A WHOLE-MINE MEDIUM-FREQUENCY RADIO COMMUNICATION SYSTEM I 

Harry Dobroski, Jr. 

Pittsburgh Research Center 

Bureau of Mines 

U.S. Department of the Interior 

Pittsburgh, PA 15236 

Dr. Larry G. Stolarczyk 

Di rector of Research 

A.R.F. Products Inc. 

Raton, No M. 87740 

ABSTRACT 1 

A rnedium-frequency (MF) cornmunication systern has been built and hds been 

tested in underground rnines. It provides radio coverage to vehicles and 

personnel anywhere in the mine via parasitic coupl ing into existing mine 

conductors. The systein consi sts of base stations, repeaters, and vehi cul ar and 

personnel transcei vers. Ai r-core antennas for vehicles and cl amp-on antennas 

fur base stations and repeaters were designed for inaximum coupl ing efficiency 

usiny existing mine wiring. Personnel transceivers are of a unique vest 

design that features d low physical profile, good user co~nfort, and efficient 

antenna design. 

In-mine tests show impressive resul ts. Range between r u b b e r - t i r e d 

vehicles and base stations is often measured in ~ni les. With repeaters, greater 

range is possible. 

INTRODUCTION 1 

Ever si nce the emergence of radi o for practical surface communi cati ons, 

attempts have been made to apply it to underground mining. The Bureau of 

ivlines was one of the early researchers in this area. As early as 1922, Bureau 

of Mines experiments showed that radio propagation in mines was possible but 

not practical . Equipment was bul ky and insensitive, and only very large 

antennas could be used. The desire for in-mine radio communications is 

obvious. Such a system would permit mine personnel to receive or initiate 

messages without physically going to a telephone. The benefits would be safer 

and more productive ~ni ni ng operations. 

RADIO PROPAGATION IN MINE ENVIRONMENTS 1 

Although radio transmission on the surface of the earth is well 

understood, transmi ssi on in an undery round envi ron~nent general ly is not. 

Compl ex interactions occur between the radio wave and the envi ron~nent. 

Characteristics of the geology (stratified layering, boundary effects, conductivi ty, etc.) and the mi ne compl ex (entry dimension, conductors, el ectromagnetic interferences, etc.) had to be measured and understood before a 
I practical mine radio systern could be built. To this end, considerable 1 resedrch has been di rected(1) - l. 

In a confined area such as a mine, radio waves can propagate useful 

di stances only if the envi ronlnent has the necessary electrical and physical 

properties. The "envi ronrnent" takes into account the natural geol ogy and 

rnan~nade perturbations such as the mine complex itsel f. As an example, if the 

wavelength ( A) of a radio wave is srnall cornpared with the entry dilnensions, a 

wavegui de mode of propagation is possible. Attenuation depends prilnari ly upon 

the physical properties of the entry such as cross-sectional area, wall 

roughness, entry ti 1 ts, and obstacles in the propagation path. Secondary 

effects such as the di el ectri c constants and earth conductivity a1 so infl uence 

attenuati on. 

Mi ne radi o systems based upon thi s effect are avai 1 abl e cornmerci a1 1 y. 

These are UHF systems operating around 450 MHz which provide useful but 

1 irnited coveraye. In high coal [2 m (6.5 ft)], line-of-sight ranges of 300 m 

(1,000 ft) are often possible. Range is reduced severely in non1 i ne-of-si ght, such as when going around a coal pi1 1 ar. In lower coal , or when 

obstacles exist in the propagation path, range is reduced even Inore. For this 

redson, conventional UHF radio systems requi re an extensi ve network of leaky 

feeder transmi ssion cab1 es and repeaters to becorne useful . Even so, range 

frorn the cable is r~ot usual ly in excess of 9 to 16 m (30 to 50 ft) , and 

equiplnent cost is very high. Clearly another approach is desirable. 

1 HISTORICAL OBSERVATIONS 

An important cont ri buti on to underground radio cornmuni cations was made by 

the Chamber of Mines of South Africa. As early as 1948, programs were in 

place to develop radio systems for deep mines, primarily gold mines (2). The 

result was that by 1973, an advanced 1-W single sideband (SSB) portable radio 

systern had been developed that apparently v~orked we1 1 . The Bureau of Mines 

procured several of these units for evaluation. Performance in U.S. coal 

mines was not satisfactory. There were several reasons for thi s. Fi rst, U.S. 

mines are highly electrified, producing considerable electromagnetic interference (EMI) not norrnally found in the South African mines, which completely 

desensitized SSB radios. Second, 1 W was not enough power. U.S. mines are 

r~iostly room and pi1 lar, which means that any radio systern would have to have 

reasonable range froin local conductors. Thi rd, geological electrical parameters were less favorable in the United States. For these reasons, the South 

African systern was not acceptabl e. 

The Bureau's approach was to first determine the actual propagation 

characteristics of MF in U.S. mines, and then to relate the propagation to the 

underground envi ronment such as the geol ogy , entry si ze, exi sti ng conductors, 

and EMI. Several exhaustive in-mi ne measurement and analysis programs (3-5) 

were conducted. These proyrarns formed the foundation for the first true 

understanding of how MF propagates in a stratified medium of various 

electrical parameters, which are often interlaced by man-made conducting 

structures (rai 1 s and power lines) and artificial voids (entryways). 

'under1 ined numbers in parentheses refer to items in the list of references 

at the end of this paper. 
Figure 1 is a simplified geometry of an in-mine site that illustrates one 

of the most important findings of the measurement prograrn, the "coal seam 

mode." For this mode to exist, the coal seam conductivity (%) must be 

several orders of magnitude less than that of the rock (ar). A loop antenna 

that is at least partial ly vertical ly oriented produces a vertical electric 

field (E,) and a horizontal magnetic field (H+). In the rock, the fields 

diminish exponentially in the Z-direction. In the coal seam, the fields 

dirni ni sh exponential ly at a rate determi ned by the attenuation constant ( a), 

which in turn depends upon the electrical properties of the coal. An inverse 

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

wave propagates between the highly conducting rock layers bounding the lower 

conductivity coal seam. The fact that the coal rnay have entries and crosscuts 

i s of ~ni nor consequence. 

In the presence of conductors, the picture changes considerably. In this 

case, the effects of these conductors can totally dominate the effects of the 

geol ogy. In general , the presence of conductors (rai 1 s , troll ey 1 i nes , water 

pipes, air lines, phone lines) is always of advantage. 

MF can couple into, and reradiate from, continuous conductors in such a 

way that these conductors become not only the transinission lines, but a1 so the 

antenna system, for the signals. The most favorable frequency depends to some 

extent on the relationship between the geology and existing conductors. The 

frequency effects are quite broad. Anything from 500 kHz to 800 kHz wi 11 

usual ly be adequate. 

SYSTEM DESIGN 

The MF system described here is based upon vehicular and personnel 

t ranscei ver units , base stations , and repeaters. It appl i es prior fundamental 

research in the area of MF and util izes the existing mine wiring network 

(power cables, trol ley line, etc.) to achieve whole-mine coverage. The basic 

system configuration is shown in figures 2 and 3. 

Figure 2 illustrates a minewide repeaterlbase station concept known as 

the global maintenance net. In this configuration, rnobi le units (persons 

using transceiver vests and/or vehicul ar transceivers) can maintain local 

communications by operating at frequency fl. The range of communications in 

this case is solely dependent on point-to-point radio propagation, aided by 

parasitic coup1 i ng. A transmission on f2 causes repeater action to occur, 

permitting the two mobile units to be separated very large distances. To 

achieve this repeater action, it is only necessary for the transmitting unit 

to reach the repeater, either directly or by parasitic effects to the repeater 

1 ine coupler. Communications with a base station are also possible. 

Figure 3 illustrates a local repeater concept constituting a local 

cellular net. This local repeater is known as a "cellular repeater" because it 

illuminates a "cell" or area of the mine, such as a working section, only. 

The antenna for the cellular repeater is a dual wire loop attached to timbers 

or the rib. An interface to the mine telephone system permits communications 

"off section." 
The systern design is distributed in the sense that each net can be operated 

independently of tile other. In practice, a net can be easily installed by 

couyliny a base station (at the portal ) to electrical conductors in the 

wi repl ant (phone 1 i nes , power 1 i nes, etc . ) . Mobi 1 e trdnscei vers operating on 

the dssiyned net frequency co~nmunicate with edch utiler arid the i3dse. Other 

nets use different frequencies and are installed in the salne Nay. 

Mobi 1 e Transcei vers --- -- -.---- 1 

Two types of mobile transceivers have been developed for the systern. 

These transceivers consi st of vest units for i ndi vidual s and vehi cul ar units 

for rolling stock. Functionally the two are equivalent, differing only in 

power levels and physical configuration. These transceivers are shown in 

fi yures 4 and 5.. 

An important human factor problem was solved by the vest design. By 

placing the radio ci rcui t modul es in pockets on the vest, the wei yht and bul k 

of the transceiver have been evenly distributed. The loop antenna is sewn 

into the back of the vest. The pockets are located where medical records show 

less frequency of injury. Sound is directed toward the ears from epaulet 

speakers. A hinged control head is conveniently located on the front. The 

desi yn a1 lows the miners to maneuver in tight quarters and yerforrn normal 

~ni ni ng tasks without catching the radio on obstructions. 

The vehicular unit can be conveniently placed on any mine vehicle. It is 

used in conjunction with a special loop antenna of advanced design that 

produces high magnetic moment. Mechanical ly, the antenna is enclosed in high 

strength lexan and is attached to the vehicle via special brackets. The lexan 

wi 1 1 not break even i f severely fl exed by impact. 

Fi xed Station Antennas 

Besides the mobi 1 e transceivers discussed above, the system a1 so consists 

of fixed transceivers such as repeaters and base stations. (See figures 2 and 

3.) For proper system operation it is necessary that these fixed transceivers 

have very efficient antennas so that the local wire plant can be properly 

illuminated and signals on the wire plant are properly received. This 

efficiency is paramount for who1 e-mi ne coverage. 

The cell ul ar repeaters use dual -1 oop antennas (for transmi t and recei ver) 

attached to the rib or posts in such a way that there is little danger of 

darnage in normal mine activities. The transmit antenna produces a large 

rnaynetic moment that provides the signal for local c e l l u l a r coverage, which is 

usual ly aided by parasitic coupl i ny and reradiation effects. The receive 

antenna is similar. 

The global repeater and base station use a newly designed RF line coupler 

(see figure 6) that permits very efficient coupling to the mine wireplant. 

Like a current probe, the coupler can be easily clamped around local 

conductors. MF signal current fl owi ng through the wi repl ant coriductors 

produces a coupl er output signal (vo), which is appl i ed to the input of the 

base station or repeater. 
Base Station and Repeater 

The base station is intended to be placed where mine management finds it 

most advantageous, usually in the surface office complex or with the 

dispatcher. If desired, the base station can be controlled remotely via signal 

lines that allow the control console to be placed in a surface building for 

convenience, while the actual base transceiver is placed in the mine where it 

can more efficiently couple into the local wiring. Both the global repeater 

and the base station uti 1 ize the RF 1 ine coupler for rnaxirnum efficiency. The 

cellular repeater is generally located in a workiny section. It enables the 

vest to operate as a mobile pager telephone by switching voice signals between 

the locdl pager telephone network and the vest. Vehicular radios can a1 so 

operate in this mode. 

The system was developed around an interchangeable set of plug-in radio 

ci rcui t ~nodul es. The same recei ver, synthesi zer, and transmitter modul es are 

used in the vehicular transceiver, base station, and repeaters. Servicing the 

equipment only requi res troubleshooting to the board level . Since the 

equi pment uses the sarne radio ci rcui t ~nodul es, the perforrnance speci fi cations 

of all transceivers are sirnilar. The signal ing used depends upon the specific 

network requi rernents. A1 1 receivers are designed with an adaptive noiseoperated squelch network that allows every transceiver on the net to hear the 

same message (party line). 

The transmitters are designed with both sub-audible (100 Hz) and in-band 

(1000 Hz) tone oscillators. A sub-audi ble tone is used in the vest 

transceiver to cause the cellular repeater to switch the message (page) to the 

pager telephone network. The repeater incl udes both a noi se-operated squelch 

and a sub-audi ble tone squelch for use in telephone switching. Sub-audible 

tone signaling is useful in identifying "stuck on" transmitters that that can 

block the communications net. In-band signaling is useful in emergency 

situations. 

Svstern Speci fi cati ons 

Emi ssi ons 

Type ........................................................ Narrowband FM 

Occupied BW........................................................lO KHz 

RF frequency.. ....... ..................................... 60 to 1,000 kHz 

Peak deviation................................................. 25 - kHz

Mod frequency ............................................. 0 to 2500 Hz 

Recei ver 

Type ....................................................... Superheterodyne 

Sensitivity .~e.e..e~e~e~ee~e~~~~m~

pV (12 dB Sinad) 

Selectivity ......................................... 8-pole crystal filter 

IF BW 3 d6.................................................12 kHz (min) 

70 dB...............................................ZZ kHz (max) 

RF BW............................ ......................... 60 to 1,000 kHz 

Squelch .................................. .........Noise operated and tone 
Transmitter 

Type ....................................................P ush Pull Class B 

Output power 

Antenna 

Magnetic moment 

Vest ........................................................... 1 AT^? 

Vehicular .................................................... 6 ~~m2 

KF Line Coupler 

Transfer impedance (ZT) 

1-in Coupler 

350 kH~...................................~........~....10.0 ohms 

520 kH~.............................................~......11.2 ohms 

820 kH~.....................................................17.8 ohms 

4-in coupler 

520 kH~...................~....~.~.....~.~~...~..~.~........lO.6 ohms 

PERFORMANCE RESULTS 

To ascertain the actual performance of any mine communication system, it 

is necessary to install that system underground and measure performance over a 

1 ong period of time. Short-term testing is unacceptable and inconclusive. 

The Bureau of Mines addresses this problem by joining with mining companies in 

Memorandums of Agreement to acquire this long-terin performance data. 

Four western inines who have agreed to participate in these performance 

tests are as follows: (a) York Canyon Mine (Kaiser Steel Co., coal); 

(b) Escalantes Mine (Ranchers Exploration Corp., silver) ; (c) Star Point Mine 

(Plateau Mining Co., coal ), and (d) San Manuel Mine (Magma Copper Co., 

copper). Because of space limitations, it is not possible to describe in 

detail the performance of the MF system in a1 1 mines. This will be the 

subject of a future paper. Instead, system performance will be described for 

the York Canyon Mine, where the system has been i n s t a l l e d the longest time. 

York Canyon Mine 

The York Canyon Mine is a combination strip and deep operation located 

near Raton, N. Ma The underground operation mines the York seam, which is 

approximately 6 ft thick. The mining operation is primarily via longwall, and 

all coal is transported out by belt. The geology in this area is highly 

faulted with numerous slips that cause considerable productivity problems. 

The geography is hi1 ly such that the overburden ranges from about 200 to 800 

ft. Primary access to the mine is via four drift entries. 
Facing these entries, one finds the left one is fresh air intake, the 

second is a neutral air supply way, the third is a neutral air beltway, and 

the right is a return airway. Ventilation fans exist in the portal areas and 

at distant ventilation boreholes. A simplified map of the portal area is 

shown in figure 7. 

One of the tremendous advantages of medium frequencies is the ease at 

which a we1 1 -designed system can "i 1 luminate" the existing mine wiring. 

Figure 7 illustrates this for the York Canyon Mine. A single line coupler 

placed around the existing 25-pair telephone line was the only device 

installed. This coupler was physically located about 25 ft into the entry and 

connected to a base station in the maintenance shop via a single-pair line. 

Measurements show that the 1 i ne coupl er functions as a transformer 

consisting of "N" turns of primary and numerous single turns of secondary. As 

an example, if one had several conductors of similar electrical properties and 

placed one through the line coupler, a current il would be induced into this 

line. If a second conductor was also included, it too would have a current il 

induced. If a dozen such conductors were included, each would have il 

induced. It becomes apparent then that to achieve the best performance from a 

1 ine coupl er, it is necessary to include the maximum possible number of 

conductors. A 25-pair phone line, like the one at York Canyon, is a bonanza 

for an MF system. In less favorable circumstances, a little innovation is 

necessary or mu1 tiple couplers can be used. Obviously a reciprocal effect 

exists on receive. 

Precise measurements of signal strength in an underground mine are 

extremely difficult and time consuming. Therefore, for this report performance of the system was based not on signal strength, but rather on the 

talk-listen range of the transceivers (between base and vests, vehicle 

transceivers). This is a more meaningful measure of performance t.o the mine 

operator anyway. To re1 ate tal k-1 i sten information to more quantitative data, 

the refer to the section on system specifications. 

With the base station and repeater configured as in figure 7, communications between the base station and vehicular or ve.st transceivers were 

possible the entire length of the supply way [2,800 m (9,000 ft)]. The vest 

would occasionally produce a poor transmission, but never a poor reception. 

The vehicular transceiver never experienced a problem. In the adjacent 

entries (fresh air on left, belt line on right), the vest would experience 

sl ightly more transmi ssion problems to the base but rarely any reception 

problems. Within a few feet of mine conductors (phone 1 ine, be1 t fire control 

1 ine, etc.) , the vest would experience no problems. Ranges of 4,700 m (15,000 

ft) with the vest were achievable in this manner. Substantial improvements 

are anticipated with the installation of a repeater. 

The MF system has been in operation at the York Canyon Mine since late 

1981. Various hardware improvements have been continual ly made, but performance is essentially as stated. The mine personnel use the system on a daily 

basis and have found it to be re1 iable and time-saving. 

Problems so far have been mostly nuisance in nature. For example, 

1 ead-in cables on vehicular transceivers would often break. Sometimes when 

vehicles were brought outside in the rain, the enclosures would leak, causing 

ci rcui t fai 1 ures. These and simi 1 ar problems are bei ng sol ved. 
CONCLUSIONS 

A whole mine radio system has been developed for underground mines based 

on rnedi um frequency principles. The program was initiated because of a need 

for more effective communications for improved productivity and safety. The 

system permits underground personnel to maintain communications almost anywhere in the rnine, and with the surface complex. As a consequence, 

comrnunication is possible to the point of equipment breakdown, a fact of 

si gni ficance in irnprovi ny productivity by reducing downtime. 

Parasitic coupling into existing mine conductors insures extensive 

coveraye. No special cornmunications cab1 e need be instal 1 ed. Base stations 

and repeaters can be installed in minutes in locations of prirne mine power. 

Backup batteries are incl uded for additional re1 iabi 1 i ty. 

Portabl e uni ts consi st of vehi cul ar and personnel t ranscei vers. The 

vehi cul ar transcei vers uti 1 i ze a newly desi yned antenna of exceptional 

efficiency and durabi 1 i ty. The personnel transceivers uti 1 i ze a unique vest 

concept that permits a low profi 1 e and affords excel 1 ent user convenience. 

Extensive tests have been run in numerous underground coal and metal 

mines. Performance has been very yood in all cases. These tests will 

continue into 1983 to ascertain how well the design perfvr~ns under long-term 

conditions of the harsh rni ne envi ronment . 
REFERENCES 

1. Artiiur D. Little Propagation of Radio Naves in Coal Mines, Task Order 1, Task 

F. Contract H0346045. 8ulvlines Ope11 Fi le Rei>t. 46-77, 1977, 183 pp. ; avai lable for 

cor~sul tdti on at the Hureau of lrli nes 1 i brari es in Pittsburgh, Pa., Denver, Col o., 

Spokane, Wash., Twin Cities, Minn., and Murgantown, W. Va.; and at the Central 

Librdrj, U.S. Ilepartl~ient of the Interior, Washington, D.C. Printed cc~py available 

f rorn the National Technical Inforilldti on Service, Springfield, Va. 22161, PB 265 

858/AS. 

2. Austin, B. A. Underground Radio Corllrnunication Techniques and Syste~ns in 

South African Mines. Proceedi1igs, Electro!nagrletic Guided Waves in Mine Envi ronlnents Workshop, Go1 den, Colo., 19723, pp 87-102. 

3. Col 1 i ns Cornrnercial Telecornlnunications. Electromagnetic Noi se and Propagatiorl 

in Low Coal [\lines at Mediuln Frequencies. Contract H0377053. HuMines Open Fi 1 e No. 

63-82, 1982, 94 pp.; available for consultation at the Hureau of tlin?s libraries 

in Pittsburyh, Pa., Derlver, Colo., Spokane, Wash., Twin Cities, Minn., and 

ivlorgantown, W. Va.; and at the Central Library, U.S. Ilepartrnent of the Interior, 

Washingtorl, D.C. Printed copy avai 1 able frorti the National Technical Inforrnation 

Service, Sprinyfield, Va. 22161, NTIS No. PB 82-202656. 

4. Cory, T. S. Propayation of EM Signal in Underground Mines. BuMines Contract 

H0366028, Col 1 ins Radio Group. HuMines Open File Rept. 136-78, 1977, 158 py. ; 

avai 1 able for consul tation at the Bureau of Mines 1 ibraries in Pi ttsburyh, Pa., 

Denver, Colo., Spokane, Wash., Twin Cities, Minn., and Moryantown, W. Va.; and at 

the Central Li brdry, U.S. Department uf tile Interior, Washington, D.C. Printed copy 

avai 1 able frorn the National Technical Infor~nation Service, Springfield, Va. 22161, 

PI3 289 757/AS. 

5. Lagace, R. L., A. G. Ems1 ie, and M. A. Grosslnan. Model ling and Data Analysis 

of 50 to 5,000 kHz Radio dave Propagation in Coal lilines, Task Order 4 UuMines 

Contract H0346045, Arthur 0. Little, Inc. t3ulYlines Open Fi le Kept. 83-80, 1980, 109 

pp.; available for consultation at the Rllreau of I'llines libraries in Pittsburyh, 

Pa., Denver, Colo., Spokane, Wash., Twin Cities, Minn., and Morgantown, W. Va.; and 

dt the Central Library, U.S. Oepartinent of the Interior, Washington, D.C. Pririted 

copy avai 1 able froin the National Technical Inforrnation Service, Springfield, Va. 

22161, PI3 80-209455. 
Coal 

or 

entry 

FIGURE 1. - The coal seam mode 

q station 

?-LI Console 

FIGURE 2. - Global repeater concept 
FIGURE 4. - Vest transceiver for personnel 

134 
1 r RFI~~L- , I coupler 

I I 

4 Wire plant 

I 

EQUIVALENT 

NETWORK ACTUAL NETWORK 

FIGURE 6. - RF line coupler for base stations and repeaters 
FIGURE 7. - Portal area of the York Canyon Mine