IMPROVED RAIL HAULAGE COMMUNICATIONS 

Harry Dobroski, Jr. 

Electrical Engineer 

Pittsburgh Mining and Safety Research Center 

Bureau of Mines 

U.S. Department of the Interior 

Pittsburgh, Pa. 

ABSTRACT 

Communications with moving tracked vehicles in a rail haulage mine 

pose a difficult problem. These communications take place from dispatcher to vehicles or from vehicle to vehicle via the trolley line, 

which is a very poor communications line. As a result, deadspots and 

high-signal areas can occur anywhere along the line, or signal can decrease 

simply as a function of distance. 

An analysis is made of the trolley line as a communications line, 

and methods of improving its characteristics are discussed. Data from 

inmine experiments are presented, showing actual improvements in communications. 

Recent Bureau developments in new hardware are also discussed. These 

include an adaptive speaker control for existing carrier phones and a new 

generation of improved carrier phones. 

INTRODUCTION 

Although trolley carrier phone systems leave much to be desired for 

haulageway communications, the fact remains that they represent the only 

practical means of dispatcher-vehicular communications. Most problems 

associated with these systems are ti-ansmission line related; i.e., a 

trolley line was never intended to be a good communications line, and it 

certainly is not. However, techniques do exist for improving overall 

communications. These techniques can be easily implemented, and the 

results are often excellent. 

In addition to the transmission line problem, basic carrier current 

hardware is archaic in design. Simple improvements can be made to this 

hardware that make vastly improved communications possible. The most 

significant improvement is that of incorporating magnetic field pickup 

instead of voltage pickup on the trolley wire. 

COMMUNICATIONS ON THE TROLLEY LINE 

Trolley carrier phones presently serve as the most important communication system in rail-haulage mines. These phones are frequency-modulated 

(FM) transceivers based on tuned radio frequency (TRF) circuitry and 

usually operate at 88 or 100 kHz, although many other commercially 
available frequencies are possible. The proper operation of the carrier 

phone system is essential for both operational and safety reasons. Up 

to now, there have been no real alternatives to this system for rail 

vehicular-dispatcher communications. The problems, and sometimes frustrations, of these systems were tolerated simply because there was no 

alternative. 

The usefulness of a trolley carrier phone system stems from its ability 

to use the trolley line as the transmission path. Since all rail vehicles 

are physically connected to the trolley line for power, this transmission 

path is convenient. 

However, anyone familiar with mine communications is aware of the 

fact that trolley carrier phone communications are highly variable in 

quality. There are times when good communications over great distances are 

possible. So at best it can be said that trolley carrier phone communications are tolerable, but vast improvements are needed. 

The problem, of course, is more of a systems problem than a hardware 

problem, although improved hardware would certainly help. It is interesting 

to note that except for changing from vacuum tube circuitry to solid-state 

circuitry, there have been no basic improvements in the hardware since it 

was introduced. 

The real systems problem is associated with the trolley line itself. 

Its prime function is to distribute dc power to vehicles and fixed loads. 

It was never intended to function as a transmission line. 

Figure 1 illustrates the theoretical attenuation rate of an unencumbered 

trolley line functioning as a transmission line, assuming typical values of 

conductivity of the surrounding medium. The attenuation rate of a carrier 

phone system operating at 100 kHz is seen.to be only 1 db/km. On this basis 

trolley carrier phones having a dynamic range of 70 db (25 V to 8 mV) should 

be able to communicate over such a line for 70 km (43 miles). In practice 

only a small percentage of this range is achieved. 

A trolley line, since it is really first and foremost a power bus, is 

shunted by many mine loads. These loads are vehicle motors, vehicle lights, 

water pumps, track block lights, personnel heaters, rectifiers, etc. A large 

rectifier, for example, can have a radio frequency (RF) impedance as low as 

2 ohms. When one considers that there can be hundreds of such loads on the 

trolley line, with some of them being mobile, it becomes apparent why the 

70-km range is not achieved. 

To learn more about the characteristics of unencumbered trolley lines, 

measurements were made on such a line in an operating coal mine. These 

characteristics are as follows: 
Characteristic impedance, Zo 200 ohms 

Attenuation rate 1.6 db/km 

Inductance/length 1 vH/~ 

Capacitance/length 30 pf/m 

Velocity of propagation .64 of free space 

TRADITIONAL SOLUTIONS 

Because of the problems in achieving good communications, several 

time-honored solutions have been tried to improve things. Perhaps the 

most widely used solution is the addition of so-called Z-boxes. The Z-box 

is simply a passive coupling device that permits RF signals to be transferred from one transmission line to another while blocking dc. Tuning is 

possible to achieve the best signal transfer. The technique is almost 

always that of transferring signals from a trolley line to a phone line in 

an attempt to compensate for low signals on the trolley line. In certain 

instances the approach works well; in other similar instances it can 

actually degrade communications. Sometimes it only transfers a poor communications area to somewhere else. The addition of Z-boxes to a trolley 

carrier phone system should be done with caution, because it rarely 

achieves the desired results. An explanation of this erratic behavior 

will be offered further in this paper. 

When the most important communications link is from dispatcher to 

vehicles, rather than vehicle to vehicle, good results can often be 

achieved by connecting the dispatcher's transceiver to a phone line that 

runs in the same entry as the trolley line. This scheme is often superior 

to that where the dispatcher is connected to the trolley line also. But, 

as before, if there are Z-boxes connecting the two, inconsistent results 

can still occur. 

Yet another solution is that of providing the dispatcher with a 

remote transceiver. In this scheme'the transceiver is situated in a 

favorable location and only the audio and keying lines are run to the 

dispatcher. This method is especially effective when the dispatcher is 

located on the surface, or in a remote area of the mine. 

One method of reducing the influence of RF shunting loads is to 

devise a method of isolating those loads. Although the power rectifiers 

represent the most severe loads on the line, they are often easily 

improved. Figures 2 and 3 show two possibilities. 

If the rectifier is set back far enough from the haulage entry, tuning 

the leads is often possible. In cases where the leads are too short to 

exhibit sufficient inductance for tuning, large open air coils can be 

built and tuned.. These and other schemes are discussed in detail in 

several Bureau of Mines reports. (l) * 

* Numbers in parenthesis refer to References at the end of the paper. 
IMPROVED SOLUTION, THE DEDICATED WIRE 

The dedicated-wire technique for improving rail haulage communications 

shows great promise for both rail haulage and whole-mine communications. 

This technique has been around for some time, although why it worked was 

not clearly understood. 

In the last section, it was shown that coupling to phone lines with 

Z-boxes, or attaching the dispatcher's transceiver to the phone line, 

sometimes improves carrier phone communications. This is because a phone 

line is actually a type of low-grade dedicated wire. It is not as good as 

a true dedicated wire, because there are phones attached sometimes along with 

other RF loads. In addition, the phone line has numerous splices, uncontrolled branches, taps, untwisted lengths, etc., that increase attenuation. 

So the best way to get around this problem is to make sure the dedicated 

wire is truly dedicated, and that means an independent wire, perhaps 14 

gauge, run down the entryway with the trolleyline on the wide side, but not 

connected to the trolleyline in any manner. 

Such a wire, since it is unloaded, has a very low attenuation rate 

of about 1 db/km (the sane as an unloaded trolleyline, since the geometry 

is the same). Therefore, if a signal is impressed on the dedicated wire, 

the signal remains high. Since the trolleyline and dedicated wire are 

located in the same entry, there is a mutual electromagnetic coupling 

between them. The effects of loads on the trolleyline are transferred to 

the dedicated wire, and the high signal on the dedicated wire is transferred 

to the trolleyline. Fortunately, if the separation between the two is 

large enough, the loading effects of the trolleyline are only weakly transferred to the dedicated wire, so that the attenuation rate Stays low. But at 

the same time, the high signal levels on the dedicated wire are strongly 

coupled to the trolleyline. The net result is that communication is now 

possible in areas where it was not possible before. 

Figure 4 illustrates the beneficial effects of the dedicated wire in an 

entryway with a trolleyline. Curve A shows what happens if transmission 

and reception take place on a dedicated wire only. This is possible between 

fixed stations but obviously not so if vehicles are involved. Curve B 

shows the voltage on the dedicated wire, from a transmission on the trolleyline. Curve C shows the reverse, the voltage on the trolleyline from a 

transmission on the dedicated wire. Curve D shows the signals received on 

a trolley wire if transmission takes place on the trolley wire. 

The theoretical benefits of a dedicated wire to improve trolley 

carrier phone communications have been proven in practice. In U.S. 

Steel's Robena mine, a large section of haulageway toward the Willow 

Tree shaft was such that communication from vehicles to dispatcher was 

not possible. The solution, suggested by a carrier phone manufacturer, 

was to install a dedicated wire tc the wide side, connected directly to 

the dispatcher. 2-boxes were also installed, although their benefit is 

unknown. As a result, good communications was established into an area 

where it was not possible before. 

In Consolidation Coal' Montour #4 mine, the problem was even more 

serious. A remote dispatcher's transceiver was installed in the offending 

area, connected to the dispatcher by audio and keying lines. This remote 

transceiver helped some but not enough. Communications were still poor. 
Even connecting the transceiver to the phone line and installing Z-boxes did 

little to improve the situation. Consol and the Bureau agreed to undertake 

a carefully controlled experiment to determine whether or not a dedicated 

wire would be of value. 

In November and December 1977, a Bureau team and Consol personnel made 

measurements in the mine. These measurements were made from a jeep equipped 

with a Sierra* model 303A tuned voltmeter. The trolley line was encumbered 

with many loads. Initial measurements without a dedicated line showed that 

it made little difference if the Z-boxes were connected. However, the 

installation of a dedicated wire with the dispatcher's remote transceiver 

connected to it resulted in vast improvements (figure 5) especially in areas 

of initially low signals. The dedicated wire was properly terminated in 

200-ohm resistors, and proper splitting networks were installed at branch 

points. As a result, communications along the entire entry (several miles) 

where the dedicated line was installed changed from one of near uselessness 

to one of good quality. 

It should be emphasized that dedicated wire communications is not 

restricted to the situation where improved trolley wire communications is 

desired. Indeed, the greatest potential may be in mines that do not even 

have a trolley line. 

One example is Consolidation Coal's Oak Park mine in Eastern Ohio 

Cfigure 6). In this mine standard carrier phones are equipped with loop 

antennas such that they transmit and receive on this antenna instead of by 

coupling to a trolley wire, which does not exist. This is a belt haulage 

mine; and the vehicles are used for personnel and supplies. This system 

permits whole-mine vehicle-to-dispatcher and limited vehicle-to-vehicle 

communications. 

Other mines, such as Repbulic Steel's Kitt mine, are installing 

similar systems. 

MEDIUM-FREQUENCY SYSTEMS 

Bureau of Mines research has shown that medium-frequency (MF) (300 kHz 

to 3 MHz) systems have a very high potential for solving the ultimate 

problem: Whole-mine communications. Whole-mine communications as described 

here means the ability for two or more vehicles or personnel to communicate 

with each other or with a base station, no matter where they are in the mine. 

Medium frequencies work well in the mine environment because they are 

low enough to not suffer severe attenuation while going through conducting 

mediums but high enough to couple parasitically onto metallic structures in 

the mine such as power cables or water pipes. Once coupled, linear attenuation on these structures is also low. Several highly controlled measurement 

programs() in the mine environment have been conducted to determine MF 

characteristics in detail. 

*Use of brand names is for identification purposes only and does not imply 

endorsement by the Bureau of Mines. 
Data so far have shown that in wire-free areas, the determining 

factor on range is the mine geology, the most favorable of which is lowconductivity coal and high-conductivity overburden and underburden. 

Under these conditions a parallel-plate waveguide exists, trapping the 

MF into a cylindrical spreading mode of propagation with low loss. 

Ranges of up to 350 m were measured under these conditions. In areas 

containing wires, ranges of many kilometers are not uncommon. 

The Bureau is presently designing base stations, repeaters, and 

portable and vehicular radio to take advantage of the favorable MF 

propagation modes. The relationship between MF systems and dedicated 

wire techniques is obvious. A whole mine MF system with base stations 

and repeaters might be configured as shown in figure7. 

THE LOW- IMPEDANCE LINE 

A bridging load R on a transmission line reduces the line voltage 

R to the fraction R+z,/~' Hence, the loss of signal is not only a 

function of the valug of the bridging load R, but also of the characteristic impedance Zo of the line. It is also apparent that a lower value 

of Zo would be beneficial. 

Since Zo for a trolleyline is around 200 to 300 ohms, a convenient 

way to lower Zo must be found. For a transmission line, Zo = m. 

Therefore, one obvious approach would be to add artificial distributed 

capacitance to the line. 

Figures 8 and 9 show an experiment that was recently done to test 

this low-impedance technique. A 4,460-ft (1.36 km) length of 300-ohm 

transmission line was deployed on the surface, a signal source was 

applied, and signal level was measured along the line, first unloaded, 

and then loaded with three modest loads. The three loads caused a 

signal loss of 55.5 db, but when 2,000-pf capacitors were attached to 

the line every 235 ft, only 12.5 db of loss was encountered. 

At this writing inmine tests are being planned to determine whether 

or not this technique can be applied to a mine trolley line. In Consolidation 

Coal's McElroy mine, a section of trolleyline will be isolated and the characteristic impedance measured. The line will be loaded at various points and 

signal measurements will be made. The line will then be equipped with shunting 

capacitors, and the measurements will be repeated. If successful, the lowimpedance line technique will be an alternative to the dedicated wire in 

situations where the installation of the dedicated wire is not desired or 

practical. 

HAULAGEWAY HARDWARE, IMPROVEMENTS AND DEVELOPMENT 

As mentioned before, trolley carrier phones have not undergone any basic 

improvements since they were first developed except for the change from vacuum 

tube to solid state circuitry. Also there seems to be an industry reluctance 

to move in the direction of more state-of-the-art circuitry. Whether or not 

reluctance is justified, there are several simple improvements that can 

be made to standard carrier phones that are of great value. 
Under Bureau contract (4) - an improved trolley carrier phone is being 

developed. Although it is still a simple TRF device, it contains several 

important different features. 

The first feature is that it can receive both in the conventional manner, by voltage pickup from the trolleyline, and also via loop antennas. 

Inmine measurements have shown that standard carrier phones cannot communicate in the vicinity of heavy RF loads such as rectifiers because the RF 

voltage on the line is too low. However, RF current and the magnetic field 

(H-field) are generally high in the vicinity of such loads. Therefore, the 

situation is complementary. When RF voltage on the trolleyline is low, the 

current is high, and vice versa. The Bureau carrier phones permit manual 

selection between voltage pickup and magnetic pickup. A voting circuit 

could be installed to do this automatically. Inmine tests have consistently 

shown that magnetic pickup always exceeds voltage pickup, no matter where 

one is in the mine. It is also a cleaner signal. Because of these findings, 

there appears to be no reason why a carrier phone even needs RF voltage 

pickup from the line. Carrier phones with magnetic pickup would likely be 

extremely immune from dead spots. 

Other features that these carrier phones have are phase lock loop FM 

detection, preemphasis/deemphasis, and a combination conventional-tone 

squelch circuit. 

Another hardware improvement that was evaluated for trolley carrier 

phone use was an adaptive speaker volume control. Most of the time the 

adjustment of the volume control on the speaker is a real problem on rail 

haulage vehicles. When the vehicle is in motion, the noise is so high that 

the operator sets the volume control to maximum. However, when the vehicle 

stops, the acoustic environment is very quiet. An incoming message at this 

vital time can virtually knock one over. Conversely, if the volume control 

is set low because the vehicle is stopped, failure to turn it up while in 

motion can be dangerous, for the operator may miss a vital message from the 

dispatcher or another vehicle. 

The adaptive volume control is a small circuit that can be built 

directly into the speaker housing. A dynamic microphone samples the ambient 

noise, and a preset level detector activates a relay that applies full power 

to the speaker if the threshold is exceeded. Otherwise no action takes 

place and operation is via the volume control. A level detector inhibits 

the relay drivers from changing the relay setting during the reception of 

incoming messages, thereby eliminating latchup problems. 

CONCLUSION 

Several techniques have been discussed concerning ways to improve rail 

haulage communications. One method concerns using a low-attenuation dedicated wire, preferably not connected to the trolleyline by coupling devices. 

Strong signals on the dedicated wire couple strongly to the trolleyline, 

while unfavorable loads on the trolleyline couple only weakly back to the 

dedicated wire. Another method is to reduce the influence of loads on the 

trolleyline by lowering the line characteristic impedance. Other solutions 

have also been included, such as tuning, Z-boxes, use af phone lines, and 

remote location of the dispatcher's transceiver. 
The benefits of medium frequencies were also discussed, and systems 

based on dedicated wires and medium frequencies were presented. 

REFERENCES 

1. "Improvements for Mine Carrier Phone Systems," H0346045, A. D. Little, 

Inc. Final Report available from National Technical Information 

Service, Springfield, Va. 22161, NTIS No. PB 273 292/AS. 175 pp. 

$9. 

"Trolley Wire Phone System Guidelines," 50166010, A. D. Little, Inc. 

Final Report available from National Technical Information Service, 

Springfield, Va. 22161, NTIS No. PB 273 479/AS. 170 pp. $9. 

2. "Propagation of EM Signal in Underground Mines," Collins Radio Division, 

Rockwell International. Final Report, distribution pending. 

3. "Improved Hardware for Trolley Wire Phone Systems," H0166144, Gai-Tronics 

Corporation. Pending report. 

4. "Adaptive Speaker Control for Mine Carrier Phones," 50377098, A. D. 

Little Inc. Pending report. 
THEORETICAL ATTENUATION RATE FOR TYPICAL 

TROLLEY WIREIRAIL 

ATTEN. 

RATE 1 

(d Bik m 1 

50 100 200 400 800 

FREQUENCY (kHz) 

FIGURE 1.--Theoretical Attenuation rate for an unencumbered trolleyline 
Capacitor tunes Trolley wire 

Feed wire feeder wires / 

Rectifier 

k- 50 feet 9r more--4 

FIGURE 2.--Isolating a rectifier by tuning the feed wire inductance. 

Added C 

'4, , Trolley wire 

I '( 4 

. A - 

Rectifier 

FIGURE 3.--Isolating a rectifier by adding L and C. 
VOLTAGE LEVELS VERSUS DISTANCE FOR DEDICATED WlRE 

-30 

VOLTAGE 

(dB re 

Tx VOLTS) -40 

FOR 

TRANSMISSION 

ON : 

DEDICATED 

WlRE 

TROLLEY 

WlRE 

DEDICATED 

WlRE 

TROLLEY 

WlRE 

5 10 15 20 

DISTANCE FROM TRANSMITTER, Km 
I I 1 

Trolley wire voltage 

with separate dedica

Trolley wire voltage with - 

telephone line as low grade 

dedicated wire 

- 

I 1 , L 

5,000 l0,OOO 15,000 

DISTANCE FROM DISPATCHER'S TRANSCEIVER, ft 

Signa I levels v s distance 
FIGURE 6.--Dedicated wire system for communicating 

with battery-powered rail vehicles. 

1 

I ,-- - J 

Base slation 

or 

repsil t e r 

4) 

Coupler 

FIGURE 7.--Whole mine MF system using dedicated wires. 

29-1 3 

Splitter 

- - Dedicated wire \ 

I I 

- 

v 

I I 

I I 

I I 

I 

I 

I 

I Phone line 

I I 

A \ 

L,--J T 

Tranceiver 

P Audio irnd keying lines 

to disputche r 
50 R - SHUNT 30 a 20 R 

LOADS 

RELATIVE 

VOLTAGE 

(dB) 

UNLOADED 

- 

- DATA I THEORY I 

-70 [ I I 1 I I 

0 4460 

DISTANCE ALONG LlNE - FEET 

FIGURE 8.--Effects of loads on a simulated trolleyline. 

k- SIMULATED LENGTH = 4460 FEET 

2dtPF 

50 n SHUNT 30n 

LOADS u20R 

RELATIVE 

VOLTAGE 

dB 

DATA 

- THEORY 

-70 1 

0 4460 

DISTANCE ALONG LlNE - FEET 

FIGURE 9.--Beneficial effects of reducing line charactezistic impedance.