AN ASSESSMENT OF LEAKY FEEDER RADIO SYSTEMS 

IN UNDERGROUND MINES 

James C. Cawley 

U.S. Department of the Interior 

Bureau of Mines 

Pittsburgh Research Center 

Pittsburgh, PA 15236 

ABSTRACT 

A summary is presented of existing leaky feeder communications 

systems currently in operation in North America. Background information pertaining to previous use of leaky feeder technology is 

discussed along with the survey methodology. A review of some 

pertinent facts concerning propagation in tunnels and specific 

reference to mine tunnel propagation is presented. A summary of 

the characteristics of each mine visited is presented in tabular 

form, and methods of procurement and maintenance are discussed. 

The role of user training in efficient system use is presented. 

Seven sites representing eight mines are included in the survey. 

Economic and technical topics are considered. 

INTRODUCTION 

Leaky feeder radio systems have become a well established technology for use in areas where normal radio propagation cannot be 

supported, such as railroad tunnel applications, or where a special 

geometry must be superimposed on the radiated signal as in Highway 

Advisory Radio Information systems. As a natural outgrowth of 

railroad and other transit applications, leaky feeder radio systems 

have found appl i cati ons i n underground mi ni ng . Extensive use has 

been made of leaky feeder radio systems in Great Britain where the 

National Coal Board (NCB) has been involved in experimentation and 

application of such systems for over a decade. Leaky feeder radio 

systems have become the backbone of British mine communications. 

Similar applications are also found in France, West Germany, and 

Belgium. 

The Bureau of Mines installed a whole mine leaky feeder system 

in the United States at Bethlehem Steel's Grace mine in Morgantown, 

Pennsylvania (1 )*. Technical ly , the system functioned as predicted, and i t-was anticipated that leaky feeder radio systems 

* Underlined numbers in parentheses refer to list of references at 

the end of the paper. 
would find rapid acceptance in industry. During the next few years, mines began to implement the leaky feeder concept. It seems logical, based upon the NCB and Bureau of Mines experiences with leaky feeder 

systems that such systems should earn a respectable share of the U.S. 

mine communications market, especially in light of the trend toward 

nontrolley haulage in newer U.S. mines. In order to assess the state of 

leaky feeder communications, the Bureau has sponsored a study of leaky 

feeder radio sys tems in underground mines (2). - 

The study uncovered an interesting array of information concerning 

system configuration, uses, miner acceptance, installation and maintenance costs and mine management's attitude toward the systems. The 

objective of the study program was to investigate as large a sample as 

possible of mines employing leaky feeder technology. Since no central 

source of information could be found, leads were obtained from radio 

equipment and cable manufacturers as well as those supplied by Bureau 

personnel. Economic as well as technical data were gathered during the 

survey. Where hard data were not obtainable, best estimates were 

attached by the survey team in order to evaluate all mines on a common 

basis. After all obtainable leads had been compiled, telephone contact 

was made with each mine to learn if leaky feeder communications were 

employed at the site. From this data base, it would be possible to 

select up to nine mines for information-gathering visits. 

PROPAGATION IN TUNNELS 

The use of radio in underground mines has been frustrated due to 

relatively poor propagation characteristics associated with mine tunnels. 

Researchers have modeled the underground tunnel as an imperfect waveguide 

whose walls partially absorb and partially reflect incident electromagnetic 

radiation. Propagation, therefore, is a function of both the tunnel 

dimensions and the surrounding material comprising the tunnel floor, 

ribs, and roof. Measurements of radio propagation characteristics along 

straight tunnel s correlates to what one would expect of such a waveguide. 

Attenuation, expressed in db, varies linearly with distance and exhibits 

large losses around corners, bends, or any deviation from a straight 

path. Surface irregularities of the walls also contribute to path 

losses. Typical results of propagation losses in mine tunnels are shown 

in Table 1. 

These measurements, made by Goddard (3), show a significant difference in communication range with antenna pclarization. This fact is 

most noticeable in mine tunnels having one dimension significantly 

greater than another. Antenna polarization aligned with the longest 

tunnel dimension will yield the greatest communication range, a property 

exhibited in Table 1. The more nearly a mine tunnel approximates a 

square the less noticeable this polarization effect will become. The 

waveguide propagation mode requires tunnel dimensions greater than a 

wavelength of the radiated signal in order to support propagation. Range 

is also a weak function of both receiver sensitivity and transmitted 

power. 
Table 1 .--Propagation characteristics in a coal mi& 

7 ft x 14 ft tunnel cross section 

Freq. Polarization Coupling Attenuation Bend Approx. range (ft) 

loss loss Straight One 

(MHz) (db) (db/100 ft) (db) line bend 

200 Vertical 65 15 - 480 - 

41 5 Vertical 78 6.5 35 908 369 

41 5 Horizontal 40 5.9 35 1644 1051 

1000 Vertical 65 4.3 36 1674 837 

1000 Horizontal 57 2.5 38 3480 1960 

The total propagation loss (between isotropic antennas) is determined from the table using the following relationship: 

where L = total propagation loss, 

Lo = fixed (coupling) loss, 

A = attenuation/100 ft, 

D = distance between transmitter and receiver (in 100 ft) , and LC = loss associated with a 90" bend. 

The above equation should only be used for distances of D > 100 ft. To 

appreciate the significance of the relationship between propagation 

losses and effective communication distances between handheld portable 

radios, consider the following: 

The effective radiated power of a typical handheld portable is 1 W (0 

(0 dbW), and its receiver sensitivity is 1 pV into 50 ohms (-137 dbW). 

Therefore, the maximum tolerable propagation loss is 137 db. Using 

this information, Table 1 can be used to determine the maximum communication range for this mine along a straight tunnel and around one 

90" corner, assuming the same receiver sensitivity and transmitted power 

are used. It should be emphasized that the propagation numbers shown 

in Table 1 are highly variable between mines. Overburden characteristics may cause significant increaseseor decreases in propagation 

characteristics. Each mine should have measurements run to determine 

optimum frequencies and ranges of communications as part of any radio 

system procurement. Mines in our survey used 150-MHz and 450-MHz 

systems. The 150-MHz radios were generally used where there was no 

desire to communicate away from the main haulage tunnel. Line attenuation and coupling losses are minimized at 150, making it an attractive 

choice for large systems. The 450-MHz radio can be used for haulage 

communications and has significantly greater range when used in areas 

away from the leaky feeder cable as a point-to-point transceiver. One 

manufacturer now has 800-MHz equipment available which should enhance 

communications in tunnels of smaller cross-sectional area. 
SURVEY DESCRIPTION 

This project involved data collection, data evaluation, the drawing 

of conclusions, and recommendations concerning the usefulness and degree 

of imp1 ementation of 1 eaky feeder radio systems in the underground mines 

of North America. Previous studies of leaky feeder cables have resulted 

in numerous recommendations for their use in mines, and a number of such 

systems have been installed and are currently in use, some for a number 

of years. In view of this, it was felt that there should be a body of 

experience available that may or may not support earlier speculation 

on the usefulness of leaky feeders in a mining environment. 

After an exacting questionnaire was developed, a two-person survey 

team visited up to nine selected mines known to employ some form of 

leaky feeder radio system. The two team members were selected with 

backgrounds specifically oriented toward the needs of the study. One 

team member is a communications engineer with extensive experience in 

two-way radio systems; the second member has extensive experience in 

mining, mine safety, and human factors relative to mining operations. 

During these visits, mine personnel responsible for the maintenance 

and/or management of the radio system were interviewed. 

A typical mine visit consisted of an in-depth interview with the 

individual at the mine responsible for radio system maintenance. The 

interview generally lasted about 2 to 3 hours and covered general mine 

information, technical and maintenance information, operational 

information, cost information, and a subjective discussion of the 

benefits and problems encountered in the use of leaky feeder radio. A 

detailed review of system configuration coupled with a performance check 

of system components such as repeaters, base stations, etc., was performed where permitted. Table 2 summarizes the specific characteristics 

found in each mine surveyed. 

SYSTEM PROCUREMENT 

The method of procurement was the same for nearly every system surveyed. Requests for quotation had been sent to several radio equipment 

manufacturers, but only one manufacturer would respond; therefore, this 

manufacturer installed all of the leaky feeder systems studied in the 

survey. As a rule, the mine operators had not prepared any detailed 

functional description other than expressing the desire for "good 

communications" in the covered areas. Systems were specified and 

installed, often with a performance guarantee, but the survey team 

could uncover no evidence that the level of performance was ever specified in writing. 

A typical system in the survey consisted of about 3 miles of 

1 /2-i nch 1 eaky feeder cab1 e with three repeaters. Twisted, shielded 

pairs interconnected the repeaters, and a voting comparator drove the 
Table 2 ... Communications systems summary (3) 

. 

Descriptions Mi nes Total s 

ABCDEFG 

Type of mine: 

Hardrock ...................... X X X 3 

Coal .......................... X XXX 4 

Surface communications: 

Two-way radio ................. X X X XXX 6 

PBX telephone (mine owned) .... X X X X 4 

Underground communications: 

Pager-phone (no dial) ........ X XX 3 

Pager-phone (dial ) ........... X 1 

Leaky feeder radio system: 

Leaky feeder cable ......... X X XXXX 6 

Twi n-1 ead cab1 e ............ X 1 

Radio paging via leaky 

feeder ..................... X 1 

Carrier current radio system . 0 

PBX dial telephone system .... X X 2 

Characteristics of leaky feeder 

system: 

Mu1 tiple repeaters ........... X X X XX 5 

Receiver voting scheme ....... X X XX 4 

Hardwire repeater interconnect X X X X X 5 

150 MHz band ................. X X X 3 

450 MHz band ................. X XXX 4 

Type of radio equipment used: 

Standard radio repeater ...... X X X X X X X 7 

Standard mobile radios ....... X XXX 4 

Railroad mobile radios ....... X X 2 

Repackaged mobile radios ..... X X 2 

Portable handheld radios ..... X X X 3 

Radio pagers ................. X 1 
repeaters in parallel from another audio pair carrying the voted signal. 

(See figure 1 .) No system surveyed used the "one way" or daisy chain 

concept as do NCB installations. Initial purchase prices for each 

system ranged from a low of $35,000 at mine E for a single repeater, 

five mobiles, and approximately 7,000 ft of 112-inch leaky feeder cable, 

to a high of over $800,000 at bine A for six repeaters, 50 portables, 37 

mobiles, and 15 to 18 miles of 718-inch leaky feeder cable. Mine E 

purchased six systems simultaneously. (See figures 2, 3, 4.) The 

typical system depicted in figure 1 would cost about $70,000 if configured with 10 mobile radios and 10 portables. 

In general, unless there is some overriding maintenance consideration, it is always cheaper to configure a leaky feeder system with 

112-inch cable instead of 718-inch cable. Figure 5 illustrates a 

hypothetical 4-mile 1 eaky feeder system using 112-inch cable, requiring 

a repeater in every 5,000-ft section and a 718-inch cable requiring a 

repeater in every 10,000-ft section. Obviously, if 718-inch cable is 

used in a mine system other considerations, such as repeater reliability, downtime, or difficulty in repeater maintenance must be considered of sufficient importance to offset the sizable cost difference 

shown in figure 5. 

System specifications during the procurement phase was an area 

uni versa1 ly neglected by mine operators. Consul ti ng firms with expertise in leaky feeder appl ications are avai lable, and mine operators 

would be wise to have a system acceptance specification as well as a 

system operational specification prepared as part of a bid package. The 

cost of such planning would be cheap insurance to those contemplating a 

sizable leaky feeder installation. 

SYSTEM MAINTENANCE 

System size, as mentioned in the previous section, varies considerably. Obviously, this has a direct influence on the budget 

required for system maintenance. The quality of maintenance was another 

factor that varied a great deal. The in-house maintenance program at 

mine A proved to be the highest quality maintenance found during the 

study but it also proved to be the most expensive at $28,000 annually. 

Mines B, C, and D had contracted maintenance to an outside firm but 

did routine equipment cleaning and cable repairs on an in-house basis. 

Maintenance expendi tures at these mi nes averaged approximately $7,200 

annually. Mines F and G utilized only contract maintenance. Portable 

equipment was removed by mine personnel and serviced in the contractor's 

repair shop. Repeaters were repaired on site only as necessary. This 

arrangement cost each mine approximately $5,000 annually. 

The systems operated by mine A and mine E present an interesting 

contrast and vividly illustrate the importance of quality maintenance 

to the success of a leaky feeder radio system. Each system is discussed in more detail in the following paragraphs. The ore haulage 
system at mine A consisted of one inbound track and one outbound track 

each approximately 10 miles long. Both tracks occupied the same tunnel. 

The leaky feeder system provided re1 iabl e communications along the 

haulage tunnel and ore loading areas. The mine is relatively new, 

and the leaky feeder system was installed as the original communication 

system. Due to the continuous communication provided by the leaky 

feeder system, five inbound and five outbound trains may use the haulageway simultaneously. Mine management stated that the Mine Safety and 

Heal th Administration (MSHA) , U .S. Department of Labor, has approved 

a 10-train haulage system only because of the reliable, continuous 

communication provided by the leaky feeder radio system. 

outs 

down 

During the first year, mine A contracted all maintenance to an 

ide organi zation. Thi s arrangement proved unsati sfactory because 

time was excessive. Due to the remote location of mine A, outside 

service personnel might not respond to a call for 4 to 8 hours. In the 

event of a communications failure, ore haulage was limited to one inbound and one out-bound train, reducing mine output by 80 percent 

for the duration of the failure. Obviously, this situation could not 

be tolerated. 

Today, mine A has an in-house staff of one supervisor and six FCClicensed technicians who maintain the leaky feeder system along with the 

mine telephone system, pager-phone system, and a surface radio system. 

It was observed during the site visit that the quality and professionalism of the electronic maintenance staff was outstanding. A regular 

schedule of preventative maintenance and performance testing is performed every 6 months or as part of normal servicing. This system was 

installed simultaneously with the opening of the mine. No former basis 

of comparison exists for economic consideration; however, management 

states that production would be limited to onefifth of its present level 

because of a two-train operating limit. Mine management fully supports 

the leaky feeder system and is convinced that the $800,000 procurement 

can be soundly justified. 

A totally different experience occurred at mine E. Six single 

repeater systems Kere purchased for installation in six small, shortlife coal mines. A typical system is shown in figure 4. Of the six 

systems originally installed, only two remain. Several of the mines 

have been depleted, and one has converted to the use of carrier phones. 

Neither of the two remaining systems is operationally up to standards. 

The survey team checked the repeater transmitter (normally rated 70 W) 

in one mine and found the output to be only one-ha1 f W. The leaky 

feeder cable itself was in poor repair owing to poor splicing techniques 

and the method in which the cable was handled by maintenance crews. 

When moving sections of cable, the crews would cut it down and use a 

utility jeep to drag the cable to its new destination. As a result, 

much of the re-used cable was stretched and had severely abraided 
jacketing, allowing moisture to seep in, damaging the conductors. A 

lack of proper maintenance instruction was also evident in the splicing 

techniques used* The mine maintenance personnel would strip the cable 

to be spliced, twist the center conductors together, and permit the 

outer conductor to remain open. Not only did this cause line impedance 

problems but it also provided a place for moisture to enter the cable. 

A time and material maintenance contract had been in effect with a 

1 ocal repai r organi zation. The local repai r organi zation had convinced 

the mine personnel that only FCC-1 icensed technicians could perform 

routine tasks such as changing microphones. An additional problem 

occurred when personnel changes in the maintenance department left no 

one to supervise radio system maintenance. The outside maintenance 

organization was allowed to be summoned by virtually anyone and with 

no subsequent verification that the calls had been made or that service 

had been adequately performed. Mine management terminated maintenance 

on the system as repair costs surpassed $15,000 monthly. A new supervisor has been appointed to oversee the system, but management has 

decided to abandon all leaky feeder systems. 

The difference between mine A and mine E seems to be one of the 

commitment made by mine management to maintain the system. With the 

exception of mine E, maintenance costs of the systems studied were not 

excessive. The system at mine E performed satisfactorily until it was 

allowed to degrade. Comments such as "dislike" and "distrust" are heard 

from the miners, as well as other expletives. Mine A management considers 

the system to be "uneconomical " and "total ly unsatisfactory. " 

USER TRAINING 

There was no organized training of mine personnel in the use of 

portable or mobile equipment, and several problems were recognized as a 

result. Primarily, when miners did not understand the purpose of the 

system, they tended to speculate that mine operators only wanted to 

"spy" on them. Reacting to a perceived threat to independence, a miner 

in this situation will tend to abuse equipment more than one who has a 

clear understanding that the system will be of personal benefit by 

increasing information flow during emergency situations as well as save 

time during major equipment outtages. Routine location of key personnel 

underground was one of the main uses of the studied systems. 

Sufficient recognition must be given to the miner's and, to a 

lesser extent, the first line supervisor's resistance to change. This 

resistance can be lessened by a short training class explaining the use 

of the system and how the equipment can be operated efficently. Emphasis 

should be placed on the fact that the radio system will save needless 

traveling within the mine and that its purpose is not surveillance of 

i ndi vidual personnel . 
Several mines reported that after an initial period of adjustment miners who carried portable radios regarded them as a status symbol, 

designating them as VIP's in the mine operating structure. In these 

situations, there was significantly less abuse of radio equipment. 

CONCLUSIONS 

(1 ) All of the systems studied were supplied by a single radio 

manufacturer. Little variation was evident in the systems and 

they tended to be routine adaptations of surface radio technology. 

(2) Five systems used 1/2-inch leaky feeder cable. Mine A used 7/8-inch 

leaky feeder cable, and mine B used RG-86U, an obsolete 200-ohm 

twin-lead. 

(3) It is less expensive to configure a system with 1/2-inch than 

7/8-inch leaky feeder cable. 

(4) In-house techni cal expertise was general ly nonexis tent, and consul ting expertise was not solicited during the planning phase of the 

procurement. Such expertise would benefit the mine in the 

purchasing phase. 

(5) In all cases but one, the systems were purchased without 

genera ti ng operating or acceptance speci f i cations . 

(6) The mines did not sufficiently investigate the suppliers of 

available equipment. 

(7) In all cases studied, the miners were not properly trained in 

the use of the radio equipment. This gave rise to the following 

probl em areas : 

a. The systems were not used to their full potential. 

b. Miners were unaware of the cost of a portable radio and abused 

the equipment. This tended to increase maintenance costs. 

c. The miner's natural resistance to change caused rejection or 

excessive cri ticism of the system. 

(8) Miners frequently viewed the radio as a means for mangement to 

tighten its control over them. 
BIBLIOGRAPHY 

1. Chufo, Robert L. and Paul D. Vancura. "The Installation, Operation 

and Maintenance of an Underground Minewide Radio system;" broc. of 

1976 IAS Annual Meeting, pp 11 96-1202. 

2. Goddard, A. E., Radio Prop., "Measurements in Coal Mine at UHF and 

- VLF, " In Proc. Thru-the-Earth El ectromagneti cs Workshop, August 15- 

17, 1973, ed. by R. G. Geyer, Colorado School of Mines, Golden, 

Col o. , December 1973, pp 54-61 . 

3. Updyke, Dana, Howard C. Turnage, and Wayne Muhler, PhD, "Evaluation 

of Leaky Feeder Communication Systems in Underground Mines," 

Draft report, contract 501 99048, At1 antic Research Corporation. 

June 1980. (Contact author of this paper concerning further 

information of avai labil i ty of this information. ) 
KEY 

El Capital cost 

El Annual maintenance 

cost 

D E 

MINE 
Omni directional antenna 

at drift mouth Yogi antenna 

..---.-.-...-..-.-. .-..-.-.-.-.-,-.-.-.-.- + / 

.-.,

O7~epeater No. I r--- 1~e~eater No. 2 I---- Repe

L-,,,, I ------ I 

Control 

I I 

Comparator and control 

console at hoist house 
N 

V1 

6 

t-' 

W 

a surface repeater) 

Rer.p*trel \lo. 4 Repeater No. 5 

I - I 

I'TX" -. I 

I -+ TX4 I 

I - TX~ I +RX- I I I ----,---I 

I - TX6 I + 1x7 , 

C - - - - -- -1 

I Repeater No. 3 

3 System confiqurat,ion 7f mine A. 

) ompa pa- I 

1 ttttttf r 

I -CUM~~C.DF- I 

xxxxxxx I 

I aaqaaaa I I 

Typical of No. 1 - 6 

Control 

co.nsole 

(Load~ng drifts) 

----- i 

Voting comparator d 

Control 

located at portal - con,sole (Procesqlng phnt) 

(Repeater No. 7 is 
L-n.--o-n--l 

Repeater 

I' I ,: 

I 

i I i lo I 

i ig i Control I ..-.-.-. 

console I b 

(Lamphouse) .CI.o.CI..o.-. - I 

4. System configuration of mine E. 

jr 'I 

.-. 0.-.-.-.-...-.-.-.-.-.-.-.-a 

2,600 f t. 

\.- .... -.-.-... .-.-.-... 0.-..-.-...-.-...-.-.

4 

- -----.--.--q 

I 

! F 

TX--I 11 I Interface to I ! 

oi 

gi 

GAI -Tronics I Liz I 

I 

! i 

I ye -- page phone I I I i 

r 1 

! 

I 

I 

i 

I ! i I ~DUP~ I ;i 

+ 
I 1 I I I 

/ $74,000 

- / - 

/ 

- 

- - 

- - 

- 

- 

Cable costs 

- 

Repeater cost 

$5,000 I 1 I J 

10 20 

SYSTEM LENGTH, feet x 1,000 

407 

5. System procurement costs vs system size for 1/2- and 7/8-inch 

leaky feeder cable.