MEDIUM-FREQUENCY PROPAGATION IN COAL MINES 

H. Kenneth Sacks 

Supervisory Electrical Engineer 

Robert L. Chufo 

Electrical Engineer 

Pittsburgh Mining and Safety Research Center 

Bureau of Mines 

U.S. Department of the Interior 

Pittsburgh, Pa. 

ABSTRACT 

Medium-frequency (MF) waves can propagate in a coal seam, bounded 

above and below by conducting rock, in an approximate transverse electromagnetic (TEM) transmission-line mode with the electric field vertical 

and the magnetic field horizontal. The theory of this coal seam mode 

is applied to recent MF data for a number of mines in order to classify 

the propagation characteristic of the mines in a simple way. 

When conductors such as rails, trolleylines, or power cables are 

present in the vicinity of the measurement path, it is found that the 

magnetic field versus range plots have two distinct regions with very 

different slopes. This effect is attributed to coupling of the cylindrically spreading coal seam mode to a low-attenuation conductor guided 

mode. As a result, voice communications range between mobile transmitters can extend several thousand meters. The use of MF for mine 

communications is discussed. 

INTRODUCTION 

MF wireless radio satisfies communication needs similar to those 

met by ultra-high-frequency (UHF)/leaky feeder systems.(l)* However, MF 

systems make use of existing mine wiring for range exten5ion and do not 

require the installation of leaky feeder cable in the extensive complex 

of mine tunnels. 

It is known that UHF radio suffers from the severe disadvantages of 

high losses when communicating around corners and through barricades.(2) - 

UHF signals are confined within the boundaries of mine tunnels. MF 

signals, on the other hand, travel relatively well through the coal and 

rock. 

* Numbers in parenthesis refer to references at the end of this paper. 
MF wireless radio is intended to satisfy the communication needs of 

roving miners as well as the haulage communication requirements of 

trackless/trolleyless vehicles. The miners involved are typically 

section foremen, maintenance workers, ventilation crews, and machine 

operators. An effective MF radio system would answer the following 

communication needs: 

1. Miners working in basic room-and-pillar sections and in sections 

with special needs, such as longwalls and sections having full-dimension 

haulage systems. 

2. Miners operating vehicles in mines with trackless and trolleyless haulage systems in which traditional trolley wire carrier systems 

are not useful. 

3. Members of rescue teams during mine rescue operations, thereby 

removing the need for inconvenient trailing wires. 

The underground mining industry's extensive experience with trolleywire carrier-phone systems has shown that a mine's electrical wiring, 

tracks, trolley wires, phone lines, and air lines can be used to carry 

low-frequency signals throughout undergrountl mines, even to locations 

some distrance from the trolley wire-rail haulage system. Early in 1975 

the Bureau of Mines bought and tested underground some prototype 335-kHz 

portable transceivers that were manufacturetl in South Africa. The 

wireless communication ranges obtained with these units were quite good 

and, in fact, much greater than originally anticipated from theoretical 

predictions. However, the tests left some doubt as to whether 335 kHz 

was the most favorable operating frequency for U.S. mines. 

To better define MF communications technology, the Bureau of Mines 

initiated a measurements and hardware development program. Its output 

would be a MF unit to meet the portable wireless communication needs 

that remained unsatisfied by UHF radio. To be successful it is necessary 

to determine which operating frequencies are best suited to wireless 

applications in U.S. room-and-pillar coal operations and to design 

associated portable equipment rugged and small enough for use by the 

mining industry. 

PROPAGATION MEASUREMENTS AND EXPERIENCE IN COAL MINES 

The initial set of comprehensive measurements were made in a 

conductor-free area of Consolidation Coal's Ireland mine in the Pittsburgh 

seam. A 60- to 2,000-kHz frequency range was covered. An analysis of 

the data showed the following: 

1. Longer communication ranges tha.n ariticipated were achieved (up 

to 500 m) in conductor-free areas. 

2. Maximum ranges occurred at higher frrequencies than expected 

(between 300 and 900 kHz). 
3. Propagation was not influenced by the network of tunnels in the 

coal seam. 

4. The best performance occurred when the planes of the loop 

antennas were oriented vertically and parallel to each other. 

Subsequent measurements at other mines have shown that propagation 

in conductor-free areas is strongly dependent on coal seam properties. 

Figure 1 shows the results of measurements in six coal seams and clearly 

demonstrates the wide variation in propagation loss from seam to seam. 

Original work indicating the existence of a low attenuation mode in 

a coal seam was published in 1976 by J. R. Wait.(3) Further analysis 

has been recently done by A. G. Emslie and R. L. ragace (4) in which the 

coal seam is treated as a waveguide propagating a TEM wave. The model 

assumes the mine can be represented as three layers consisting of coal 

bounded by rock overburden and underburden and has shown good agreement 

between theory and experiment in the frequency range of 100 to 1,000 

kHz. 

OPERATING FREQUENCIES IN CONDUCTOR-FREE AREAS 

The data analysis and theoretical modeling of coal seam waveguide 

radio wave propagation presented above provides a firmer foundation than 

previously available on which to base the selection of operating frequencies 

for wireless mine communication in the MF band. Large variations in 

signal attenuation rate have been found between the three coal seams 

investigated which are widely separated geographically. The Pittsburgh 

seam measured in northern West Virginia was found to be the most favorable, 

while the Herrin No. 6 seam measured in southern Illinois was found to 

exhibit extremely high loss. 

Figures 2 and 3 show theoretical calculations for received field 

strength at 200 and 400 my respectively. The parameter values are the 

average values deduced from the mine data shcwn in Figure 1 for the 

Pittsburgh, Pocahontas, and Herrin seams. 

Also plotted on these figures for comparison are magnetic field 

noise levels versus frequency. These noise levels represent the upper 

bound envelope of average rms values found in active areas of coal mines 

and a lower bound noise level representing the intrinsic receiver noise 

for a narrow band frequency modulated (FM) radio receiver having an 

intermediate frequency (IF) bandwidth (B) = 12 kHz, noise figure (F) = 6 

db, and loop antenna effective turns area (NA) = 1 m2. The active mine 

noise levels were taken from data measured at spot frequencies in three 

coal mines by Bensema, Kanda and Adams (5) - (6) - (7) - of the National Bureau 

of Standards. 
Examination of Figures 2 and 3 reveals that performance is best for 

the combination of lowest coal conductivity and highest rock conductivity. 

Increasing coal conductivity increases the shunt loss in the coal seam 

waveguide, while decreasing the rock conductivity increases the series loss, 

both of which increase the signal attenuation rate. Furthermore, as this 

attenuation rate increases, the frequency behavior changes from a broad 

bandpass filter type of characteristic centered around 1 MHz for low attenuation rates like those found in the Pittsburgh seam, to an increasingly 

attenuated low pass filter characteristic having a steadily decreasing cutoff 

frequency as the attenuation rate increases to values like those found in the 

Herrin No. 6 seam. This trend toward the attenuated low pass filter behavior 

is, of course, more accentuated for the longer communication ranges, i.e., 

40 m versus 200 m. 

The most favorable frequencies for communicating these ranges can be 

estimated by comparing the signal level curves with the noise level curves 

plotted on each figure. The most favorable operating bands are seen to be 

those where the slopes of the signal and noise curves are equal or nearly 

equal. Since the signal curves also represent levels for the largest, 

intrinsically safe transmit moment, an absolute measure of performance 

can also be estimated by noting the number of db that the signal level 

curve lies above the noise level curve at the frequency of interest. 

For example, moderate performance (occasional message repetitions required) 

requires that the average rms carrier-to-noise ratio at a FM receiver be 

at least 10 db or better for the mine noise condition prevailing during 

the message transmission. 

The equivalent receiver noise curve is probably the most representative 

for communication in conductor-free areas located at least one or two entries 

(i.e., separated by one or two pillar widths) away from mine electrical conductors carrying high noise currents. On the other hand, the active mine 

upper bound noise curves most likely give an overly pessimistic view. In 

this latter instance, these levels occur in the immediate vicinity of highpower machinery and electrical conductors, locations where the signal will 

also be enhanced by the presence of the conductors. Thus an active area 

noise curve approximately half way between the two noise extremes is 

probably more indicative of worst case "conductor-free areau noise levels. 

These levels are likely to occur in transition regions between conductorfree and conductor-present locations. 

Figure 2 reveals that conductor-free area portable-to-portable radio 

ranges on the order of 400 m will be attainable only in highly favorable, 

low-attenuation seams like the Pittsburgh seam. Furthermore, the performance will occur only at operating frequencies between about 900 kHz and 

2 MHz and under noise conditions approaching the receiver-noise-limited 

case. In addition, Figure 3 illustrates that conductor-free ranges of 

200 m will not be attainable in high-loss seams such as the IIerrin No. 6 
seam, even at frequencies below 100 kHz and under receiver-noise-limited 

conditions. The outlook is somewhat improved, but not much, for moderateto-high-loss seams like the Pocahontas No. 3 seam, where 200 m ranges should 

be possible over a broad band of operating frequencies between about 150 kHz 

and 1.5 MHz under receiver noise limited conditions. 

On a more positive note, the in-mine measurements of Cory and others 

have also shown that greatly improved radio communication ranges are obtainable in coal mines when the portable units are located in a tunnel having elec 

trical conductors such as a power cable, pagerphone line, or trolley wire/rail 

line. Significant range improvements have even been experienced with one and 

sometimes both of the units located in a tunnel separated by a coal-pillar 

width from the tunnel with the conductors. In some cases improvements have 

also occurred with one of the units located in a tunnel separated by two coal 

pillars from the tunnel having the electrical conductors. Since these conductors are generally located in haulageways and working sections where miners 

requiring communications are also located, this propagation condition is an 

important one. Moreover, it is one that may allow the desired radio communication ranges to be achieved and exceeded even in coal seams having high 

signal propagation attenuation rates. Figure 4 shows data taken in three 

mines near conductors that illustrate the range-enhancing effect of 

conductors. Additional data are presently being taken in the vicinity of 

mine electrical conductors and analyzed to quantitatively model and 

assess the impact of such conductors on the attainable radio communication 

range. 

HARDWARE DEVELOPMENT 

The Bureau is currently developing a MF system for mobile and 

portable communication capable of covering a small mine using belt haulage 

and battery-powered vehicles. The overall system is indicated in Figure 5 

in which a repeater is used to provide whole-mine coverage from portable to 

portable and from portable to base. Tentative specifications are shown in 

Table 1. 

An operating system is expected to be completed by the fall of 1979. 

SUMMARY 

Prototype MF wireless radios have been tested in underground mines 

representing coal seams of several different conductivities. These tests 

were conducted as part of the Bureau's program to investigate the propagation characteristics of mines. The test results in the 60- to 2,000- 

kHz range indicate a conductor-free area propagation range of 150 to 500 m 

and a range of over 1,000 m in the presence of mine conductors. The range 

at a specific frequency in conductor-free areas is largely dependent on 

the conductive properties of the coal and, to a lesser extent, on seam 

thickness and conductive properties of the surrounding rock; in areas 

with conductors these properties are much less important. 
Further underground evaluation of prototype MF wireless radios for 

both personnel and vehicular use as well as evaluation of various repeater 

schemes will be performed in conjunction with propagation measurements. 

The results of these equipment tests and propagation loss measurements 

will be used to improve the design and performance and to determine system 

design guidelines for future mine wireless radio equipment. 

REFERENCES 

1. Chufo, R. L., "Vehicular Communications 2,500 Feet UndergroundYVV 

IEEE 27th Annual Vehicular Technology Conference Record, Orlando, 

Fla., March 1977, pp 161-165. 

2. Emslie, A. G., R. L. Lagace, and P. F. Strong, "Theory of Propagation of UHF Radio Waves in Coal Mine Tunnels," IEEE Trans. on 

Antennas and Propagation, V-AP-23, No. 2, 1975, pp 192-205. 

3. Wait, J. R., "Note on the Theory of Transmission of Electromagnetic 

Waves in a Coal Seam," Interim Summary Report, Analytical Investigation of Electromagnetic Fields in Mine Environments, USBM contract 

H0155008. November 1976. 

4. Emslie, A. G. and R. L. Lagace, "Medium Frequency Radio Propagation 

and Coupling in Coal Mines," Proceedings, Electromagnetic Guided 

Waves in Mine Environments Workshop, Boulder, Colo., March 1978. 

(Special Report, USBM contract H0155.008 .) 

5. Bensema, W. D., M. Kanda, and J. W. Adams, "Electromagnetic Noise 

in Robena No. 4 Coal Mine," U.S. National Bureau of Standards 

Technical Note 654. Bureau of Mines contract H0133005, April 1974. 

194 pp. Available from Nationzl Technical Information Service, 

Springfield, Va. 22161, reference Cat. # C13.46:654, $1.50. 

6. Bensema, W. D., M. Kanda, and J. W. Adams, llElectromagnetic Noise 

in Itmann Mine," U.S. National Bureau of Standards, NBSIR 74-390. 

Bureau of Mines contract H0133005, June 1975, 112 pp. Available 

from National Technical Information Service, Springfield, Va. 22161, 

reference COM 741-1718/AS, $6.50. 

7. Kanda, M. J. W. Adams, and W. D. Bensema, "Electromagnetic Noise in 

McElroy Mine," U.S. National Bureau of Standards, NBSIR 74-389. 

Bureau of Mines contract H0133005, June 1974, 170 pp. Available 

from National Technical Information Service, Springfield, Va. 22161, 

reference COM 741-1717/AS, $8.00. 

8. Cory, T.S., "Propagation of EM Signals in Underground Mines," 

Collins Radio Group, Rockwell International Corp., Final Report 

USBM contract H0366028, April 18, 1977. Distribution pending. 
TABLE 1.--Portable transceiver specifications 

Frequency range ...................... 300-1000- to 10-kHz steps 

Channels ............................. 2 transmit, 1 receiver 

for repeater operation 

and talk around 

Modulation ........................... SSB 

Antenna .............................. Detachable loop, bandolier 

Power out ............................ 1 watt PEP 

Squelch .............................. Self-adjusting relative to 

noise background 

Weight ............................... Less than 1 kg 
FIGURE 1.--Composite plot of signal attenuation rates in db/100 ft 

for six mines in three different coal seams 
FIGURE 2.--Theoretical values of H as frequency for Pittsburgh, 

Pocahontas, and Herrin coal seams at a range of 200 m 

27-9 
M313W / 3M3dWVOM31w I er qp H 

FIGURE 3.--Theoretical values of H vs frequency for Pittsburgh, 

Pocahontas, and Herrin coal seams at a range of 400 m. 
i oodz 

FIGURE 4 .--Communication range in three seams in areas with conductors 
FIGURE 5. Medium-frequency communications using a repeater