COMMUNICATION WITH MOVING TRAINS I N TUNNELS

By Newton Monk and H. S. Winbigler Bell Telephone Laboratories, Incorporated

New York, N.Y.

Synopsis

This paper descr ibes tests conducted in the North River tunnel of the Pennsyl- vania Railroad to determine a p r a c t i c a l means f o r communicating wi th moving loco- motives and t r a i n s i n t u n n e l s . It i s shown that frequencies between about 25 and 1500 megacycles per second ape not su i t ab le fo r such communication over any subs t an t i a l d i s t ance when employing con- ventional radio techniques. Tests a r e d e s c r i b e d u t i l i z i n g o t h e r means of t r ans - mission, including the use of a s e r i e s of antennas bridged onto a t ransmiss ion l ine extending through t h e tunnel, and a closely-spaced two-wire line without an tennas ac t ing as a r a d i a t i n g and pick- up device. It i s concluded that satis- factory t ransmission in tunnels can be obta ined for a d is tance up to approxi - mately 6,000 feet using the frequencies and equipment o rd ina r i ly employed i n r a i l road mobile systems and -twin-lead cable such as RG-86pJ su i tab ly loca ted within the tunnel .

In t roduct ion

Very e a r l y i n t h e a p p l i c a t i o n of r a d i o t o the n a t i o n ' s r a i l r o a d s it was found that, f o r the frequencies used, the rad io signals did not pene t ra te in to tunnels beyond a few hundred f e e t . Thus, communication with the t r a i n s when they are within tunnels of any s u b s t a n t i a l length i s not poss ib le with the e x i s t i n g radio systems. To determine a feasible method fo r ob ta in ing communication wi th locomotives and cabooses when they are i n t u n n e l s , a number of t e s t s were made by Bell Telephone Laboratories in cooper- a t i o n w i t h the Pennsylvania Railroad i n the r a i l r o a d ' s North River tunnel. This paper descr ibes these tests and ou t l ines an arrangement which i s be l ieved to be

p rac t i cab le fo r communicating wi th t r a i n s i n t u n n e l s .

As assigned by the Federal Communi-. ca t ions Commission, f r equenc ie s i n the range of 159 t o 162 megacycles are used by t h e r a i l r o a d s i n t h e i r r o a d and yard r ad io i n s t a l l a t ions . It i s highly desira- ble that the same communication f a c i l i t i e s be employed when the t r a i n s a r e w i t h i n tunnels as are used a t other t imes. For t h i s reason most of the tests descr ibed i n this paper were made a t f requencies i n t h e neighborhood of 150 megacycles. However, a few t e s t s were made a t higher and lower frequencies.

Descr ipt ion of Tunnel

The Pennsylvania Railroad's North River tunnel passes under the Hudson River between New York and New Jersey . It consis ts of two separate tubes, each of which c a r r i e s a s ing le t r ack . One tube i s known as the "north tube" and the other as the "south tube". Normally, trains leaving Pennsylvania Stat ion in New York pass through the north tube, whi le in- coming t ra ins use the south tube. The t e s t s were made in the north tube.

The tubes measure 13,393 f e e t from p o r t a l t o p o r t a l . I n a d d i t i o n t o the entrances , t he re a r e two shafts a t i n t e r - mediate points through which access to the tunnel i s possible and where base s t a t i o n equipment can be located. One of these i s a t 32nd S t r e e t and 11th Avenue, Manhattan, and the other is i n Weehawken, N e w Jersey. A s impl t f i ed dia- gram of the north tube, showing the loca- t ion o f these shafts and o t h e r p e r t i n e n t information, i s given in Figure 1.

The tubes are s u b s t a n t i a l l y straight throughout most of t h e i r l e n g t h , w i th a

I-- I 6627 FT "' 'Ti.$ 13,393 FT

SUBAQUEOUS SECTION

5907 FT

P * WYSIM S m A L

Fig. 1 - Simplified diagran. of north tuoe.

21

Fig. 2 - Cross-section of north tube.

two-degree curve close to the eas t e rn end. S t a r t i n g a t the Manhattan p o r t a l t h e r e is. a 1.92 per cent descending grade changing t o a 0.53 per cent grade about 3,000 f e e t from the por ta l . This l a t t e r grade holds for about 2 ,000 feet under the r iver , from which point there I s a 1.3 per c e n t r i s e i n grade to the wes t por ta l .

Between the New York and New Jersey shafts the tubes are of typical under- water construction, being composed of a c i r c u l a r c a s t s t e e l s h e l l w i t h a r e i n - forced concrete l ining. The nature of the tube construction over most of this length is indicated in Figure 2. It shows, i n simplified form, the segmented c a s t s t e e l ring construct ion of the she l l , the re- inforced concrete l ining, and o ther f ea tu res . West of the New Jersey shaft the re inforced concrete tubes have an i n t e r io r con f igu ra t ion l i ke that shown i n Figure 2. East of the New York sha f t t he tubes a re somewhat wider and hiRher in t e rna l ly .

Radio Prowfa t ion

Although it was already known that the loss a t VIIF radio f requencies was extremely high in tunnels , i t was believed that knowledge of the approximate loss in the tunnel under test would provide a f i r e basis for fur ther exper imenta t ion . Accordingly, radio propagation within the tunnel, in the 152-162 megacycle band,

was f i r s t s t u d i e d . A 30-watt radio trans- mitter and a companion receiver together w i t h the neceesary control and power equipment were s e t up in a small room a t the foo t of the 11th Avenue shaft, Figure 3 i s a photograph of t h i s s t a t i o n . The radio t ransmit ter , control panel , and power panel are mounted i n the l a r g e r rack, and the r ad io r ece ive r i n t he smaller one. The radio equipment was connected through an ad jus tab le a t tenu- a tor , included on the control panel , and a sui table length of R Q - 8 f i cable t o a coaxial antenna located within the tunnel about as Indicated in Figure 4.

The radio equipment was arranged to operate on a coonon car r ie r channel which i s assigned for use by the New Jersey Bell Telephone Company in th i s area. This channel uses frequencies of 152.75 mega- cycles for t ransmission from the base e t a t i o n t o t h e mobile u n i t s , and 158.01 megacycles fo r t r ansmiss ion i n t he r e - verse d i rec t ion . By employing these frequencies , tests could be made wlth exis t ing publ ic passenger radiotelephone i n s t a l l a t i o n e on a number of t r a i n s which pass th rough ' the tunnel da i ly . A mobile s t a t ion ope ra t ing on these frequencies, which was i n s t a l l e d on one of the rail- road ' s e lec t r ic locomot ives for rad io survey purposes, also was a v a i l a b l e f o r the tests.

talking tests were conducted to and from With the arrangements described,

22

I ” I ... -I .. -.^-_”

F i g . 3 - Base station equipment at the l l th Avenue shaft.

t h e t r a i n s with a va r i e ty of antenna types, heights, p o l a r i t i e s , and or ien ta- t ions , and with several values of a t tenu- a t ion i n se r t ed between the base s t a t i o n and the antenna. Also , recordings were made of the signal s t rength received a t the base s ta t ion from the passenger t ra in or the locomotive. By not ing the instants a t which t h e t r a i n s t a t i o n passed c e r t a i n wayside signals during i t s passage through the tunnel , p lo t t ing a t r a in p rog res s curve, and coordinating t h i s information wi th t e s t times, the location of the t r a i n s t a t i o n a t any time during the t e s t s was quite accurately determined.

The r e s u l t s of t he propagation t e s t s f o r a representat ive antenna are summarized in F igure 5. This shows the d is tance from the base s t a t i o n f o r which r e l i a b l e communication could be obtained p lo t ted aga ins t the power output of the rad io t ransmi t te r . Data fo r bo th d i r ec - t i ons of transmission are included. Unavoidable changes i n day-to-day values of the mobile transmitter output and re- ce iver sens i t iv i ty p robably account for most of the observed scattering of the data. The straight l i n e drawn through the t e s t p o i n t s i n d i c a t e s that the aver- age a t tenuat ion of the rad io waves a t 152 megacycles over the pa th measured is i n the order of 18 db per hundred feet . It should be noted that this a t tenuat ion 1s

not that for an empty tunnel s ince, dur- ing the tests, the path between base and mobile s ta t ion an tennas was a t least par t ia l ly occupied by t h e t r a i n . For a 30-watt (+15 dbw) t ransmi t te r , t h i s a t tenuat ion permits a range of only about 750 feet .

A recorded sample of t he s igna l from the locomotive, as received a t the base s ta t ion, is shown in F igure 6. The

UECESS IN TUNNEL WALL l lTH W E . SHAFT

J

Fig. b - Base station antenna location and mounting.

23

FEET FROY BASE STATION

200 400 so0 em 1 0 0 0

Fie. 5 - Radio coverare vs. t r a n s m i t t e r output, a t i 511 mtgac-cles.

curve represents the build-up and decline of the rece ived f ie ld s t rength as ind i - cated by rece iver first l imi t e r cu r ren t when the locomotive enters and leaves the coverage area. It w i l l be noted tha t the received s ignal s t rength var ies about as would be expected, and no substantial i r r e g u l a r i t i e s are evident .

In order to determine the possibi l - i t i e s of u t i l i z ing f r equenc ie s of the order of 40 or 450 megacycles some fu r the r data were obtained a t these frequencies. In these cases ta lking and l i s t e n i n g t e s t s only w t e s t s , *e made. For t h e 40-megacycle

witable base station equipment ard

t r a i n s equipped for public passenger service operat ing a t approximately t h i s frequency were used. For the 450- megacycle tests, a commercial Tv con- v e r t e r was purchased, suitably modified, and i n s t a l l e d on one of the 150-megacycle- equipped trains to convert the incoming signals from 450 megacycles to the ca r r i e r frequency of the 150-megacycle rece iver . These tests indicated that the average radio a t tenuat ion over the tes t pa th a t 40 megacycles was about 3.6 db per 100 f e e t and that a t 450 megacycles approxi- mately 12 db p e r 100 feet .

The above data together wi th the r e s u l t s of t u n n e l t e s t s made b observers as gleaned from the l i teraturee are p lo t ted in F igure 7. The points on t h i s curve are by no means accura te bu t a re be l ieved to Ind ica te the o rder o f magni- tude of the l o s s i n t h e r a d i o path i n a ra i l road tunnel when occupied by a t r a i n . It w i l l be noted tha t the loss increases rap id ly wi th f requency in the VHP range and decreases again as the frequency i s increased. Th i s suggests a change-over from f ree-space to waveguide transmission. Computing the c r i t i ca l cu t -of f f requency of a c i r c u l a r waveguide having the approx- imate dimensions of the tunnel under test, we f i n d t h a t the cut-off frequency varies somewhat wi th the mode of transmission but i s In the order of 50 megacycles. However, i f the dimensions of the wave- guide are reduced to take account of the presence of the t ra in within the tunnel and abso rp t ion a f f ec t s , the cu t -o f f f r e - quency is subs tan t ia l ly increased . Figure 7 suggests that t h i s cut-off frequency i s about 275 megacycles. The curve ind ica tes a l so that 150 megacycles

*See Bibliography a t the end of t h i s paper.

I ST L IY u4

I ST L I Y PA

I C 0

I20

8 0

4 0

0

YOBILE ST4TlON DIST4YCE E4ST OF l 4 S E ST4TION - FEE?

160

120

8 0

4 0

0

375 43) 47 2 505 539 572 606 640 674 708

W l l L E 17471011 OISTINCE WEST OF 04SE STITION -FEET

Fig. 6 - Recorded s igna l r ece ived from locomotive.

2h

20

I 8

J I6 z

14 LL 0 c 12 0

IO U u

P a

Z 6 9

4

2

IO 20 50 100 200 Mo loo0 2000 M O O FREQUENCY - M C

Fig. 7 - RF transmission loss in train-occupied tunnels.

i s almost the worst frequency which could be chosen for th i s type o f t ransmiss ion .

drawn s lana l

S t i l l another deduct ion can be from Figure 7. The minimum usable . for radio transmission of t h i s

type i s known t o be about 120 db below one watt. Assuming t h a t a 30-watt radio transmitter i s employed, a t o t a l a t t e n u - a t i o n of approximately 135 db i s the maximum which can be tolerated. There- fo re , t o t r ansmi t a distance of one mile, the length of a long f r e igh t t r a in , the l o s s i n the tunnel must not exceed about 2.5 db per hundred f e e t . Allowing f o r a small margin to i n su re good transmission under a l l condi t ions, the curve of Figure 7 Indica tes that frequencies above about 25 megacycles and below around 1500 mega- cycles are not su i tab le for rad io commun- ica t ion In tunnels over any subs tan t ia l d i s tance by conventional means.

It should be emphasized t h a t t h e curve of Figure 7 shows the loss which i s experienced when a t r a i n i s i n the tunnel . A few t e s t s made wi th walkie-talkies showed tha t when a t r a i n i s not present the a t tenuat ion i s somewhat less. Tests made by other observers employing small hand c a r s o r t r o l l e y s a l s o have indicated lower losses , par t icular ly a t the higher frequencies.

Parallel Antennas

The above data ind ica ted tha t rad io coverage in the tunnel could be provided by a s e r i e s o f base s t a t ions l oca t ed within the tunnel and spaced about 1000 f e e t apart. Obviously, such an arrange- ment w i t h 13 or 1 4 base s t a t i o n s i n e a c h of the two Hudson River tubes did not appear very a t t ract ive. However, the tes ts suggested another possibi l i ty: a transmission line extending through the

tunnel fed a t one end and having antennas bridged onto i t a t s u i t a b l e i n t e r v a l s . This arrangement would have the advantage of employing only inert equipment in the tunnel . Unfortunately, to provide cover- age f o r a tunnel of any substant ia l length by such means would requi re a t ransmission l ine of very low loss (0.5 db o r less per 100 f e e t ) i n o r d e r t o de l ive r su f f i c i en t .power t o the more dis tant antennas. Coaxial cables such as 7/'8-inch "Styroflex" o r "Heliax" hav- ing a t t enua t ions of t h i s o rder o f magni- tude are avai lable but they are qu i t e expensive. Nevertheless, a few tes t s to determine the f e a s i b i l i t y o f t h i s method of providing coverage within the tunnel were i n i t i a t e d .

For these tests 2000 f e e t o f RG-8/U coaxial cable was i n s t a l l e d i n the tunnel , Although R G - 8 i U cable has a loss a t 150 megacycles of some 2.7 db per hundred f ee t and would no t be s a t i s f a c t o r y f o r a permanent i n s t a l l a t ion o f t h i s type of any appreciable length, it is inexpensive, e a s i l y i n s t a l l e d , and r ead i ly ava i l ab le . It was thought that the method could be t es ted ou t wi th t h i s type of cable w i t h - out the expense and diff icul t ies which would be incurred w i t h low-loss st ructures .

No special precautiorls were taken in i n s t a l l i n g t h e RG-8/U cab le . It was mounted i n the tunnel by suspending it a few inches below a l ight ing condui t located on the wall about eight f e e t above the catwalk. This condui t is v i s i - b le in F igure 10. One end of the cab le was terminated i n the base s ta t ion equip- ment a t the 1 1 t h Avenue s h a f t . From th i s point the cable was extended westward w i t h f ive antennas, spaced a t approxi- mately 500-foot intervals, connected to i t . These antennas, w i t h t h e exception of t h e one a t the d i s t a n t end which

2 5

terminated the cable, were simple rode nine inches in length designed to provide a r e l a t i v e l y low bridging loss to the cable .

The f irst observation w i t h t h i s arrangement Indicated that good t r ans - mission was obtained throughout the en- t i r e l e ,ng th of the cable . The second and fourth antennas were then removed to p ro - vide greater spacing between the antennas and the test was repeated. The r e s u l t s were the same, The remaining antennas were then removed in turn and successive t e s t s were made. It was found that an exce l l en t c i r cu i t cou ld be obtained with no antennas a t a l l . I n view of t h i s re- su l t , t he cab le was extended another 1000 f e e t . However, good transmission could not be -obtained beyond the or iginal 2000 f e e t . In an e f f o r t t o o b t a i n g r e a t e r coverage, antennas were bridged a t the 2000-foot point and every 250 f e e t beyond. With t h i s arrangement, the range of satis- factory t ransmission was increased to 2600 f e e t . It was concluded from these tests that fo r r e l a t ive ly sho r t t unne l s up t o about 2000 fee t i n l eng th , communication can be obtained with a s ing le base s ta t ion a t one tunnel entrance connected to an RO-8/U cable running the length of the tunnel , * I f base station equipment can be l o c a t e d i n the middle of the tunnel, this d is tance can be increased to about 3800 f e e t . Bridged antennas will permit those d is tances to be extended to about 2600 and 4900 fee t , r e spec t ive ly .

Transmission Line

The a b i l i t y of the train s t a t i o n t o pick up the s igna l in the R G - 8 / U cable and vice-versa immediately led to specu- l a t i o n a s t o how the coupling was being effected. Consequently, a t e s t was made i n which the inner and outer conductors of the coaxial cable were shorted to- gether and energized, against ground, by the base s ta t ion t ransmit ter . With t h i s arrangement, the coverage was only about one-fourth of tha t obtained wi th the cable connected normally. In addition, labora- t o ry t e s t s Ind ica t ed that R G - 8 / U cable,

which has a s ing le braid ou te r conduc'tor, has r e l a t i v e l y high c ross t a lk i n to a similar and closely associated cable . From these data, i t was concluded that the f ie ld radiated d i r e c t l y from the coaxial cable was mainly responsible for the coverage obtained. It appeared reasonable to suppose, therefore, that a t ransmission l ine having an appreciable ex te rna l f i e ld bu t with considerably lower attenuation than the RG-8/U cable would extend the coverage materially.

Accordingly, theore t ical studies of s ing le and tm-wire t ransmiss ion l ines were initiated. Because of physical space l imitations, such lines must be mounted c lose to the w a l l of the tunnel. This r e s u l t s i n a b s o r p t i o n e f f e c t s which grea t ly increase the a t tenwt ion over that f o r such l ines In free space. Since i t was f e l t that accurate estimates of such absorption effects could not be made, it was concluded that a c t u a l f i e l d t e s t i n g would be required to determine the best arrangements. A s ing le-wire l ine of the Ooubau type was Considered, but a t a f r e - quency as low as 150 megacycles i t was f e l t that a two-wire l i n e would be more p r a c t i c a l . The absorpt ion loss and the f i e l d s t r e n g t h a t a given dis tance from a two-wire l ine both decrease as the spacing between the w i r e s i s decreased. Therefore, a compromise between these f a c t o r s was indicated. Other considera- t ions included first cos t and t h e a b i l i t y of t he l i ne t o s t and up under the weather cond i t ions l i ke ly t o be experienced i n the tunnel. With t h e s e f a c t o r s i n mind, a survey of ava i l ab le two-wire l i n e s having low a t t enua t ion a t 150 megacycles was made. RG-86B s o l i d d i e l e c t r i c pa ra l l e l -pa i r cab le , which i s similar t o but heavier than television twin-lead cable , and which has an adver t i sed f ree- space loss of about 0.6 db per hundred, fee t , appeared to meet the requirements best . Accordingly, 1000 f e e t of t h i s cable was ins ta l led in the tunnel and a s e r i e s of t e s t s made wi th i t .

The arrangement employed for these tests is shown in Figure 8. The base station

Fig. 8 - Testing arrangemsnt for RG&/U cable.

26

equipment was connected direct ly to 1500 f e e t o f RG-8/U coaxial cable which ex- tended westward along the tunnel wall. The RG-861U parallel-pair cable was then connected t o the end of the coaxia l cab le through a spec ia l impedance-matching transformer (balun). This was employed not on ly to match the 50-ohm impedance of the coaxia l cab le to the 200-ohm impedance of the parallel-pair cable but a l s o t o c o n v e r t from an unbalanced t o a balanced l ine. The RG-86f l cable was terminated a t the far end i n a second impedance -matching transformer and a res i s tance of 50 ohms. Thus, by subs t i - t u t i n g a 50-ohm measuring se t for the res i s tance , measurements of the cable loss could be made. A schematic of the impedance-mawhlng device employed, which was constructed local ly , i s shown in F igure 9.

R-259

~

Fig. 9 - Impedance-matching transformer (balun).

The RG-8% w i t h i t s loss of

approximately 0 db, served two usefu l purposes . Firs t , i t excluded the possi- b i l i t y of any f i e l d r a d i a t e d d i r e c t l y by the transmitting equipment from reaching the RG-861U cable . Second, i t provided attenuation between the base s t a t i o n and the RG-86/U cable . T h i s a t tenuat ion , as well as add i t iona l pad loss, was required to determine the limits of acceptable transmission.

The RG-86fl cable was suspended f.rom the exposed wall conduit previously referred t o by means of marline twine in such a manner that i t hung approximately 6 t o 10 inches below the condui t . In t h i s pos i t ion , the para l le l -pa i r was about 7 f e e t above the catwalk, 12 feet above the rails and 6 t o 10 inches from the wall of the tunnel. Th i s placed i t about 6-1[2 f e e t away from the car and locomotive antennas, in a plane about 1 foo t below them.

With t h i s tes t setup, two-way t a l k - ing and l is tening tests were made between the base s t a t i o n and a number of t r a i n s . Next, the loss of the parallel-pair cable in p l ace was measured and found t o be 1.3 db per hundred feet , a subs t an t i a l i n -

crease from the ind ica tad loss o f 0.6 db per hundred f e e t when the cable i s i n f ree space . From these data it was determined that wi th transmission powers of 30 watts, s a t i s f a c t o r y two-way communi- ca t ion would be possible with the R G - 8 6 f l cable for a d is tance of approximately 6000 feet .

For a f i n a l series of tests, the R Q - 8 6 4 line was mounted as i n d i c a t e d i n Figure 10, i n what was considered to be the most favorable location from the standpoint of coupling which physical and c learance requihuents in the tunnel would permit. I n this pos i t ion the cable was about 6 inches c loser to the c a r and locomotive antennas, and about 1-1/2 feet higher than for the previous tests, per- mi t t ing a c l ea r l i ne o f s ight to the c a r antennas and a partial one to the loco- motive antenna. However, no s ign i f i can t improvement i n coverage range was ob- served.

It was concluded from these tests that s a t i s f a c t o r y communication could be established wi th locomotives, cabooses, and t ra ins in the Pennsylvania Rai l road I s North River tunnel w i t h R G - 8 6 f l wire su i t ab ly Installed throughout the length of the tunnel and three 30-watt base s t a t i o n s . One of the base s t a t i o n s would be located a t each portal and one a t the Weehawken shaft, which i s about 6000 feet from the west p o r t a l and 7400 f e e t from the east p o r t a l . The s t a t i o n a t t h e e a s t p o r t a l would be connected t o about 4400 f e e t of wire, t h a t a t the Weehawken shaf t to about 3000 feet extending eastward and 2400 feet extending westward, and the s t a t i o n a t the west po r t a l t o abou t 3600 feet, This arrangement should provide considerable margin over. the limiting distance of 6000 f e e t from the base sta- t i ons as determined by the tests.

Although the tests were conf ined to a spec i f i c tunnel, i t i s believed that the so lu t ion a r r ived a t would be general ly appl icable to o ther tunnels . To be sure , the e f fec t o f the tunnel walls upon the a t tenuat ion of the wire may vary somewhat with the physical s t ructure of the tunnel as may, a lso, the coupl ing between the wire and the train antennas. However, these factors should not be subs t an t i a l ly d i f f e ren t i n o the r t unne l s and should result o n l y i n r e l a t i v e l y minor d i f f e r - ences in the range of coverage a t ta inable with the RG-86/U wire and a single base Sta t ion .

Conclusion

Th i s paper has desc r ibed t e s t s looking toward a p r a c t i c a l s o l u t i o n t o the problem of providing communication w i t h r a i l road ro l l i ng s tock i n t unne l s .

Fig. 10 - Location of RG-&/U cable on tunnel w a l l .

The r e s u l t s of t h e t e s t s may be summar - ized as follows:

1. Frequencies between about 25 and 1500 megacycles a r e n o t s u i t a b l e f o r r a d i o communication with locomo- t i v e s or t ra ins in tunnels o f any sub- s t a n t i a l l e n g t h by conventional means.

2 . Communication i n t h e 152-162 megacycle band between base s t a t i o n s and t r a ins i n t unne l s , c an be accom- plished by connecting a base s t a t i o n t o a closely spaced two-wire line ex- tending through the tunnel and mounted about s ix inches f rom the tunnel wall i n a posi t ion to provide as close cou- p l i n g t o the antennas on t h e t r a i n s as clearance or o the r r a i l road r equ i r e - ments w i l l permi t .

3 . An arrangement of the type descr ibed in (2) above employing RG-86/U wire and a 30-watt base s t a t i o n will provide covera e f o r a dis tance of approximately E 000 f e e t i n t h e North River tunnel of the Pennsylvania Railroad.

4. It i s believed that the cov- erage which could be obtained wi th t h i s arrangement i n other tunnels would not be ma te r i a l ly d i f f e ren t .

It should be pointed out t h a t o u r e f f o r t s were d i r ec t ed toward de - termining a p r a c t i c a l means fo r p ro - viding radio communication wi th r a i l road ro l l i ng s tock i n t unne l s . T h i s was t o be accomplished u t i l i z i n g equipment and f a c i l i t i e s known t o be ava i lab le and without employing special mobile s ta-

tion arrangements. It is not suggested tha t the arrangement arrived a t i s the best s o l u t i o n t o t h e problem nor i s it even suggested that RG-86/U wire i s the best poss ib le wire to u se . Fu r the r research and experimentation might wel l revea l a more e f f i c i e n t method or type of wire l i n e . However, such a study would be time-consuming and expensive. In view of the p rac t i ca l answer already arrived a t , i t does not appear to be j u s t i f i e d a t the present time.

In conclusion, the authors wish to express the i r apprec i a t ion t o t he i r a s soc ia t e s , Messrs. H. J. Bergmann and R. V. Crawford, fo r t he i r sugges t ions and a s s i s t ance i n conduc t ing t he t e s t s , and t o Mr. F. 8. Llewellyn for h i s advice and encouragement. Also, the cooperation of the engineers and other personnel of the Pennsylvania Railroad i s g r a t e f u l l y acknowledged.

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Bibliography

Ernest Dahl, "Rock Island Radio Tests", Elec t ronics , Vol. 18, pp. 96- 102; May 1945.

J. P. Shanklin, "VHF Railroad Com- munication in Tunnels" , Communications, Vol. 27, pp. 16-19; June 1947.

G. H. Leversedge, "The Problems of Radio Communication wi th Moving Tra ins" , B r i t i s h IRE J l . , Vol. 7, pp , 157 -163 ; Ju ly -AU@s t 1947.

J. B. Love11 Foot, "Transmission Through Tunnels", Wireless World, Vol. 56, pp. 456-458; December 1950.