114 PHILlPS TECHNICAL REVIEW VOL. 1, No. 4

THE V.R. 18 TRANSMITTING AND RECEIVING EQUIPMENT

By C. ROMEYN.

can be reptoduced immediately the recordingprocess has been completed (e.g. after 1/5 of asecond). This property is of the greatest impor-

Fig. 12. Microphotograph of the sound-track of a 1000-cycle note on the "Philimil" tape, with the same magnifi-cation as in fig. 8. The coating is devoid of all grains andthe edges of the sound-track are sharply defined. The groundnoise is therefore much reduced.

tance and value when recording sound-films. Theproducer has now no longer to wait for thedevelopment of the light-sensitive film in order to

Introduetion

With the progressing development of commer-cial flying, the need for some means of inte{com-munication between an aircraft in flight and theairport very soon became apparent, and the firstpassenger and commercial airplanes, although stillvery small, were already equipped with wirelessapparatus. With the steady and radical improve-ments in technical methods and apparatus duringthe last ten years both flying and wirelesstechnology have made rapid strides. The impor-tance of wireless intercommunication during flighthas progressively increased and at the present dayit is impossible to conceive of a passenger orcommercial aircraft being without wireless

decide whether the sound record conforms withhis requirements. After each scene has beenrecorded he can listen in to the playback of thesound-track immediately and decide on the spotof the sound-track for documentary and otherrepeated.For broadcasting purposes also, the immediate

reproducibility of the sound-track is of the greatestutility. The exchange of programmes betweenstations, the postponement of the transmission ofcurrent items of news (races, speeches, etc.) to amore suitable time of the day, the production ofradio plays, all these are much facilitated by thePhilips-Miller system, while the tonal qualityexceeds that obtainable with the wax disc. Thehigh fidelity of reproduetion also offers a methodof copying sound-records which in certain circum-stances may be very convenient, viz, by makinga new record of the reproduced sound track on asecond "Philimil" tape. This "mechanical" copyingcan he carried out at the Sa111etime as repro-duction, so that a direct duplicate can he obtainedof the sound-track for documentary and otherpurposes.

".1 apparatus. Reports of weather conditions alongthe aircraft route, landing instructions, direction-finding signals, etc., have become indispensahleto the pilot.

It is the task of the wireless industry to providesuitable apparatus capable of meeting the specialrequirements for use in modern aircraft. That anaircraft radio equipment in many respects mustdiffer fundamentally from a permanent andstationary ground equipment is obvious. In thepresent article the V. R. 18 aircraft transmitting-receiving equipment designed by Philips IS

described. This equipment has been speciallyevolved to meet the various requirements for useaboard aircraft, yet in its design attention has,moreover, been given to certain specialised needs

APRIL 1936 AIRCRAFT RADIO EQUIPMENT V. R. 18 U5

considering the application of this equipment forthe Douglas air liners operating on the Nether-lands East Indies route of the K.L.M. air services.

General Characteristics of a Wireless Equipmentfor Aircraft Use

The V.R. 18 equipment (see fig. 1) consists mthe m ain of the transmitter, receiver, aerial andrequisrte sources of power, as well as a series ofauxiliary components such as the control box, theaerial lead-through, etc. In both electrical andmechanical characteristics, all components have tobe designed to meet special requirements. Th us allparts must be as light as possible and take np theminimum of space without constituting an obstruc-tion. Furthermore, the equipment must be installedin such a way that it is not exposed to seriousvibration or hard jolts. To permit the interior tobe tested readily and quickly, easy dismantling ofboth the transmitter and the receiver is moreoverdesirable.

Fig. 1. Complete wireless equipment V.R. 18 for aircraft.(In the Douglas machines the various components aremounted in different places.) Z = Transmitter. 0 =Receiver. B = Control box. ZO = Converter furnishinganode voltage of transmitter. 00 = Converter furnishinganode voltage of receiver. S = Switch for changing overfrom trailing aerial to fixed aerial.

Intercommunication hetween aircraft and air-port is 'nowadays performed almost exclusively by

tel e g r a phi c means. In this connection it IS

interesting to review briefly the historical develop-ment of the methods employed. During the earlyyears of flying intercommunication with aircraftwas carried out solely by means of the tel e -ph 0 n e. This instrument alone could be used atthat time, since the pilot who had to operate thewireless apparatus already had both hands fullyoccupied in controlling the flight of the machineand it was thus impossible for him to work a Morsekey. The disadvantages of using telephony became,however, steadily more apparent. In the first place,to cover the same range a telephone transmitterhas to have a greater power than a telegraph trans-mitter; .yet the most serious drawback of telephonyis that it requires a wider frequeucy band, since,owing to modulation, an additional side hand istransmitted at hoth sides of the carrier-wave fre-quency. In view of the increasing number of trans-mitting stations, which were concentrated in a'comparatively small geographical area, the fewfrequency bands available were soon taken up. Theonly practical means for avoiding intensive mutualinterference of stations was to adopt the tele-graphic method of intercommunication. Thismade it necessary to provide a wireless operatorfor each aircraft in addition to the pilot. Thisaddition to the crew would, however, have becomenecessary for other reasons, even without changingover from the telephone to the Morse key. Thegreater demands made on the pilot by the morecomplex problems associated with navigation(flying at night and through fog) were alreadymaking the care of the telephone an onerousadditional responsibility, while on the other handwireless intercommunication for the self-samereason, viz, the much-increased number of reportsrequired for safe navigation, itself demanded closerattention. Moreover, it had become practicable tocarry a special operator, as larger aircraft werebeing built in which more room was provided inthe pilot's cabin.

Intercommunication between aircraft and air-ports is thus at the present time almost exclusivelybased on tel e g r a p h y. By international agree-ment the wave-lengths of 600, 944, 918 and 932 mhave been allocated for wireless aircraft trans-mitters, of which the 600-m wave-length is onlyallowed for transoceanic flights.

The Transmitter

The transmitter of the V.R. 18 equipment isconstructed for continuous-wave and tonic-traintelegraphy. In the former a high-frequency oscil]-

116 PHILIPS TECHNICAL REVIEW VOL. 1, No. 4

ation IS radiated at intervals corresponding to theMorse signals. These oscillations are generated bya control stage 5 (see fig. 2) containing an oscill-ating valve (called the control valve) and a tuning-circuit. The oscillations generated are amplifiedby an amplifying stage V comprising two valvesconnected in parallel. In the anode circuit of thisstage the amplified energy is fed to the transmit-ting aerial A. This method of wiring ensures a veryconstant frequency, as aerial reaction on theoscillator (control stage) IS very small. Forfrequency adjustment the tuning-curcuit of thecontrol stage is provided with a variable conden-ser, which by a snap action can be fixed in fourstandard positions. These four settings corespondto the above-mentioned standard internationalwave-lengths, but can be altered to conform toany subsequent alteration in the agreed wave-lengths.

5

Vg

-Va + Va

Fig. 2. Simplified circuit diagram of the transmitter V.Z.IS.S is the control stage, which generates the desired frequency,V the amplifying stage containing two valves in parallel,A the aer-ial circuit with aerial reaction, tuning variometerand ammeter. The vo ltage-drop at the resistance in parallelwith the Morse key SZ is applied as negative grid bias Vgto all valves and inhibits transmission. By means of theMorse key this resistance is shorted and, as a result, thetransmitter enabled to transmit in synchronism with theMorse signals.

A high negative bias V IS applied to thegrids of the control and amplifying valves, whichinhibits the oscillation. During transmission thisnegative bias is removed in synchronism with theMorse signals (see fig. 2).By means of a rotary interrupter, which is in

series with the Morse key, the radiated high-frequency oscillation can, moreover, be Interru pted1000 times per second. This method of trans-mission, tonic-train telegraphy, is used for makingthe connection with some station. Owing to thegreater width of the frequency band as a resultof modulation with the 10'00-cycle frequency,

tuning is rendered more simplé; But as soon as theconnection has heen set up, the wireless operatorchanges ovcr to continuous-wave telegraphy, sincethe latter cal~ses less interference owing to theabsence of side hands.The Douglas air liners on the East Indies route

of the K.L.M. have to cross regions in Asia whereairports are few and far between. In order to keepin communication with at least one airpost a power-ful transmitter is required, and for this reasonthe power output of the V.R. 18 equipment inthe aerial circuit has been rated at 75 watts. Therange when using a trailing aerial (see below)is then at least 600 km for continuous-wave tele-graphy and 300 km for tonic-train telegraphy. Incertain circumstances the range may he consider-ably greater and on several occasions this equip-ment has been ahle to transmit over distancesexceeding 6000 km. During flights in the Europeanzone, where a large numher of well-equipped air-ports are situated close to each other, a fairly smalltransmitting power is, on the other hand, sufficientfor efficient intercommunication, provided atmos-pheric disturbances are not too serious. In fact, atransmitter with an excessive power output isundesirable for this zone, since it may interferewith the wireless signals being transmitted simul-taneously from an airport to other aircraft inflight. The transmitting power of the equipmentcan therefore be regulated, the aerial power heingreducible to either a half or a quarter hy increas-ing the negative bias of the amplifying valves.

The interior of the transmitter, with the chassispulled out, is shown in fig. 3.

Fig. 3. V.Z. IS transmi tter with rhassi« drawn out and frontwall removed. The housing and chassis are made of dur-alumin.

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o 11 E

I: zIII I: IL ~

Fig. 4. Simplified circuit diagram of receiver V.O. 18: The circ~'ït shown is th~i:of a. Iive-vulve superheterodyne receiver, where A is the first tuning circuit, H thehigh-frequency amplifying stage, 0 the modulating and oscillating valve, Osc the,oscillating circuit, which is tuned to an oscillation with a frequency differing byan. almost constant amount (differcntial or intermediate frequency) from the tuningfrequency of A and, H. The three variable condensers of these three stages aremounted on a common shiift. Fl is the first and F2 the second intermediate-frequencyband filter; bv means of à variable coupling the band width passing through thesefilters can be varied .. The intermetliate-frequency amplifying stage M is situatedbetween the two filters. The intermediate-frequency alternating voltages are- appliedto the diode D; which also contains in the same glass-body the three-electrode valve

{ of the Iow-Irequeney amplifying stage L. Z is the beat oscillator which generates anoscillation differing from the intermediate frequency by a specific (variable) frequency(usually 1000 cycles). The oscillation of the beat oscillator is also applied to D,so that at the exit of the rectifying stage an oscillation with the differentlal frequency(1000 cycles) is to be heard -. E is the terminal stage with the output transformer,The oscillating circuit I is 'tuned to the intermediate frequency and hence short-

, ciréuits any (disturbing) .carrier wave emanating from a long-wave transmitter with áfrequency equal to the intermediate frequency. By means of the switch S-the automaticvolume control" reacting on all preceding valves H, 0 and M, can be switched on" (left position) or replaced hy manual control (r'ight position). To generate the rectified•output voltage Jor the automatic volume control a special detector d is provided.This is necessary because the diode- D, being coupled to the heat oscillator Z, already .,furnishes' an output voltage when no signal at all is received at the receiver, and wouldthus in this case already reduce the sensitivity of the receiver.

/5610

The Receiver frequency. If this frequency together with , th~intermediate-frequency signals i\'l passed to the

When crossing those areas .of Asia sparsely rectifying, !'t~ge, the Morse signals become audibleprovided with airports an aircraft must. needs be o~ the differ!'!ntial' frequency between' the two.equippedwith a verysensitive receiver. The super- The frequency of the heat.oscillator can be regu-heterodyne method, adopted' in the V.R. 18 equip- , lated, sq that the pitch of the Morse signals can

'ment is particulá~ly, suitable, .for obtaining the : be adjusted as required. This simplifies the separa-high sensitivity required. A 'lay-öut of the circuit tionj of. stations operating on wave-lengths. closeemployed (simplified). is shown in fig. 4. An octode together .• THe recei~er is rated for wave-Iengths(0) serves as a converter valve. It is preceded by a h,~twe~n 520 and 1300 m. . , 'high-frequency amplifying stage (H). The inter,:, The, apparatus, on a' wave-length of 600 m,mediate frequency generated in the converter varv~ ;', furnishes a power output of 10 milliwatts with ais again amplified {in M), tl{en rectified (i~' D) ,,0.75 -u-volt . amplîtude of. the incoming signal "),and amplified once' more in' the, low-frequency . This is' roughly the maximum sensitivity whichstage (L). A small heat ,oscillator (Z) is provided: can he attain~d at.present. ' .,to render the' contiriuous-wave signals audible,:- ~'IT9-e sensith~j~y~:or:the receiver' can be regulated(since no audible frequencies per se are obtained by hand, or an automatic volume control (see Sfrom these signals after' tlre.rectifying stage). This in 'fig. 4) m~y he put !nto action witl~ the aid 'ofoscillator' generates .osciflations with a frequency ---,~) Measured in accordance with the usual definition ofwhich differs only slightly from. the intermediate sensitivity,

117

118 PHILlPS TECHNICAL REVIEW VOL. 1, No. 4

which the receiver adjusts itself continuously to afairly constant output power and the volume canthen be adjusted by hand to the required value.Volume control is particularly useful whenapproaching radio beacons, as without it thevolume has to be continually readjusted.A further requirement which has to be met In

the receiving apparatus is the possession of a highselectivity. This feature is necessary to set up areliable channel of communication in areas whereair traffic is heavy, and is also of great value forflying in the tropics in order to reduce the effectsof atmospheric interference. The circuit of thesuperheterodyne receiver permits a very highselectivity to be obtained in a very simple manner,for the intermediate frequency is constant so thata large number of invariable oscillating circuitscan be tuned to it (see P, and F2 in fig. 4). Insome cases, however, a high selectivity is undesir-able, particularly during wireless telephone recep-tion (hy cutting off the side bands, speech becomesdistorted or even unintelligible) and particularlywhen picking up stations; in this case the wirelessoperator listens whether he is being called by anyairport and must therefore listen as it were to allstations at the same time. To permit. this to bedone the selectivity of the apparatus is variable;the band width can be adjusted to 3.8, 5.5 and 7.5kilocycles. The smallest band width is used forreceiving continuous-wave telegraph transmitters,the medium width for telephony and tonic-traintelegraphy, :111(1 the largest for picking up stations.A photograph of the receiver with the chassis

pulled out is reproduced in fig. 5.

Fi~. 5. V.O. 18 receiver with chassis pulled out. Thehousin g and chassis arc made of duralnmin.

The Aerials

The Douglas all' liners are equipped with atrailing aerial, i.e. a wire 60 m long which isloaded with a weight at the lower end and canbe paid out during flight through a lead-throughin the body of the aircraft. The paying-out andwinding-in of the aerial is performed by a winch.An electrical counterpoise is provided by the metalfuselage of the aircraft.In many cases a trailing aerial cannot be used.

The time required to fly from one airport toanother at a small distance, e.g. from Schiphol toWaalhaven, is only a few minutes at the highspeeds attained by the Douglas machines. Thistime is not sufficient for paying out the' aerial.Nevertheless, unbroken radio cornmunication isnecessary, for the aircraft must receive its landingiristructions from the airport during flights.Moreover, it is desirable when landing to take inthe aerial in good time and yet remain in com-munication with the airpost up to the last minute.For this purpose each machine is equi.pped withan additional aerial which is fixed permanen tlyabove the body. This is also naturally the onlymeans whereby Ianding can be controlled fromradio-beacons.The Douglas air liners must be capable of flying

in all weathers. Should they fly through stormclouds, a long trailing aerial will increase the(langer of being struck by lightning. The aerialis therefore taken in and transmission and recep-tion must then be maintained with the aid of thefixed aerial. To keep in communication with theairport, which in this case may be at a greaterdistance, and in spite of the lower radiation ofthe fixed aerial, which has only one quarter of the"effective height" of the trailing aerial, every carehas been taken that the maximum possible portionof the available energy is radiated. In view of this,not only the trailing aerial but also the fixed aerialhas, therefore, been carefully adapted to theamplifying stage, by introducing a special aerial-loading inductance and aerial coupling. A singlemanual movement of the switch provided is allthat is required to change over from the trailingaerial to tbe fixed aerial.

Power Supply

Perhaps the nature of the power supply bestbrings out how far improvements in aircraft designhave had a fundamental influence on the designof the aircraft wireless equipment. In the past anoutboard generator was used with an auto-regulat-

APRIL 1936 AIRCRAFT RADIO EQUIPMENT V. R. 18 119

iug air-screw (the anode voltage and the filamentvoltage bad to be kept constant; in other words,the speed of the air-screw had to be independentof the "wind" velocityover a wide range, i.e. ofthe flying speed). As the flying speed was in-creased, which was achieved mainly by giving alland even the smallest components of the aircrafta stream-line design, the use of an outboard gener-ator was no longer permissible lil VIew of itshigh air resistance.

To act as a source of power for the wholeequipment, the 12-volt starting battery is now usedwith the V.R. 18 set. The filaments are fed directlyfrom the battery. The anode voltages for thetransmitting and receiving valves are furnishedby two couverters which are driven from thebattery. The anode voltage of the receiving valvesmust be properly smoothed and any interferenceeliminated. Formerly the necessary supply wastherefore furnished by a dry battery; the converternow used for this purpose is of special design witha very low interference from the commutatorbrushes and in which all causes of interferencehave been most carefully eliminated by means ofcondensers and chokes.

The converter for the transmitting valvesfurnishes a uni-directional voltage of 500 volts at300 milliamps and the converter for the receivingvalves 200 volts at 40 milliamps. The startingbattery is recharged during flight by a dynamo,which is driven from the aero-engines. Comparedwith the outboard generator, the battery offers theadditional advantage that the machine can forsome time continue to send out wireless messagesfrom the ground, for instance after a forcedlanding.

Installation of Equipment

The transmitter with the associated converter isaccommodated in a corner of the luggage cabin.It is suspended by shock-absorbing cables and isthus adequately protected against jolts and vibra-tion. The receiver is set up on spring rubber

supports lil front of the operator's seat (fig. 6),together with the control box, on which arearranged, among other items, the Morse keys,various switches and the aerial ammeter. For nightflying, dial illumination is provided, being capableof regulation and so placed that it does not inter-fere with the pilot's field of vision. The componentsof the transmitter and receiver are mounted onspecial chassis, which can be taken out separately.Like the boxes and the converter honsings, thesechassis are made of duralumin or aluminium inorder to keep the weight as low as possible. Thewhole equipment (transmitter, recei.ver, two con-verters, winch and aerial lead-through, control box,switches, cable and telephone) weighs about 92 Ibs.

Fig. 6. Installation of the receiver (above) and the controlbox (below) in front of the wireless operator in the pilot'scabin of a Douglas air liner.