Transoceanic Telephone Service-Short-Wave Equipment

Technical Features of the New Short-Wave Radio Stations ofthe Bell System

A. A. OSWALD*Member, A. I. E. E.

Synopsis.-The application of short-wave radio transmission receiving stations of the American Telephone and Telegraph Com-to transoceanic telephone circuits is developing apparatus and sta- pany located respectively at Lawrenceville and Netcong, New Jersey,tions designed specifically to meet the needs of these services. This and it outlines some of the radio problems encountered in the stationpaper describes from the radio point of view the important technical design.features and developments incorporated in the new transmitting and * * * * *

SHORTLY after transatlantic telephone service was entire papers. It will be convenient to deal with theopened in January, 1927 the long-wave radio transmitting and receiving stations separately and incircuit between New York and London was each case to consider briefly the system and apparatus

supplemented, first by an experimental short-wave of one channel before describing the general stationradio link in the west-east direction and later by a plan.short-wave link in the east-west direction.' From this TRANSMITTING SYSTEMbeginning, as an auxiliary to the long-wave circuit, the

shot-avsste hs ee imroedstadiy.o. ha The four channels at Lawrenceville are equipped withsindependent transmitters using certain auxiliary ap-its average performance throughout the year now more paratus in com'mon. Each channel involves a -radionearly approaches that of the long-wave system and it transmitter with itS associated power plant and wirehas become an important part of the transoceanic

facilities. The relative merits of the two systems, their e.. ~~and adjusted for the specific wavelength ass'lgnmentscombined usefulness, and their transmission features of the channel.are the subject of another paper and will not be dis-cussed here.2 For the present purpose it will be suffi- The general method of transmission, with the excep-cient to note that there are now in operation between tion of directional sending, is the same as that employedNew York and London, one long-wave and three short- for program broadcasting stations in that the radiatedwave two-way circuits and that within a few weeks signal contains the carrier and both sidebands. Systemsa short-wave circuit will be available between New in which one or more of these components are sup-York and Buenos Aires. pressed at the transmitter appear to offer further means

. . . ~~~~~ofimproving short-wave transmission and theThe radio transmitting units for the New York end Inecessary apparatus for the practical application ofof these four circuits are located at the new stationsuhyteswnoprigatfqecesnteodr

whichtheAerica Telehone nd Teegrap Com such syst'ems when operating at frequencies in the orderwhich the American Telephone and Telegraph Com-. .pany has recently established at Lawrenceville, New of 20,000 kilocycles is undergoing development. How-

Jersey. The receiving units areconcenever, throughout the development of the transmittersas now installed at Lawrenceville the possibility ofcong, New Jersey. The factors entering into the selec- future major modifications in the method of transmis-

tion of these station locations are outlined in another sion has been kept in mind. For this reason the modu-paper3 and therefore need not be mentioned furtherT psly bf. lator-amplifier system was adopted. In this system the

description of the transmitting and recesving sysem signal which is to be radiated, is prepared by modulationdescipton fte tansittig ad rceiingsysemsprocesses at relativelv low power levels and thereafterand apparatus, a discussion of technical features in the vstation layouts, and an outline of the major problems amplified the requisite amount. The amplifier and its

power plant, representing a large proportion of theencountered in the station design. Comprehensive .--.0._A' . .-investment in equipment, can be continued in servicetreamen ofindvidal nit isproerl let fr oherwith no appreciable alterations, even though the system

*Bell Telephone Laboratories, New York, N. Y. of transmission and the modulating apparatus undergo-1. 0. B. Blackwell, A. I. E. E. JOURNAL, VOl. XLVII, PP. radical changes.

369-373,May 1928.ThgeeasceeotrnmsinssowinF.2. R. Bown,page 624.Thgeeasceeo rnmso 1sow E.3. F. A. Cowan, page 638. 1. After passing through the line terminal and controlPresented at the Winter Convention of the A. I. E. E., New York, apparatus, which includes standard repeaters, the voice

N. Y., Jan. 27-31, 1930. currents are further amplified and employed to mrodulate629

30-28

630 OSWALD: TRANSOCEANIC TELEI'HONE SERVICE Transactions A. T. E. E.

the plate voltage of an oscillator consisting of two transmission line. The antenna itself is both a very250-watt tubes connected in a push-pull circuit and efficient radiator and a highly directive one.oscillating at the frequency of the carrier which is to be TRANSMITTING EQUIPMENTtransmitted. The frequency of such an oscillator, ifnot carefully controlled, will wander outside of the At the transmitting station the apparatus for eachnotcarfulycntolld, illwaneroutideof hechannel comprises, (1) wire terminal equipment andassigned frequency band, thus causing interference channel 2cm ises, (wreqntrnarl dequipmentanwith other services and it will also suffer variations repeaters, (2) a voice frequency control desk, (3) theduring the modulation cycle which contribute to fading radio transmitting set containing the oscillators,duringpthenome nodulationcyclewhichcontere istentor ing modulators, and power amplifier, (4) a power controlphenomena encountered at the distant receiving board, (5) rectifying apparatus and filters for supplying

RECTIONAL direct current at 10,000 volts, (6) motor-generatorsllANTENNA for providing various circuits with direct current, (7)

LINE water circulating pumps, tanks, and cooling units.AND CONTROIL AMPLF0R TRANSMISSONLINWThe wire terminal equipment and repeaters at the

COOSNCITUROLUTLCERD r n{gJtransmitting station are standard units mounted onOSCILLA n TWOrelay racks beside the voice frequency testing apparatus

AMPLIFIE common for all channels.The voice frequency control desk provides facilities

PIEZO-ELECTRC HARMONIC HRMONIC by which the attendant can monitor the incomingOSCILLATOR 'ENERKTORI CENERATOR'Z

voice currents and the outgoing radio signal. MeansFIG. 1-BLOCK SCHEMATIC OF TEANSMITTING SYSTEM are provided for observing the volume of these signals.

Oscillators are provided for the purpose of quicklystation. In order to reduce these effects the oscillator checking the performance of the system during line-upis held in step at the desired carrier frequency by means periods and for sending Morse signals over the radioof a second oscillator which is electrically removed from link when required. The control desk is also equippedthe reactions normally influencing and tending to vary with apparatus for direct telegraph communication withthe frequency of the controlled oscillator. Every the technical operator at New York.precaution is taken to maintain accurately the fre- The radio transmitter consists of seven independentlyquency of the second oscillator and among other things shielded units mounted on a common sub-base to formit is governed by a piezo-electric quartz crystal whose a single assembly, 4 ft. by 20 ft. by 7 ft. high. Some oftemperature is regulated closely. the units are subdivided into several small shielded

Since it is impractical to use crystals cut sufficiently compartments. Very effective electrical screening orthin to oscillate directly at frequencies in the range shielding between the various parts of a short-wave10,000 to 20,000 kilocycles, thicker crystals of lower transmitter is essential. Otherwise stray fields intro-frequency are used in combination with harmonic duce unwanted feedback couplings which producegenerators which multiply the crystal frequency firstby two or three and then by one or two as the caserequires. By virtue of the wide differences between theinput and output frequencies of the harmonic generatorsthese intermediate steps tend to isolate the crystaloscillator from the other radio circuits and thus aid instabilizing the frequency.The modulated radio frequency output of the con-

trolled oscillator is applied to the grids of a two-stagepower amplifier employing water-cooled tubes designed FIG. 2-FRONT VIEW OF SHORT-WAVE RADIO TRANSMITTERfor operation at these frequencies. The first stage OF TYPE USED AT LAWRENCEVILLEcontains two tubes and the second stage contains six.The tubes are arranged in push-pull circuits, the entire distortion effects and spurious oscillations. Fig. 2 is asystem being carefully balanced to ground. The front view of the transmitter. Beginning at the leftcarrier output power from the last stage is 15 kw. With there are two units for speech amplification, one for100 per cent modulation this corresponds to 60 kw. at radio frequency generation and modulation, one unitthe peaks of the modulation cycle. In other words, a each for the first stage, the interstage circuit, and theradio telephone amplifier of this type, rated at 15 kw. last stage of radio-amplification, and a double-sizedwhen provided with a sufficiently large d-c. power unit for the output circuit. It is interesting to notesource, could be used as a 10,000-kilocycle continuous that the over-all length of this assembly is as much aswave generator of 60 kw. capacity. five-eighths of a wavelength at the highest frequencyThe radio signal delivered by the amplifier is con- in its operating range, which is 9000 to 21,000 kilo-

veyed to the antenna by means of a 600-ohm open wire cycles. Each transmitter is required to operate at

April 1930 OSWALD: TRANSOCEANIC TELEPHONE SERVICE 631

several assigned frequencies within this range and to for each stage of the amplifier. This is necessary tochange in a few minutes from one to another. This is prevent distortion by interstage modulation caused bydone by changing coils and varying condensers in the the common impedance of the rectifier. Effects of thisoscillator and amplifier circuits and switching to differ- nature become important as the requirements placed onent quartz crystals. Except in cases where two assigned unwanted modulation products become more stringent.frequencies are in harmonic relationship, it is necessary

* 1 1 01 P 1 r* ml ~~~TRANSMITTING ANTENNASto provide a crystal for each of the frequencies. The tRasMItINGwAnTeNaScrystals are mounted in an oven and continuously The antennas at Lawrenceville all have compara-maintained at 50 deg. ± 0.05 deg. cent. by recording tively sharp directional properties. Such antennasregulators. In order to avoid long interruptions to are readily realized when dealing with radio waves ofservice in the event of a crystal failure or other circum- very short wavelengths. Although the fundamentalstance requiring the opening of the oven and the sub- principles involved in producing these directionalsequent re-establishment of temperature equilibrium, effects have been known for many years, economicthe ovens and crystals are provided in duplicate. limitations effectively prevented their application toThe electrical problems which are encountered by the transmitting antennas for long wavelengths. These

engineer designing a power amplifier for these high limitations are altered immensely in the case of an-frequencies arise largely from the inherent stray or tennas for short wavelengths and, when the usefuldistributed capacities and inductances which are far propagation properties of short waves became known,less important at lower radio frequencies. For example, great stimulus was given to the development of an-between the anodes of the amplifier circuit there exist tennas for directional sending and receiving. The samecapacities, which are composed of capacities within the type of antenna can be used, of course, for both purposestube itself, the direct capacities between the tube water but, since the objectives when sending and receivingjackets and mounting plates and the like. The total are somewhat different, the tendency has been tovalue of this composite capacity in the last stage is develop arrangements adapted to each case.approximately 100 m. m. f. This value cannot be Directional transmission is a very large subject andappreciably reduced by any change in design which now will only be touched upon sufficiently to describe in aseems desirable. The reactance of 100 m. m. f. at very general way the antennas at Lawrenceville. There20,000 kilocycles is about 80 ohms. Thus the engineer are many possible arrangements and combinations andis confronted at the outset with a generator (the tubes) the engineers must choose from these the ones mostwhich has an internal impedance in the order of 2000 suitable for their purpose. In general all of the schemesohms but across whose terminal is shunted inherently depend upon producing interference patterns whichan 80-ohm reactance. Fortunately, this obstacle can be increase the signal intensity in the chosen direction andsurmounted by introducing resonance effects but reduce it to comparatively small values in othernevertheless it places very important limitations on the directions.design of the associated circuits. These problems One of the methods of obtaining a sharply directivebecome more difficult with increase of either power or characteristic is to arrange a large number of radiatingfrequency. Increase in power requires higher voltages elements in a vertical plane array, spacing them atand currents and thus larger elements, spaced farther suitable distances and interconnecting them in such aapart. The augmented bulk increases both stray ca- manner that the currents in all the radiating memberspacities and unwanted inductance of leads. Higher are in phase. A simple way of accomplishing this resultfrequencies increase the magnitude and therefore the and the one which is now being employed at Lawrence-relative importance of these effects. ville depends upon the manner in which standing wavesThe power control board has nine panels equipped are formed on conductors. It is generally known that

with the necessary instruments and apparatus for current nodes and current maxima will recur along acontrolling and distributing all power to the transmit- straight conductor whose length is an exact multiple ofter. The motor-generators, pumps, fans, oil circuit one half the wavelength of the exciting e. m. f. and thatbreakers, and other apparatus are remotely controlled the phase difference between successive current maximafrom this point. A system of relays and signal lamps is 180 deg.' Such a conductor when folded in a verticalprovides protection and indicates the location and plane as shown in Fig. 3 and with its length adjustedgeneral nature of any trouble. With the exception of slightly to compensate for the effects of folding, satisfiesthe application of high-voltage direct current, the entire the aforementioned requirements for producing direc-system startsup and shuts down in the proper sequence tional radiation. The arrows in Fig. 3 indicate thein response to the manipulation of a master control relative directions of current flow and the dotted lineswitch. indicates the current amplitudes along the conductor.

Direct current at 10,000 volts is supplied to the It will be noted that the instantaneous currents in allanodes of the power amplifier tubes by a transformer 4.Thi asue.fcus htth cnutrinsaefeand rectifier using six standard two-electrode thermi- from objects affecting its electrical properties and that the endsonic tubes. The rectified current is filtered separately are free or properly terminated to produce reflections.

632 OSWALD: TRANSOCEANIC TELEPHONE SERVICE Transactions A. I. E. E.

the vertical members are in the same direction and that reflector "curtains" which are suspended between steelin the cross members their directions are opposed. Due towers appropriately spaced. Aside from other con-to these current relations and the physical positions of siderations, which will be mentioned in connection withthe elements, the cross members radiate a negligible station layout, the size of the antenna is influenced byamount of energy whereas the vertical members com- the complex and variable nature of the wave propaga-bine their effects for the directions perpendicular to the tion through space. At present this determines theplane of the conductor. In other directions destructive degree of directivity which is most useful for theinterference reduces the radiation from the vertical average conditions.5members. The system is equivalent to four Hertz The closed loops of each unit corresponding to Fig. 3

greatly facilitate the removal of sleet. In addition tof( _____________loading the antenna mechanically ice, having a dielec-

tric constant of 2.2 at these high frequencies, adversely%it\4>i affects the tuning. At Lawrenceville sleet is removed__I1g _ J 1 by heating the wires with current at 60 cycles. This is

-- _laccomplished without interfering with the service by/ / V employing one of the less familiar properties of a trans-

Ill11| x mission line. The same property also is used to effectimpedance matches wherever the transmission linesare branched. If a line, exactly one-quarter wavelength

g=et=_ long, of surge impedance ZO is terminated with a loadI ,I ZR, the sending-end impedance Zs is equal to Z 2/ZR.

If ZR is a pure resistance the sending-end impedance is-'1 _ _1a pure resistance. Hence a quarter-wavelength line

may be used to connect two circuits of different im-pedances and these impedances may be matched bycontrolling the value of ZO either by varying the diar-

FIG. 3-CONDUCTOR BENT TO FORM ONE SECTION OF SIMPLE eter of the conductors or their spacing. Likewise, ifDIRECTIVE ANTENNA ZO is fixed and ZR is made very small, then Zs will be

The type used for transmitting at Lawrenceville extremely large.

Oscillators driven in phase, and arranged in two groups In Fig. 4 two units of the type shown in Fig. 3 areone half wavelength apart, the two oscillators of each excited through transmission lines 1 and 2 of equalgroup being placed one above the other. Both compu- length in order to give the correct phase relations in thetation and experiment have shown that with thissystem of radiation there is an improvement of approxi- jmately 6 db. In other words the same signal intensityin the chosen direction is obtained with one-fourth ofthe power required by a one-element radiator. A secondsimilar conductor system placed directly behind the Afirst in a parallel plane one-quarter wavelength away, 2will be excited parasitically from the first conductor andwill act as a reflector, thereby creating a unidirectionalsystem. It has been found that the reflector furtherreduces by 3 db. the power required to maintain a given Lsignal intensity in the desired direction, thus bringing B0C,Lthe total gain for the system up to 9 db. This is also RADIOin agreement with the theoretical computations.

It is obvious that the system in Fig. 3 can be extended FIG. 4-ANTENNA SLEET-MELTING CIRCUITvertically to include more radiating elements by in-creasing the length of the conductor and it can be radiating elements R. The lines are joined in parallelenlarged horizontally by placing several units along- by condensers of low impedance at radio frequenciesside each other, care being taken to obtain the desired and they are connected in series for 60-cycle currentsphase relations by transmission lines of the proper by the quarter-wavelength line A which, being short-length. In this way large power savings may be circuited at the one end, presents a very high impedanceeffected. At Lawrenceville the maximum gain is about to radio frequency currents at the other end and there-17 db. (a power ratio of 50) over a vertical halfwave fore behaves like an anti-resonant circuit. The quarteroscillator. The enlarged system lends itself readily to 5. J. C. tSchelleng, "Some Problems in Short Wave Tele-mechanical support and forms so-called exciter and phone Transmission," I. R. E. Proceedingys, June 1930.

April 1930 OSWALD: TRANSOCEANIC TELEPHONE SERVICE 633

wavelength line B serves as a transformer and is transmission lines. It should be recognized, of course,adjusted to match the impedance at the junction of that in the early applications of a comparatively newlines 1 and 2 with that of the radio transmitter. The art, it is impossible to approach anything like accuratequarter wavelength line C is effectively short-circuited evaluation of all the factors entering into economicfor radio frequencies by the condenser D and acts the balances and furthermore very considerable weightsame as A. These quarter wave lines consist of short needs to be given to the probable future trend oflengths of pipe mounted on frames under the antenna developments.curtains as shown in Fig. 5. At Lawrenceville all of the antennas for the three

TRANSMITTING STATION channels to England are arranged in a straight lineAmong the first radio problems encountered in the about one mile long. The direction of this line is

design of a transmitting station for several channels perpendicular to the great circle path to Baldock,are those concerning the size, shape, and number of England, where the signals are received, (Fig. 6). The

antennas for the fourth channel are similarly arrangedin a line 1500 ft. long and they are directed for trans-mission to Buenos Aires, Argentine.

Placing several antennas in a single line reduces thecost of the supporting structure, and all the antennashave a clear sweep in the direction of transmission. Bylocating them in proper sequence with respect to wave-lengths it is possible without objectionable interference,to place the antennas end-to-end and thus use support-ing towers in common. Due to the wide difference inwavelength between adjacent antennas and their right-angle position with respect to the line of transmission,their proximity has no appreciable effect different fromthat of the towers. The proper selection of towerspacing in respect to wavelengths makes it possible to

}444M

FIG. 5-SECTION OF ANTENNA SYSTEM AT LAWRENCEVILLE +X< 130AM

Showing lower portion of curtains and quarter wave transmission lineused as transformers and anti-resonant circuits /20.5 M

TRANS. 11M

antennas, their directions of transmission, their relative I&2 32.7MGREAT CIRSCLEpositions from the point of view of mutual interference, TRANMI1TT STATIO ct 22M TO LONDONand their grouping around the transmitters. AMMERICAN TELEPHW & TELERAPHThe number of antennas required for each channel is 1-E 1.63 M

determined by the hours of operation and the average TN 30.7 Mgrade of service which the system is expected to render. TaNSFor service covering a large portion of each day sev- 0.7M

eral wavelengths are necessary. Transmitters Nos. # > J15.6M1, 3, and 4 at Lawrenceville each are assigned threefrequencies. No. 2 has five assignments in order to /improve the likelihood of at least one channel beingavailable throughout the entire day at all seasons.The size and shape of the antennas are, of course, FIG. 6-ARRANGEMENT OF ANTENNAS AT LAWRENCEVILLE

determined by the directivity wanted, by the type TRANSMITTING STATIONemployed, the frequency assignments, and by considera-tions of cost. They are governed also by the necessity erect a uniform supporting structure. This has theof connecting several antennas to the same transmitting advantage of flexibility and will permit future altera-set. This involves both the spacing and arrangement of tions of either the location or size of a given antenna.antennas to avoid adverse mutual reactions and it At present, each antenna occupies the space betweenrequires that attention be given to the losses in the three towers.connecting transmission lines, which are by no means In order to avoid undue loss in the transmission linesnegligible. Operating economies suggest concentrating the radio transmitters are grouped in two buildings.all the transmitters at one point but the cost per kilo- The buildings each contain two transmitters and arewatt hour of modulated high-frequency power must be identical in layout, in so far as the radio equipment istaken into account when considering the use of long concerned. Building No. 1 has additional space for the

634 OSWALD: TRANSOCEANIC TELEPHONE SERVICE Transactions A. 1. E. E.

central wire terminating and testing equipment. This a suitable level for efficient demodulation and is appliedapparatus is contained in an electrically screened room to the grid of the first detector. By means of a beatingwhich effectively prevents high-frequency fields from oscillator whose frequency is suitably adjusted, theinterfering with the proper functioning of the apparatus. first detector steps the signal carrier frequency down to a

RECEIVING SYSTEM fixed value of 400 kilocycles from one in the range 9000to 21,000 kilocycles which depends, of course, on theShort-wave reception iS characterized by less diffi- distant transmitting station assignment. The inter-

culty with static than that encountered with lonw es, mediate frequency signal at 400 kilocycles then passesOn the other handit suffers interference frosures through a combination of amplifiers and filters whichsuch as the ignition systems of passing airplanes andfuteexldthunaedierrnc.Tew tdautomobiles, which ordinarily do not disturb long-wave snaterreaches the scwanddeterere Tisnded

systms.Freuenty te icomig rdiowave sufersignal reaches the second detector where it is demodu-systems.~ ~Frqenl th inomn rai wae sufe Iated and the voice currents reproduced. The latterwide and rapid swings in intensity and there are varia- lat ed and appliedto teptelephoe lnestions in the apparent direction of arrival. On accountof the extremely high frequencies the apparatus and A portion of the output from the intermediate~~~~~amplifier which would normally go to the second de-antenna structures are very different from those for the tectorgrid,i dietd nd r ale s tenlong~~~~waes ohrietegnal scee f.eeto tector grid, is diverted and further amplified. It iS then

long wimilave oirectherwiseffectsandthuenrlsm ecption supplied to a device which automatically tends to main-aethodsimiar, directional effebots.anddoubledetectiontamn the receiver output volume constant by controllingmethods being employed for both.The radio wave is collected by means of a directional the bias potential of the first detector grid circuit. The

antenna array whose prime function is to improve theratio between the desired signal and unwanted noise or OSCILLATOR LINEDIRECTIONALTEMN IO

other interference. This it does in two ways: viZ., (1) ANTENNA AND REPEATER

by increasing the total signal energy delivered to thereceiver and (2) by discriminating against waves whose NSMISSION FIRS SECND NL

directions of arrival differ from the chosen one. In-creasing the total energy collected from the incomingmessage wave permits the detection of correspondingly STAGES FREQUENCY ATLI AND MONITORweaker signals because there is an apparently irreducible AMPLIFIER

minimum of noise inherent to the input circuits of thefirst vacuum tube in the receiver6 and this noise es- AUTOMATIC TWO STAGES

GANINTERMEDIATEtablishes a lower limit below which signals cannot be CONTRX FRECINCYAMPLIERreceived satisfactorily. Since, under many conditions, FIG. 7-BLOCK SCHEMATIC OF RECEIVING SYSTEM1the directions of arrival of static and other disturbancesincluding unwanted radio signals are random, it is time constants are adjusted so that this gain controlobvious that sharp directive discrimination aids very does not respond to the normal variation in signalmaterially in excluding them from the receiver. On power corresponding to speech modulation. Otherwise,the other hand, the antennas are not sharply resonant of course, there would be serious distortion effects.systems and they do not distinguish between waves This device partially offsets the ill effects of widefrom substantially the same direction and closely fluctuations in signal intensity but it does not overcomeadjacent in frequency. This duty is left to the circuits the deterioration in signal quality which usuallyof the radio receiver. accompanies the low field strengths during such fluctua-

Having collected the signal with a directional antenna tions.the energy is conveyed to the receiving set by means of RECEIVING EQUIPMENTconcentric pipe transmission lines of small diameter. At the receiving station the apparatus for eachThe use of concentric conductors simplifies the pre- channel comprises (1) the radio receiving set, (2) avention of direct signal pick-up by the lines, it reduces power plant for the receiver, (3) wire terminatinglosses and prevents external objects from influencing equipment and repeaters. The latter are located at athe transmission properties, thus allowing the line to be central point in the station along with certain voiceburied in the ground or placed a few inches above the frequency testing apparatus used in common by allsurface where it will have no appreciable adverse channels and supplied with power from a commoneffect on the antenna performance. source.

Referring now to Fig. 7, the radio currents arriving A radio receiving set which embodies the above de-over the transmission line are first amplified by two scribed system and of the type installed at Netcong isstages of radio amplification involving tuned circuits shown in Fnig. 8. It consists of a large number of indi-which discriminate further in favor of the wanted vidually shielded units mounted on panels and assem-signal. The signal delivered by the radio amplifier is at bled on three self supporting racks of the type commonly

6. J. B. Johnson, Physical Rev., July1928. employed in the telephone plant. This permits the use

April 1930 OSWALD: TRANSOCEANIC TELEPHONE SERVICE 635

without modification of certain standard pieces of radio amplification is provided in triplicate and theequipment, such as jack strips, fuse panels, meter switching is done between the first and second stage.panels, audio frequency filters, and the like. It also Thus the antennas are permanently connected to thepermits the removal and repair or substitution of units set and their adjustments remain undisturbed. Thewith a minimum of delay. The set is required to re- circuits of the second stage require tuning when theceive signals at three fixed frequencies in the range 9000 frequency is changed. Hence to tune the receiver onto 21,000 kilocycles. This involves connections with any one of the assigned frequencies the attendant merely

moves the dials of the second stage to predetermined* ~~~~settings, switches the grid circuit to a first stage which

li|| is already tuned and connected with the proper antennaand he adjusts the beating oscillator to obtain an inter-mediate frequency of 400 kilocycles. Screened gridtubes are used for the first two stages of amplification.A key shelf is provided with telephone and telegraphfacilities. The power plant consists of standard 24-voltand 130 batteries, rectifier charging units and auto-matic regulators.

RECEIVING ANTENNASIn discussing antennas for directional sending it was

mentioned that an identical antenna could be used forreceiving purposes, but since the requirements in thetwo cases are not the same, quite different structureshave been developed, although the methods of obtainingdirectivity are alike. In the sending case the reductionof random radiation ceases to be profitable when theincrement thus added to the energy, which is radiated

A in the direction of the distant receiving station, is arelatively small part of the total. In the receiving case,although the response to the wanted signal may not beincreased appreciably by further improvement in thedirective pattern, the reduction in noise and interfer-ence from random directions justifies additional im-provement. Expressed another way, the objective inthe transmitting case is a high gain compared to anondirectional antenna, whereas in the receiving casethe objectives are, first, a high average signal-to-noiseratio and, second, a gain sufficient to override the noiseinherent in the receiving set. Satisfying the firstaccomplishes the second.Improvement of the average directional discrimina-

tion means a nearer approach to ideal conditions.Whereas steel towers, sectionalized cables, guys and thelike, when properly located relative to the conductorsof a sending antenna, do not cause any appreciablepower loss, their presence near the receiving antennamay prevent the realization of the extreme directiveproperties which are wanted. Moreover, there is needfor much greater rigidity in the positions of the con-

B ductors. For this reason the antennas at Netcong areFIG. 8-SEHORTWAVE RADIO RECEIVER (A) FRONT VIEW (B) supported on wooden frames constructed like large

REAR VIEW crates.

three antennas through three separate transmission Due to the variable conditions surrounding thelines. The tuning of the antenna and transmission line propagation of short waves in space, the vertical angleterminations are rather lengthy processes requiring of arrival of the signal wave at the receiving stationprecise adjustments. In order to facilitate quick frequently changes considerably throughout a twenty-changes from one operating frequency to another four hour period and is not always the same from day towithout intricate tuning operations, the first stage of day. In order to combat this variable condition, it

636 OSWALD: TRANSOCEANIC TELEPHONE SERVICE Transactions A. 1. E. E.

appears desirable to select an antenna arrangement manner in which the signal waves arrive althoughwhich does not have sharp directional properties in a costs cannot be wholly neglected. The useful length isvertical plane passed through the horizontal direction limited by the fact that random fading occurs at dis-of arrival. The type of antenna selected for Netcong tances as short as ten wavelengths and it is doubtful ifmeets this requirement by having only a single hori- an antenna this long would realize the computed im-zontal row of quarter-wave vertical elements in one provement. The cost per decibel gained is small for theplane. Another solution, of course, would be to provide initial steps, but it mounts very rapidly as the lengthseveral antennas of different characteristics and to shift of antenna increases. The height also is limited by costabout from one antenna to another as the conditions and by the necessity of allowing for considerablewarranted.

Fig. 9 is a general view of one of the Netcong receiv-ing antennas. Like the transmitting antennas, the 4 -1- 4conductors are arranged in two parallel planes one- v CUROT --

quarter wavelength apart in order to obtain a uni- A Aldirectional system. The conductor in each plane is I) l AN11-IOOt Xbent and terminated as indicated in Fig. 10 but is muchlonger than that shown. The vertical members aremarked A. As in the transmitting case the directional FIG. 10-DIAGRAM OF SIMPLE DIRFCTIVE RECEIVING ANTENNA

variation in the vertical angle of arrival as discussed ina previous paragraph.

. ~~The antennas at Netcong are six wavelengths longand the lowest conductors are about 10 ft. off of the

'SP~ I 1ground. The gains over that of a half wave verticalantenna are in the order of 16 db. (power ratio of 40).The average improvement in signal-to-noise ratio is ofthe same order. There are certain null points towardthe sides and rear for which the ratio of directionaldiscrimination is very large.The transmission lines are constructed of inner and

outer copper tubes respectively 3/16 in. outside diam-

FIG. 9-ONEOF

RECEIVING ANTENNAS AT NETCONG. (24.7 RECEIVING STATIONAIIRCAN TELEPHONE & TaEGERAPH CO.METER WAVELENGTH) SNTCONC NJ

GREAT CIRCLE

effect depends upon the manner in which standing waves CENTRAL TERNAL CT Loccur along the conductor. A signal wave arriving 33.2M /broadside to the array, induces voltages in the vertical RECEPVERmembers which are identical in phase and amplitude. . 4TM "K *4G30R /

Because the vertical members are interconnected R4 33alternately at the top and bottom by members of one- 3 RECEN R

quarter wavelength and the last horizontal members eRrEsAT CtR_ETO

are one-eighth wavelength, the net effect of the induced.IA

voltages is the establishment of standing current andvoltage waves along the conductor. The receiver isconnected at a voltage anti-node and the current which FIG. 11-ARRANGEMIENT OF RECEIVING ANTENNAS AT NETCONGflows through it is proportional to the sum of the volt- RECEIVING STATIONages induced in the vertical members. In the case of asignal wave arriving from the horizontal directions eter and 5/8 in. inside diameter. The tubes are heldparallel to the plane of the array, the voltages in the concentric by torroidal shaped insulators made ofvertical members are in successive quarter-phase Isolantite, a ceramic product similar to porcelain andrelationships, no standing waves are produced, and no well adapted for high-frequency voltages. This samecurrent flows through the receiver. Because current material is used for insulating purposes throughout thenodes occur at the center of each horizontal member, transmitting and receiving antennas. Transmissionthe loss by reradiation from these members is negligible. lines are supported a few inches above the ground andThis is an important feature which contributed to the are connected to earth at short intervals. The linesselection of this type of antenna for Netcong. vary in length from 200 to 1500 ft. One of the inter-The size of the antenna is determined largely by the esting problems in connection with their design is the

April 1930 OSWALD: TRANSOCEANIC TELEPHONE SERVICE 637

provision of means for allowing variation in length with order of frequency are separated more than this. On thetemperature. Ordinary expansion joints introduce other hand, to avoid adverse reactions no two are placeddifficulties with electrical contacts and impedance adjacent and end-to-end as at the transmitting station.irregularities. To avoid these the lines are made 10 per The end-to-end separation at Netcong is in the order ofcent longer than otherwise necessary and they follow a four wavelengths. The areas surrounding antennas are

sinuous course which permits the necessary bending. cleared of trees and kept free of all overhead wires or

Sharp turns are not permissible because experiments conducting structures to avoid reflection effects whichhave shown that they cause reflection disturbances. disturb the directional characteristics of the antennaThe measured loss in 1000 ft. of line at 20,000 kilocycles systems.is 2db. The locations of antennas are also influenced ma-

RECEIVING STATION terially by the necessity of avoiding interference fromthe ignition systems of internal combustion engines.

The radio problems encountered in the layout of the This imposes a requirement that the station site bereceiving station, in general, include most of those isolated from air routes and roads carrying heavyalready mentioned in connection with the transmitting traffic. The antennas are placed as far as possible fromstation, but their solution in some instances is quite secondaryroadsnwhichcrosstheirpline of reception.different. In addition there are requirements imposed Te layoat Ntcons shownin Fig1.by sources of radio noise both within the station itself, The layout at Netconga S shown inFog. 11. Thereand in the surrounding area which is beyond the controlof the station. receiver building for each group. A headquarters

building located at the road entrance contains the wireThe number of antennas is determined, of course, by terminating equipment, line repeaters, and voice fre-

the frequency assignments of the distant transmitting quency testing apparatus. The power plant at eachstation. Where two assignments are within 100 kilo- receiver and the entire central terminal apparatus atcycles it is possible to use the same antenna for both, the headquarters building are placed in electricallybut thus far, this has not been done at Netcong. shielded rooms to prevent radio noise disturbancesThe size of the antennas is not limited appreciably emanating from them and reaching the receivers directly

by the length of transmission lines because other or via the antennas.factors make it necessary to separate them rather The radio stations described herein are pioneerwidely. On this account and also because the receiving commercial applications in the development of shortapparatus and its power plant are small, comparatively wave telephone transmission. Although progress hasinexpensive units, it is economical to place the receivers been rapid and far-reaching our knowledge of thein small buildings centrally located with respect to the behavior of short waves is by no means complete. Itgroup of antennas for one channel. In this case the is reasonable, therefore, to expect that the future holdslengths of transmission lines are not controlling factors many improvements and that the information obtainedand the dimensions of antennas are governed primarily by further fundamental investigations may materiallyby the considerations previously outlined when de- alter both our views of the transmission phenomena andscribing the individual antenna. The small height of our ideas of what the apparatus and stations should be.the antenna permits them to be placed in the line ofreception of other antennas spaced ten wavelengths ormore away and of widely different frequencies such as Discussionthose of one channel. Antennas adjusted for the same For discussion of this paper see page 643.