Control and Monitoring via Medium-Frequency 

Techniques and Existing Mine Conductors 

HARRY DOBROSKI, JR. AND LARRY G. STOLARCZYK 

Abstract-Medium-frequency (MF) techniques have been successfully 

.ppUed to the problem of radio voice communications in both coal and 

new and nonmental mines. Long range is achieved by taking advantage 

R existing mine conductors that carry and propagate the signal over vast 

yeos. It is very likely that in the near future such systems will become the 

rnfemd method for general mine communications. MF techniques can 

JM) be applied to the areas of in-mine control and monitoring. 

Preliminary work in this area is described. In a western mine, locomotives 

on being operated under MF nmote control in a loading operation. In 

scveml eastern mines, MF has been evaluated as a method for monitoring 

the status of devices in nmote areas of the mines. All control and monitor 

information uses existing mine conductors, 

INTRODUCTION 

0 VER THE LAST several years, numerous attempts have 

been made to develop radio systems useful in 

underground environments such as mines. Such systems 

would provide to the underground environments the same 

advantages that radio offers to surface operations, namely 

instant communications to key personnel, resulting in improved safety and productivity, and direct managerial control. 

Unfortunately, undergound radio propagation is extremely 

difficult, and conventional approaches have not been successful. The problem clearly calls for unconventional approaches. 

The first of these unconventional approaches is based upon 

guided wave techniques where radio signals (VHF or UHF) 

are impressed upon special cables called leaky feeders that 

accept signals from and radiate signals to portable radios in 

their immediate area. By the use of repeaters and base stations, 

communications with and between the portables are possible, 

but only in the vicinity of the leaky feeder cable [I], [2]. A 

tremendous advantage would result if a system existed that 

could take advantage of existing mine conductors (power 

lines, control lines, etc.) to achieve the same effect. Coverage 

would be extensive, and no special cables would usually be 

needed. Medium-frequency (MF) techniques offer such advantages. 

Comprehensive theoretical and experimental studies 

showed that MF (300 kHz-3 MHz) signals could effectively 

be coupled into existing mine conductors from either magnetic 

dipole antennas or special line couplers. Once impressed upon 

a conductor, the signals would propagate long distances, 

reradiating along the way [3]. The effect very closely 

simulated that of the leaky feeder cables at VHF or UHF. The 

advantage was that existing mine conductors could be used. 

Although certain existing conductors were clearly better than 

others, all were effective at MF to some degree. In addition, 

signal energy interchange exists between conductors not 

physically connected. Therefore, signals impressed upon one 

conductor are transferred to other conductors, achieving 

extensive coverage. 

Based upon this research, an experimental MF voice 

communication system was designed, constructed, and evaluated [4]. It consisted of base stations and repeaters that used 

line couplers to inject signals onto, and receive signals from, 

existing mine conductors; Mobile transceivers for vehicles and 

personnel were also designed. The overall system made it 

possible for mobile transceivers to communicate with each 

other, or with the base station, by using the existing mine 

conductor networks. Fig. 1 shows the general system concept. 

APPLICATION OF MF TO UNDERGROUND 

OPERATIONS 

The obvious application of MF to underground operations, 

and the one for which it was originally conceived, was for 

operational mine communications. By the use of a system 

similar to that shown in Fig. 1, it is possible for a mine to 

establish quickly an extensive MF radio network for safety, 

productivity, and managerial decisionmaking and control. 

Therefore, the benefits associated with communications between key persons in any complex managerial hierarchy can 

be extended to the underground mining industry. The other 

benefits to safety, maintenance, etc . , are quite obvious. 

A second application, and one in which the Bureau of Mines 

has a keen interest, is that for postdisaster scenarios. Not only 

can MF provide an effective communications link for rescue 

teams in search and recovery operations, it can also provide an 

in-mine beacon to pinpoint the location of trapped or injured 

miners, if these personnel are equipped with small transmitting 

devices. 

A third application is that of data telemetry. Because of the 

favorable propagation characteristics of in-mine MF that result 

in good signal-to-noise (S/N) ratios on existing conductors, 

MF offers interesting possibilities in this area. The balance of 

this paper will expand upon this third application. 
Lml mine wirinp -b5 ------ - 

Repeater Base 

station 

T/fl or fz T/f, or f, 

R/f, R/f, 

Vehtculor 

tronsceiver 

Cables 

Fig. 1. General MF system concept. 

THE CONTROL OF TRAINS IN A METAL MINE 

LOADING OPERATION Fig. 2. Ore loading operation. 

Because of safety and productivity considerations, a metal 

mine that used block caving mining methods needed to 

remotely control the locomotives in a loading operation. In 

this operation, rubbled ore from upper grizzly levels flowed 

from sub raises and transfer raises to a haulage level where 

train cars were filled. The transfer raises serve as temporary 

loading bins for the ore. A working space called a "pony set" 

is provided above the loading area. By the use of pneumatic 

gates controlled from the pony sets, the flow of ore from the 

transfer raises to the cars can be controlled (Fig. 2). The 

person in the pony set has to have an unobstructed view of the 

car being loaded. When loaded, the flow was stopped and the 

locomotive operator was signaled by blinking lights or air 

horns to advance the train by one car, so the next car would be 

in position. The operation was labor intensive and inefficient. 

Two persons were required to perform this single task. When 

multiplied by the number of other trains and loading operations in the mine, this inefficiency quickly resulted in 

excessive personnel costs. 

A managerial decision was made to streamline the process 

by putting the locomotive under remote radio control from the 

pony sets. In this manner the locomotive operator could bring 

the train into the loading area, leave the train, and go to a pony 

set, and from there advance or reverse the train under radio 

control while controlling the loading of the cars. A second 

person was no longer required. To achieve this, a UHF radio 

remote control system was installed. 

This system was of European design and was commerically 

available in the U.S. market. A portable transmitter which 

could be carried by a person, contained numerous control 

buttons for forward, reverse, stop, etc. When the appropriate 

button was pushed, a coded digital signal was transmitted to a 

companion receiver on the locomotive, interfaced with the 

locomotive controls, which would result in the desired 

locomotive response. The system operated at a frequency of 

452 MHz at a power level of 0.1 W, 

From the start, system performance was very poor. As Fig. 

2 shows, a person in the pony set is not actually in the entry but 

is directly above it. Line-of-sight propagation was not possible. In addition, a vast number of cars exist between the pony 

sets and the receiver on the locomotive. Conditions were such 

that the cutoff frequency of the panel was too high to permit 

reliable operation at UHF. Signal attenuation between transmitter and receiver, and other factors introduced excessive 

signal losses between the transmitter and receiver resulting in a reliable operating distance of less than 100 ft, which 

was unacceptable. The power level of 0.1 W was far too low 

for almost any in-mine application, especially one that 

requires a tunnel mode of propagation to achieve any 

reasonable range. Although tests were not conducted at higher 

powers, prior research [2] has shown that acceptable performance could probably never have been achieved at UHF. 

Since the mine had recently installed an MF system for 

communications on the main rail haulage which' performed 

exceptionally well, the question was asked whether or not it 

could also be applied to the problem of train control in the 

loading operations. Initial experiments in the loading panels 

showed that MF signals (of magnetic moment 2 A-Tom2) 

could achieve a propagation distance of 1000 ft in these panels 

whether or not train cars were positioned between transmitters 

and receivers. The conductors in the area functioned effectively as the MF distribution system. A decision was made to 

proceed with the MF train control approach. The foYowing 

section describes the initial MF system, shows the general 

system concept, and gives specifications and operating performance. 

THE GENERAL MF APPROACH 

The initial experimental system, shown in Fig. 3, consists of 

a transmitter under wireline control from fixed control boxes 

in the pony sets. The control cable (and all other cables in the 

entry) performs as both the propagation medium and the 

distribution antenna, producing local magnetic field all along 

the loading panels. A radio receiver on the locomotive 

completes the system. Table I gives the general hardware 

specifications. 

THE TRANSMITTING SYSTEM-DETAILS 

The transmitting part of the overall system was installed in 

the loading panels. This consists of various pony set control 

boxes, a control cable, modulator, MF transmitter, and RF 

line driver. 

At each pony set location, a pony set control box was 

installed. The box contains three pushbuttons for brake 

release, forward, and reverse, along with an indicator light 

emitting diode (LED). 

The pushbutton information from each pony set is sent by 

the cable to s modulator at the base station. Signals from the 

pony set pus!:buttons and a locomotive identification (ID) are 

encoded into audio frequency shift keying (FSK) and sent to 

the transmitter. The locomotive ID is generated by a special 
Switch Cloaurr Line Pony 91 Control Boxrr 

r---- --- 1 

I 

I 

I 1 power I 

I I I 1 I 

L, ------a 

Fig. 3. MF radio control via control boxes and control cables. 

TABLE I 

SPECIFICATIONS 

General 

Frequency f, (300 kHz) remote control mode 

fi (350 kHz) voice communications mode 

Power output 20W 

Digital signal Manchester coded baseband FSK 

Mark frequency 1070 k 10 Hz 

Space frequency 1270 k 10 Hz 

Baud rate 300 ~t 15 

Base transmitter station ' 

Frequency fi (300 kHz) only 

Modulator output 0-2 V peak to peak 

RF interface f, (300 kHz) line coupler 

Transmitter type f df2 type set for f I only 

hmotive transceiver 

Frequency fl/fi depending on operating mode 

Demodulator dydc range >60 dB 

RF inkxface h/f2 Imp antenna 

physical key-electronic contactor assembly that permits 

unique coding. Different physical keys are associated with 

different locomotives. The audio FSK modulates the transmitter where it is converted to MF at 300 kHz. 

The modulator is equipped with a seven-segment visual 

display corresponding to the ID key being used. It also 

supervises (as well as decodes) the three command lines for 

the three pushbuttons in the pony set control boxes. Fig. 4 

shows how this is done. 

All pushbuttons of a particular function (forward, for 

instance) are wired in parallel as shown. The last pony set 

control box also contains an "end of line" resistor. Since RI 

= R2, normally the potential at A is 6 V. Pushing any 

pushbutton will cause point A to rise to 12 V, indicating a 

command function. If point A drops below 6 V, it indicates 

problems with either the 12-V source or the control cable. 

When any of the control function detector circuits in the 

modulator are activated, an LED corresponding to brake1 

forwardlreverse and line openlshort will be lit. In this manner 

the control line is supervised. If a shortlopen fault is detected, 

a corresponding LED at each pony set control box is also lit. 

The transmitter is simply an unmodified MF transceiver, 

with an output power of 20 W. It is a two-frequency device 

(300 kHz1350 kHz) that is automatically locked to 300 kHz 

when connected with the modulator. The control frequency 

chosen at this mine was 300 kHz. 

The output .of the base transmitter is sent to a line covpler 

Function Purh Buttons 

In Pony Sat Borra 

Llnr 

Reel~tor 

Fig. 4. Supervised command control line circuit. 

clamped around the control cable. In this interesting arrangement, the control cable performs two functions. It connects the 

pony set control boxes to the modulator and also acts as the 

MF distribution antenna. The line coupler induces MF current 

into the control cable, which produces a local magnetic field 

all along the loading panel. 

THE RECEIVING SYSTEM 

The transceiver on the locomotive is equivalent to that of the 

base station but can operate on either fi (300 kHz) or fi (350 

kHz), depending upon whether it is needed for voice communications on the haulageways or remote control in the loading 

panels. A two-position switch on the attached demodulator 

unit provides the selection. In the "manual" position, 

operation is at 350 kHz for voice communications with the rest 

of the rail haulage system. In the "remote" position, 

operation is at 300 kHz, and the locomotive is set for remote 

control from the pony set control boxes. A vehicular loop 

antenna on the locomotive picks up and transfers to the 

transceiver either 300- or 350-kHz signals. 

When the "remote" inode is selected, FSK-encoded MF - 

signals are picked up by the antenna-transceiver system from 

the'locd wiring and demodulated to FSK audio signals. The 

transceiver is also locked into the "receive" mode at 300 kHz. 

The demodulator converts the incoming audio FSK to a digital 

data stream. These data are an exact copy of those sent from the 

modulator at the base. The data are decoded, validated, and 

checked for redundancy. If correct, the data are then sent to 

control relays that drive the final contactors associated with the 

desired locomotive hnctions. 

The data field associated with the system contains 15 b. The 

first 5 b contain the locomotive's ID and are determined by the 

physical key inserted into the modulator at the base. The last 

10 b concern the function to be performed, such as brake 

release. Each locomotive has its own code preprogrammed in 

the modulator. In this manner, the decoded output can be valid 

only for a unique individual locomotive. After each reception 

of data, the last 4 b of each decoded data word are compared. 

The data will cause the output relays to activate only if 1) the 

baud rate is correct, 2) the ID codes match, and 3) data from 

two decoders are identical. In addition. for increased safety, 

the MF link from base to locomoti\re must be intact for the 

control relays to be activated. The loss of signal for more that 

1 s causes time out, and the brakes are reset. 

System operation to date has been excellent. Train control is 

reliable out to 1000 ft from the transmitter in a situatioc 

where, of course: there are cars between the ty.vo. 
The auestion may be asked why this system represents a 

significant advance in train control when the same performance could have been achieved (in this mine) by carrier current 

techniques where the telemetry signals could simply have been 

capacitively coupled onto and received from the trolley line in 

the conventional manner, at a standard carrier current frequency of perhaps 88 kHz. The significance lies in several 

factors. 

1) Control is possible to locomotives that do not use dc 

trolley power, such as diesels and battery units. 

2) The locomotives need not be very near any conductors. 

Many tens of feet separation are possible. 

3) The most important feature is shown in Fig. 5. Via a 

portable MF transmitter-repeater combination, reliable control 

is possible from the person, wherever standing. In fact, 

portable vest transceivers already exists (Fig. 6) that permit 

this feature. 

OTHER APPLICATIONS 

Syrtrm 

Fig. 5. MF controI via portable transmitter repeater and existing conductors, 

The Use of MF in a Snapper Operation 

One of the most dangerous tasks in a mine with a rail 

haulage system is that of coupling and uncoupling cars of the 

train. The person who does this work is often referred to as a 

"snapper." 

The danger usually results from the fact that the snapper and 

the locomotive operator (motorman) are not in direct contact 

with each other because of the length of the train, curves in the 

loading area, and other factors. If the motorman moves the 

train when the snapper is between cars, the results can be 

tragic. Although snappers are supposed to adjust car couplers 

with long bars, and never actually place themselves between 

cars, they often do enter this area when alignment or other 

problems occur. 

Even under the best of conditions, the operation is clumsy 

and inefficient. To address this problem, many mines have 

attempted to establish some sort of communications link 

between snapper and motorman. 

The first attempt made is often to string a series of pager 

phones in the snapper's and motorman's locations. This 

approach is usually ineffective. The pager phones at the 

snapper's location are seldom where they are needed because 

the coupling-uncoupling point usually changes. The pager 

phones at the motorman's location are seldom where needed 

because the train length changes. Even if these problems did 

not exist, the system does not provide a good solution. If the 

snapper must physically stand at a pager phone location, he 

cannot "talk" the motorman into inching the train forward or 

back as needed. 

Other attempts have been made that are equally unsatisfactory. Carrier phones with loop antennas have been given to 

snappers to permit direct communications with the motorman. 

In this approach, the snapper is required to carry a 30-lb 

carrier phone from site to site. If the trolley line does not 

extend into the snapper's location, communication is impossible. Even if a trolley line or other conductors exist, coupling 

losses and antenna inefficiencies are great, and performance is 

again poor. 

Fig. 6. Portable vest transceiver with remote control features. 

UHF radio has also been tried and shown to be generally 

unsatisfactory. The long string of cars between snapm and 

motorman causes large signal attenuations, as do curves in the 

entryways, The .overall result is short range. 

An experiment was conducted to evaluate the performance 

of MF in a snapper operation in a local coal mine that was 

seeking a solution to this problem. The mine used a looparound-loading point method of loading train cars from shuttle 

cars. Fig. 7 shows the general layout. 

The locomotive was equipped with a standard 20-W MF 

transceiver and loop antenna. The snapper was given a 

standard 4-W MF vest, similar to that shown in Fig. 6. The 

system did not provide any sort of control capability from vest 

to locomotive, only a voice link between snapper and 

motorman. The entryways where the snapper worked contained only a trolley line. 

The communis:~tions range, between the motorman and 

snapper, was ty~,$:ally 1000 Ft when rail cars existed between 

the two. At 100!) ft, directional effects existed such that the 

snapper had to i:,! oriented parallel to and within 5 ft of the 
Fig. 7. Use of MF in snapper operation in coal mine. 

trolley wire to achieve reliable communications. At separations less than 1000 ft, directional effects diminished, and it 

was possible to be farther away from the trolley line. 

Because the snapper always works near the rail cars which 

represent a massive metal anomaly in the area, there is little 

doubt that even at MF these cars increase the signal attenuation. A solution would be to simply use a repeater concept as 

shown in Fig. 5. If this were done, the vest transceiver would 

have the benefit of 20 W of power instead of 4 W. Not only 

would range increase, but directiond effects would be 

eliminated. 

Status Monitoring of Mine Devices 

In many instances it is necessary to know the condition or 

status of a critical mine device such as a water sump, motor, 

etc. Although the information desired is usually simple (is the 

sump~overflowing?) itis also critical. It is usually not practical 

to run cables between the device and display location because 

of distance and other problems. Also, the information desired 

is sometimes temporary in nature. If it were possible to 

monitor and display this information in a convenient manner, 

this would be of great advantage. Medium frequencies allows 

this to be done. 

An experimental data telemetry system has been developed 

that permits the monitoring of mine devices without the need 

to deploy specid cables. It uses existing mine conductors. The 

general system is shown in Fig. 8. 

The transmitter is a self-contained battery-powered portable MF digital device that can encode and transmit 4 bits 

of data and a 12-bit address at any desired MF frequency, 

impressing this digital signal qnto local mine wiring by the 

use of line couplers or other means. When so done, the existing conductors transmit and radiate the data throughout 

the mine. The receiver, also battery-powered, receives these 

signals either from line couplers (if the receiver is in a fixed 

location) or from loop antennas (if the receiver is mobile). 

The transmitter transmits the data in short bursts of 40-ms 

duration followed by an adjustable off time which results in 

selectable duty cycles. This mode of operation has several 

important advantages: 1) circuit simplicity; 2) low average 

power consumption; and 3) low probability of data coIIision if 

more than one transmitter is used. 

The receiver can be configured to operate in several =odes. 

depending upon the nature of the &ta bein: transmitted. h thc 

Erl atlng Conductor8 

tor Duty Cyclr Pulrar 7-

I.D. + Data I 

Input OutW 

Fig. 8. Simple MF data telemeuy system. 

first mode, if alarm data exists on the four data bits from a 

particular transmitter, an immediate alarm indication (lights, 

buzzer, etc.) can be made to occur. In the second mode, the 

reception of a data wold that contains only an address sets an 

internal timer within the receiver. So long as data are received 

within the time-out period of the receiver, time out will never 

occur and no alann or loss of signal indication will be 

displayed. 

Anything that disrupts the signal from reaching the receiver 

within the time-out interval will cause this fact to be displayed. 

The disruption can be caused in several ways: 1) the 

transmitter is actually shut off by the alann condition or a 

transmitter failure; 2) a break in local mine conductors; 3) 

poor SM ratio caused by electromagnetic interferences (EMI) 

or great distances; and 4) a failure in the receiver. 

The foregoing mode of operation is nothing more than a 

supervisory, MF approach where the data consists of only the 

digital address of the particular transmitters. The Bureau's 

experimental data system has been designed to operate on the 

second mode. Limited testing on a mine phone line showed a 

95-dB signal margin remaining at a distance of 1 mi from the 

transmitter. Additional testing has not been done to date. 

The obvious application of this technique would be to have 

numerous in-mine transmitters sending data back to a fixed 

location such as an underground shop, but an even more 

valuable (and not so obvious) application exists. By the use of 

magnetic loop antennas placed on vehicles (such as the 

maintenance foreman's jeep), data, or status of devices, could 

be monitored while on the move, an application not practical 

by other means. 

CONCLUSION 

MF techniques have been used in a train-loading operation 

where radio control was required. The MF system worked 

well in an environment where a corresponding UHF system 

performed poorly. The MF systefi was based upon standard 

comrnerically available transceivers, interfaced with encoders 

and decoders. Because of the success of this system, the mine 

has improved the efficiency of the loading operation, reduced 

the number of personnel involved, and has enhanced general 

safety. 

The initial system controlled trains via fixed control boxes 

in the pony sets, connected to a base station. Future plans calls 

for control from mobile portable transceivers. If very close to 

conductor;, these transceivers couId control the locomotives 

dir~ctly. If distant from wires, or under difficult conditions, a 

repcater system could b:: ussd. 
Correction to "Control and Monitoring via 

Medium-Frequency Techniques and Existing 

Mine Conductors" 

HARRY DOBROSKI, JR 

LARRY G. STOLARCZYK 

In the above paper, ' two concluding paragraphs and the references 

were missing from the manuscript on page 1092. They appear as 

fo1lows. 

In a coal mine, MF was applied to the problem of communications 

between snapper and motorman in a rail haulage-looparound operation. With standard portable and vehicular transceivers, communications were established between the two with 1000 ft of rail cars 

intervening. Although not truly a remote control application, the 

situation is similar to that of the metal mine and performance was 

equally good. 

A simple MF data telemetry system was designed and tested that 

indicates that the integration of digital and MF techniques could be of 

benefit in monitoring critical mine parameters or devices. The system 

provides long range propagation using existing mine conductors. 

(11 D. T. Updyke, W. C. Muhler, and H. C, Turnage, "An evaluation of 

leaky feeder communications in underground mines," (contract 

J0199048, Atlantic Research Corp.) BuMines OFR 30-81, 1980, 104 

pp.; NTIS PB 81-19466. 

(21 R. L. Lagace. M. L. Cohen, A. G. Emslie, and R. H. Spencer, 

"Propagation of radio waves in coal mines," (contract H0346Q45, 

Arthur D. Little Inc.) BuMines OFR 46-77, 1977, 183 pp.; NTIS PB 

265 858lAS. 

[3] R. L. Lagace, A. G. Ernslie, and M. A. Grossman, "Modelling and 

data analysis of 50 to 5,000 kHz radio wave propagation in coal 

mines," (contract H0346045, A. D. Little, Inc.) BuMines OFR 83-80, 

1980, 109 pp. ; NTIS PB 80-209455. 

14) A.R.F. Products Inc. A Medium Frequency Wireless Communication 

System for Underground Mines (contract H0308004). For funher 

information contact Harry Dobroski, Jr., Technical Project Officer, 

Bureau of Mines, Pittsburgh Research Center, Pittsburgh, PA 15236.