficult to deploy and would require the most extensive 

modifications of the basic electronic design. 
As already discussed in an earlier section of this report 

several problems arise with the use of carrier phone systems in the 

shorting effect caused by a trolley haulage motor and the existence of 

"dead zones" due to standing waves on the trolley line, A third 

problem is the fact that if a roof fall or haulage accident severs 

the trolley line, all carrier phone communications in-by the accident 

will be disrupted, 

The first two problems have been effectively contralled 

in most situations by using devices to couple the signal onto the 

telephone lines as depicted in Figure 2. By proper positioning of 

the coupling devices both of the first two problems can be combated. 

Unfortunately, the position of dead spots can be controlled only by 

"cut and try" methods. As the trolley lines are lengthened and 

shortened, the optimum position of couplers will vary, 

One method of controlling all three problems mentioned above 

would be to use satellite remote controlled carrier phone stations 

at critical positions in the trolley systemn Control and voice signals 

could be frequency multiplexed over the telephone lines, When a 

signal is to be received at the dispatcher's office, circuits would 

automatically choose the receiver with the strongest received signal, 

Communications would be carried on by this satellite station, When 

the dispatcher initiates the call, he would choose the satellite 

station nearest the position of the station being called. 

Other ideas that have been discussed include the use of RF 

chokes in motor power supply lines to alleviate the "short circuit" 

problem. The possibility of using inductive coupling to the telephone 

line exclusive of the trolley line has been considered as a method 

to deal with the same problem, 

Despite the problems with carrier phones mentioned here, it 

appears that the low frequency carrier current method is still the 

best choice for communication with moving trolley vehicles? 

As part of the research effort, the potential use of carrier 

current signalling on power cables was also investigated. This work 

was undertaken for several reasons. The possibility of extending 

carrier phone services to the section power center and even to the 

face area could require the signals to be transported on the power 

cables- As the system studies progressed it became evident that 

there was little need for this type of application, The major 

need for carrier current on power cables appears to be for situations 

where the use of a conventional telephone line would not be economical 

and an alternate signal path would add to system reliability, For 

example, it would be desireable to bring telephone circuits outside 

the mine at different places to offer alternate circuit paths in case 
of a telephone line fault. It may be more economical to use carrier 

current over an existing borehole cable than to replace the cable 

with one containing the needed communication circuits or to install 

a new borehole. Another alternative to this particular problem 

may be the use of through-the-earth signalling. 

Although the original intent was to conduct a complete 

investigation of carrier current transmission on power cables, difficulties were encountered in obtaining the needed equipment for 

coupling to high voltage circuits. With the early termination of 

this effort, as explained in the Introduction, this work was limited 

to the study of impedances and attenuation rates exhibited by power 

cables in the low( frequency range. 

Both analytical and experimental studies were accomplished 

and compared. A mathematical model was developed which yielded 

via digital computer solution the expected attenuation per unit length 

and the complex characteristic impedance as a function of frequency. 

Good agreement was obtained between the predicted and measured values 

of characteristic impedance. Difficulty was encountered in measuring 

attenuation because the cable samples available were too short to 

exhibit measurable amounts of attenuation. 

The following conclusions were drawn from this study. The 

characteristic impedance and attenuation of power cables at carrier 

current frequencies can be predicted with good accuracy by analytical 

methods using the known cable dimensions. For the same cable size, 

the characteristic impedance of a high voltage rated cable is more 

than that of a lower voltage rated cable. The attenuation per unit 

length of a cable depends on both cable size and voltage rating. 

The higher the voltage rating, the less will be the attenuation for 

the same cable size and for the same voltage rating, larger size cables 

will have lower attenuation. values. 

The detailed results including graphs of impedance and attenuation for typical cables used in coal mines are presented in 

Reference 11. 

Pn the work discussed above, the carrier signal is placed on 

one high voltage phase relative to ground. An alternate method 

may be useable in some applications. The ground check wire is 

insulated relative to ground in the cable and must be connected at 

the load end so as to permit flow of the 60 Hz gorund check current. 

A radio frequency choke can be used at the load end to permit flow of 

the ground check current, but also exhibit a high impedance at the carrier 

current frequency. Thus the carrier current can flow through the 

ground check wire and the ground wire without inferferring wslth 

the ground check system* Although this arrangement may exhibit more 

attenuation per unit length than the conventional approach, it may 

prove better due to using a cheaper and simpler coupling system and 

being less noisy, 
well for receiving where signal levels are low. For transmitting, 

the saturation properties of ferrite materials limit the magnetic 

field strengths that can be produced. 

Another inherent problem in the use of through-the-earth 

prop,agation for voice applications is that to achieve optimimum 

operation for ranges of several hundred feet normally requires 

operating frequencies in the audio range. The use of direct 

transmission of audio is limited by the presence of severe nofse 

from harmonics of the power frequencies. The use of conventional 

modulation techniques demands the use of higher, and thus, sub-optimum 

frequencies. 

Despite these limftations this method of propagation has 

the unique advantage of propagating directly through-the-earth and 

would be useful in emergency situations where all other propagation 

techniques would not be available. Devices for such use have been 

developed for use in African gold mines (Ref. 12). If carrier phones 

were modified to permit through-the-earth communication during emergencies as discussed in the previous section of this report, portable 

devices for communicating with such carrier phones would be necessary. 
"Leaky" Transmission Line -- this method of propagation 

requires a transmission line to facilitate the propagation of sfgnals between the transmitter and receiver although no physical 

connection to the transmission line is necessary for the transmitter 

or receiver. The "leaky" transmission line may be a balanced twin 

lead, coaxial cable with a loosely woven shield, or a well shfelded 

coaxial cable with slots placed in the shield, It appears the 

best method is the use of radiating slots introduced only as often 

as required to maintain communicatfons. By the reciprocity principle 

it follows that a transmission line that can radiate can also 

receive signals, Systems of this type used thus far are of two 

configurations. In a Belgium and Brftish developed system the 

coaxial line acts as a passive transmission medium for a simplex 

channel (Ref, 13, 14). Another system developed by Motorola 

Communications, Inc., uses active repeaters connected to the transmission line (Ref. 15). 

"Wave guide" Propagation -- by using signals with wavelengths on the order of the mine entry dimension, a waveguiding 

effect can occur permitting direct transmission of radio waves. 

Work in this area is presently underway by U. S. Bureau of Mines 

Contractors. Preliminary experiments indicates UHF frequencies 

may provide useful ranges of several hundred feet in open afrwaysc 

Usefulness -- our studies indicated that hand held communicators or "walkie-talkies" will be useful only in certain areas of 

the mine. Situations identified where such devices could be used 

are as follows: 

1. Along conveyor belts -- mines that use primarily belt 

haulage could use two-way communication to roving belt patrolmen 

and maintenance men. However, it may be that low frequency paging 

to the miner combfned with the telephone system may prove more 

economical with little sacrifice of system capabilfty, 

2, haulage terminals -- many track haulage terminals 

require communication between a motorman and a miner walking nearby, 

Hand held communicators would be ideal for this application, 

3. face area -- communication between workman in the face 

area (continuous, conventional, or longwall) would be very valuable. 

Experiments should be planned to determine to what extent this need 

exists. Experience on long walls are indicating a definite need. 

In each of the above situations it 1s apparent the hand-held 

communicators are to provide two-way communication between two people 

in a given area and the need for connecting these sub-systems into 

the mine-wide communication system is very limited, Thus, these types 

of sub-systems should normally operate independent of the mine telephone system. However, provisions should 'be made for the intercannection 
to be established especially for the case of emergencies. This could 

be accomplished by including a button on each hand held communicator 

which would activate a tone to establish connection to central control. Similar provisions could be made to permit central control 

to connect into each local sub-system. 

Outside Radio -- coal mines require the use of men who 

work outside the mine for such activities as surveying, power system 

maintenance, fan maintenance, etco As a mine grows larger these 

crews become more dispersed and difficult to communicate witho Many 

mining companies have found that two-way radio can provide significant 

improvement in the utilization of outside manpower. By installing 

radio equipment in all outside vehicles and providing hand held 

communicators to crews on foot, communications with outside personnel 

can be established at any time. 

Outside Telephone -- most coal mines maintain an outside 

telephone system independent of the inside system. The outside 

systems have many different forms. They can be categorfzed into 

three types. 

(a) Independent Private Exchange -- this type of system 

is maintained totally independent of the inside system or the local 

commercial system. Usually no provisions are made to interconnect 

the two systemsc In some cases, this system and the inside system 

are interconnected and served by the same exchange or operator. 

(b) Private Business Exchange (PBX) -- this is a small 

private exchange supplfed by the local commercial telephone company 

and provides direct connection to local public telephone lfnes, 

Usually no provisions are made for interconnecting the inside 

telephone system into the PBX, In some cases a dial telephone 

connected to the PBX system is placed at the dispatchers office underground. A very few mines use the P BX arrangement for both the inside 

and outside systems. 

(c) Business Phones -- many mines, especially the smaller 

ones, utilize some form of business phone service provided by the 

local telephone company. This is popular where only a few offices 

have to be served and a private business exchange would not be 

required. 

In general there has been very little attempt to integrate 

the outside telephone system with the inside system. Apparently 

this has been because the two systems are usually vastly different 

electrically and it is not clear to many that an interconnection of 

the two systems is possible. At many recently developed mines where 

the value of interconnecting the inside and outside system was 

recognized, it was decided to use a dial system throughout the mine 

complex, 
CONCLUSIONS 

Although this program stopped short of its original goal, 

several important conclusions of a general nature were made. 

The original hypothesis of this program was that the mine 

communication problem needed to be tackled on a system basiso The 

experience gained by this project has clearly shown the validity of this 

hypothesis. Moreover, it is increasingly evident that as mine 

communication systems require upgrading the redesign of the whole system 

should be undertaken on a truly system basis with due consideration to 

newly developing technology. 

Difficulty was encountered in accurately evaluating the 

communication needs of the mining industry for two reasonso First 

each mine is unique and thus usually has its own special operating 

characteristics which affect the communication requirements, Secondly, 

because the miners and the mine management are so accustomed to 

the presently used communication methods it is impossible to evaluate 

the worth of new possibilities without trying them in operating coal 

mines. 

This study led to several conclusions about specific needs in 

mine system design. The most obvious is the general need for fmprovement of communication system reliability. Ways to achieve this are 

presented herein. There is an important need to be able to alert or 

page miners throughout the entire mine, A first step in this direction 

would be to establish such capability in the vicinity of each working 

face. 

Telephone systems in large mines become overloaded at cereain 

times during a shift. Thus there is a need for increased telephone 

system capacity. There are a variety of ways this can be accomplished, 

some of which are discussed in this report. 

All mine communication systems consist of several distinct 

sub-systems. Provisions should be made to permit manual or automatic interconnection between the subsystems when required. 

The mine communication system is obviously going to be required 

to handle more than voice communication as new electronic monitoring and 

control systems are integrated into the mining system. Thus alterations 

to existing systems and the design of systems for new mines should 

consider these future requirements. This is especially important in 

the choice of a telephone line and its installation. 

Obviously the work presented in this report is incomplete and 

effort should continue, in some form, in each of the areas discussed, 
REFERENCES 

"Mine Rescue and Survival," Final Report by Committee on Mine 

Rescue and Survival Techniques, National Academy of 

Engineering, Washington, D. C., March 1970. 

Sommerfeld, A,, "Uber die Austreitung der Wellen in der Drahtlosen 

Telegraphic," Ann. Physik, Vol. 28, p. 665, 1909. 

Ilsley, L. C., H. Be Freeman, and D. H. Zellers, "Experiment 

in Underground Communication Through Earth Strata," 

Department of Commerce, Bureau of Mines, Tech. Paper, 

p. 433, 1928, 

Felegy, E. W., and E. J. Coggeshall, "Applicability of Radio 

to Emergency Mine Communications," Bureau of Mines Report 

of Investigation 4294, May 1948. 

Layman, G. E., "Optimization of an Electromagnetic 'Through-theEarth' Communication System," Dissertation, West Virginia 

University, 1972. 

Wait, J. R., in Antenna Theory edited by R. E. Collins and 

Fa J, Zucker, Part 11, Chapter 23, McGraw-Hill, 1969. 

"Calibration Principles and Procedures for Field Strength 

Meters (30 Hz to 1 GHZ)," U. S. Department of Commerce, 

National Bureau of Standards, Technical Note 370, March 1969. 

Williams, R. Ha, and C. J. Bennfng, "Conductivity Measurement of 

the Earth at ELF," IEEE Transactions on Antennas and Propagation, p. 364, May 1963. 

Mayhugh, T. L., "A Time Average Autocorrelation Detector," 

Master Thesis, West Virginia Univerfs ty , 1972. 

Whiting, M., "A Coding Scheme for a Personal Paging System," 

Master Thesis, West Virginia University, 1972. 

Sarkar, M. K., "Carrier Current Communications Through Mine 

Power Cables," Master Thesis, West Virginia University, 1972. 

VermeuPen, D. J., and P. J. Blignaut, "Underground Radio 

Communication and Its Application for Use in Mine Emergencies ,It 

The Transactions of the S. A. Institute of Electrical 

Engineers, p. 94, April 1961. 

Liegeois, R., "Teletransmissions Underground," Proceedings of 

the First WVU Conference on Coal Mine Electrotechnology, 

West Virginia University, August 1972. 
14. Martin, D. J. R., "Radio-communication in Mines and Tunnels," 

Electronic Letters, Vol. 6, No. 18, pp. 563-564, 

September 3, 1970. 

15. Nickel, V. , and J. S. Brown, "~adiax~ and Portable Underground 

Communications Systems," Proceedings of the First WVU 

Conference on Coal Mine Electrotechnology, West Virginfa 

University, August 1972. 
APPENDIX A 

Measured power transfer between two standard loops are given 

in the following tables. The test equipment setup is described in 

Figure 17. The data fa normalized for standard loop antennas, defined 

by: radius - 1 meter, single turn, and copper resistance of the 

wire equal to 0.01 ohmlmeter. A11 measurements were made with the 

axis of the two loops parallel. 

The measurements are in dB, and being the ratio of powers, 

are equal to: 




LIG c:!? 

O CJ U .rJ 

.a IF rJk -3 

C 1 .. CJ $4 

$2 j2 G? rrl 0 

Y &4 bi! cu UGiiL::U csm3 rC=csro 
APPENDIX B 

Noise Measurements 

The data presented here was measured using the receiver 

test setup described in Figure 18. 

The magnitude of H is found from the equation 

where : 

V = open circuit receiver antenna voltage 

N = number of turns in antenna loop 

A = antenna loop area 
CC. 

d Cd e? 


&~bG-.(mac2mtOu7 

. . . I*.. . b 4 8 

C,J PJ Cr7 lr, F- L3 ri P? a fl \D r-idr4WC'J~ 
crs u; co w *:f 1- co m r-• L? ~n o + r-l I-! CY N C-4 Cq A' = C*? P) 4 l-l F4 d d !4 r-t 1-4 1-4 r-4 r-4 I4 t-4 

ti~trlrtrtrr