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184 PHILIPS TECHNICAL REVIEW Vol. 2, No. 6

POSITION FINDING AND COURSE PLOTTING ON BOARD AN AEROPLANE BYMEANS OF RADIO

Summary. In this article several radio receiving arrangements are described, with whichit is possible to plot the course and take radio bearings on board an aeroplane.

IntroduetionThe necessity of determining the posItIOn of

aeroplanes by means of radio has become moreurgent now that more and more flying is beingdone in the upper air. Originally a pilot had todepend for his navigation entirely upon the groundservice, which at his request gave his position andthe compass course he should follow. To do this theaeroplane asks one of the direction-finding stationson the ground to determine his position with atleast one other D-F station. The aeroplane trans-mits a continuous signal for some time so that theD-F stations can determine the direction fromwhich the signals are received. One of the groundstations then works out these cross-bearings onthe map and signals the resulting position to theaeroplane. Even with long practice the taking ofsuch cross-bearings of an aeroplane occupies atleast two ground stations for several minutes.If in a given region there are other aeroplaneswhich are demanding their position or their course,they must await their turn. With the increase inspeed of aeroplanes such a wait became more andmore of a disadvantage, while because of the in-creasing density of air traffic it has become moreand more unavoidable. An attempt is being madeto limit as far as possible the demand for bearingsfrom the ground service by equipping the machineswith instruments which make it possible for thepilots to determine their own course and position.If there is a radio transmitter in the line of

flight of an aeroplane, then the correctness of thecourse followed can be checked with a relativelysimple instrument, a so-called course finder, whichcan be installed in even small private aeroplanes.If, however, one's course is toward an aerodromewhere there is no transmitter in the neighbourhoodthe course can be checked by finding the positionat short intervals by means of a radiogoniometer.In order to take such radio bearings one must beprovided with a more highly perfected measuringinstrument, a so-called homing device which can beinstalled on board the larger aeroplanes. Whenbearings are being taken, however, the radio-telegraphist must neglect his ordinary radio trafficservice duties, such as receiving weather reportsand the like, and enquiring about the landingpossibilities.

Course finder

With the aid of the Philips' course finder V.P. 4,shown in fig. 1, which can be used in series with anordinary receiver, one is able to check the correct-ness of the course followed in a simple way. Theaction of this instrument is based upon the direc-tional reception by means of a loop aerial, whichis situated on or in the case of wooden constructioninside the body of the aeroplane with its planeperpendicular to fore and aft line of the machine.

Fig. 1. Course finder, type V.P. 4.

Fig. 2 shows the horizontal polar diagram ofa vertically placed loop receiving a constant signal.If the North-South direction is perpendicular to theplane of the loop, the polar diagram consists oftwo circles touching each other along the line ofthat direction as drawn in fig. 2 (figure-eight

JUNE 1937 POSITION FINDING AND COURSE PLOTTING BY MEANS OF RADIO 185

diagram). If we choose a course, directly towarda radio station, it will be received with minimum 'strength on the loop' which is directed with itsplane perpendicular to the direction of flight. Ifone flies in any other direction, the signal is received

Fig. 2. Diagram of the receiving strength of a loop aerial(figure-eight shape) and non-directional aerial (circle).The heart-shaped diagram is obtained by combining thesetwo in an appropriate way. '

with greater intensity. The strength of the signal,. however, gives no indication as to whether one mustturn to the right or theleft in order to regain thecorrect course., In order to obtain an indication of the direction

,in which one must steer, the course finder is so,arranged that the electromotive forces which aregenerated in the loop are combined with thoseinduced in the ordinary receiving aerial of the air-plane. The polar diagram of the received signalstrength on an entirely non-directional aerial is acircle as shown in fig. 2. The ordinary aerial of anairplane, it, is true, does not give a truly circulardiagram, but an oval one, which is symmetricalwith the axis of the aeroplane, so that in principlethe following considerations may he applied to it.At a given position of the combination switch theaerial and the loop electromotive forces are in phasewith each other for the right half of the polardiagram, and just in opposite phases for the lefthalf of the diagram. We therefore obtain a h~art-shaped diagram (cardioid) for the receiving strengthof the course finder, as is shown in fig. 2. When'the combination switch is, reversed, the heart-shaped diagram is obtained on the other side ofthe north-south direction. If in fig. 2 the signaldoes not come exactly from the north, but a littleto the east, then in the course finder the electro-. motive force in the ordinary 'aerial is increased bya small electromotive force in the loop. When the

, ,switch is reversed, the latter is subtracted from theformer and the signal thus becomes fainter. A signal

which in fig. 2 comes from west of north, however,becomes stronger on commutation., In this simple course finder the commutation isdone by hand, while reception is exclusively byear.A second commutator may be introduced at somedistance from the course finder, for instance on thecontrol stick. This reversing ofthe switch by handoffers no great practical difficulty, since it is onlynecessary while the course is being found. When thecourse in once found, the course finder may be dis-connected from the ordinary aerial so that the courseis kept by means of the minimum loop reception.

Direction finder-homing device

The pilot can be provided with a visual indicationof his course by providing for a periodical com-mutation. This can be done by means of an elec-trical, mechanicalor electromagnetic' circuit ar-rangement. The Philips direction finder-homingdevice V.P.K. 35 (fig~·3 and photographs on page 182and 183) is a combination of a direction finder and,a course finder, which if; furnished with anautomatic commutation ;frrangement and a visualindicator. As the name ,indicates, the apparatusserves not only for "homîng", but also for takingbearings. It is used in all the new Douglas DC, 3machines of the K.L.M . <. /

The apparatus is suitaine for:1) Normal, n onv d ir e c tq'o n a I reception whena special vertical aerial ;älone is connected to it.This aerial is introduced into a hollow aerialmast made of insulation materialon top of thçfuselage.2) Tàking bearings, whereby the angle be-tween the direction of the sending station and theaxis of the aeroplane is measured with the help •of a loop aerial, whieh is erected on top of theaeroplane body, and rotatable about a verticalaxis. In fig. 2 the figure-eight diagram of thereceiving strength is that when the loop planeis assumed to be perpendicular to the north-southdirection. At minimum reception the plane of theloop is perpendicular to the direction from whichthe signal comes. In order to obtain a sharp min-imum the action of the loop as a non-directionalaerial, the so-called aerial effect of the loop, iscompensated. The signal maystill, however, havecome from one of two opposite directions, and onecan decide which of these is the correct one by:3) Sen sed e ter m i nat ion. As with the coursefinder the signals received on the loop and' OD the

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1) In a later number of this periodical, in an article aboutthe .measurement of field strengths, a more detailed explanationwill be given of the way in which the aerial effect can becompensated.


vertical aerial must be combined to give theheart-shaped diagram of the receiving strength asin fig. 2. The loop is now, however, not fixed im-movably with its plane perpendicular to the longi-tudinal direction of the aeroplane, but may berotated about the vertical axis, so that the north-south direction in fig. 2 can be arbitrarily orientedwith respect to the axis of the machine.If, when the position of the plane of the loop

is perpendicular to the north-south direction infig. 2; we have a minimum reception on the loopalone, the signal may still come either from the

the plane of the loop is perpendicular to the axisof the aeroplane, we then hear strong dashes andweak dots, while a signal coming from the lefthalf plane gives strong dots and weak dashes.When the course is directly toward the transmitterone hears a continuous dash. Not only is the pilotable to hold his course toward a radio transmitter,byear, but also he has the course-line toward thetransmitter concerned given on the visual courseindicator which is installed on the dashboard.

5) Beacon reception of the Philips long-wave beacon, B.R.A. 101, which is installed

Fig. 3. Direction finder-homing device (Type V.P.K.P .. 35).

north or from the south. If the loop is turned slightlyto the right, the combined receiving strength onloop and vertical aerial will decrease for a signalcoming from the north and increase for a signalfrom the south. If the phase of the loop signalis reversed, the heart-shaped diagram is mirroredwith respect to the north-south direction of thediagram, and the changes in the received strengthupon turning the loop in the neighbourhood ofthe position of minimum reception are exactlyopposite.4) Hom ing. The phase of the electromotive

force generated in the loop is periodically accordingto a dot-dash rhythm having a time intervalrelation of I : 7. During the long dash interval thereceived signal strength is as shown in the heart-shaped diagram of fig. 2, while during the shortdot interval this diagram is mirrored with respectto the north-south direction. If the signal in fig. 2comes from the east, i.e., from the right when

at all the aerodromes III the Netherlands. Thisbeacon has an aerial system which, like that ofthe bearings finder, consists of a loop and a verticalrod which are both fixed with respect to the earth.The radio beacon sends with loop and rod, and thephase of the electromotive force in the loop isreversed in a dot-dash rhythm. In a directionperpendicular to the plane of the loop a continuousdash occurs, while to the right of the beacon line,which may for the present be assumed to be alongthe north-south direction of the diagram of fig. 2,the dashes are heard more clearly, and to the leftthe dots. If the aeroplane receives the beacon signaionly on the vertical aerial, then the same effectoccurs in the detector as is caused by a non-directional transmitter with the help of the auto-matic commutator arrangement in a bearingsfinder. It may thus be seen by means of the visualcourse indicator on which side of the beacon lineone IS.


In the methods of reception described in 4) and 5)simultaneous indication by eye and ear is possible.Experience has shown that some pilots can keepa steadier course' by ear than by visual indication;with others the reverse is true. All ofthem, however,appreciate the simultaneous information of sightand hearing, where one of the two acts as controland complement of the other. When receiving withatmospheric disturbances the visual course in-dicator is "restless", while in the telephone thedesired signals may be distinguished adequately,since they may be adjusted to give a constant toneof any desired pitch, which can be plainly distin-guished from the disturbances.Flying in the direction of a broadcasting station,

the carrier wave can be heard as a continuous dashwith the aid of a beat oscillator. After the beatoscillator has been switched off, the mod u Ia t ionof the transmitter concerned may be lis-tened to, without disturbance of the visual courseindication. This may be of importance for thereception of weather reports, which are sent outperiodically by many broadcasting stations.Furthermore one can check whether or not one'scourse is toward the rig h t transmitting station,for instance by means of the announcementsbetween the items of the programme.

Construction of the direction-finder homing-device

On the front plate of the apparatus variouscontrols may be seen (fig. 3). The lighting of thevarious scales for night flying can be regulatedwith knob A. By compensation of the aerial effectof the loop with knob E, one can attain a sharpmmimum. The wavelength region 250-670 m(1200-488 kilocycles) is chosen by setting theswitch D in the left-hand position, while in theright-hand posrtron wavelengths 790-2000 m(380-150 kilocycles) are received. The apparatusV.P.K. 35 is thus not only suitable for the shippingand aÏT traffic bands, but also for the most impor-tant part of the medium-wave broadcasting band.Tuning is done with knob C, while the scale in theupper left-hand corner of the front plate is thewavelength scale.

On scale E is shown the kind of reception forwhich the receiver is adjusted by means of theswitch IC On the measuring instrument F one maycontrol the strength of the signal which is addedto the visual course indicator situated on thepilot's dashboard. This signal strength is adjustedby means of knob H, while G operates the volumecontrol for radio reception. On the plate J the 'po-sition of the wavelength scale for reception from

the most commonly used transmIttmg stationsmay be noted. The whole installation may beswitched off with switch L, while with M the beatoscillator for carrierwave reception can be switchedin. In order to make a distinction between theknobs frequently used and those seldom used,the former are larger.The mixing switch N makes it possible for the

wireless operater to listen ·to the direction-finderhoming-device or the ordinary radio receiver (fortraffic and weather reports) or to both at once,while at the same time the pilot, by means of abridge circuit, receives only and without disturb-ance the signals of the direction-finder. Throughthe possibility of simultaneous listening to thereceiver on board and the direction-finder, theoperator may be called by a ground station Vlathe receiver on board during the tuning of thedirection-finder homing-device or during the takingof radio bearings.A meter for checking the strength of the signal

added to the visual course indicator is introducednot only on the front of tll~ apparatus itself, butalso on the instrument board of the aeroplane,In this way the pilot also can read the signalstrength received.

Fig. 4. Rotating loop aerial, angle indicator and arrangementfor rotating the loop, for apparatus type V,P.K. 35.

188 PHILlPS TECHNICAL REVIEW Vol. 2, No. 6

Fig. 4 shows the loop and the arrangement forrotating it. In order to make it possible to readfrom the scale at eye-height and at the same timeturn the loop easily with the hand, reading scaleand handle are kept apart. The course directionwith respect to the fore and aft line of the aero-plane can be read off from an angular scale whichturns past an index mark. On a pelorous circlewhich is adjustable with respect to the coursecircle, and whose north-point can be set accordingto the compass, magnetic bearings can be found.The positions of the scales can be read clearly bymeans of a lens. With the help of a second lens onemay read the deviation curve, which gives theamount by which the measured bearing directionmust be corrected for every position of the loop

aerial signal must be allowed to pass. For thispurpose both the valves' of the loop stage 2are blocked by a high negative grid potential.

b) The same is done for b e a con re c ep t ion.c) For blind flying the phase of the loop

signal with respect to the aerial signal mustbe reversed periodically in a dot-dash rhythm.This is attained by blocking the valves of loop'stage 2 alternately in this time relation, It isdone by ~eans of a commutator operated bythe converter for the anode potential.

d) For the determination of the directon(h~art-shaped diagram) one of the two valvesin the loop stage 1 is blocked.

e) In taking bearings (figure-eight diagram)only the loop signal must be allowed to pass.

9

Fig. 5. Diagrammatic representation of the circuit of the apparatus VK. 35.

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with respect tD the .aeroplane as a result of thedistortions of the electro-magnetic field by the bodyofthe aeroplane. The two scales are lighted clearlyby a smaillamp, care having been taken that in theé~s~ of night :flying no glaring light falls directlyinto the pilot's eyes.

Circuit' of the, direction-finder homing-device

The action of the apparatus is explained belowwith the aid of the block diagram (fig. 5).

The aerial signal is amplified by the high-fre-quency amplifier stage, the loop signal by the high-frequency amplifier stage, the, loop signal by, thehigh-frequency stage i~ which there are two valveswith grids in parallel and anodes 'connected inopposite phase. By blocking one or the other ofthe ~alves in 'these high-frequency amplifier stagesby means of the 'switch which -determines thekind of reception, 'mixing takes place in the way'required for the various kinds of reception, namely:

a) For non-directional reception only rthe

Not only one of the valves of stage 2" but alsothe valve of the aerial stage 1, is blocked.The signals thus allowed to 'pass and amplified

are combined in stage 3, where the high-frequencysignals are also converted to a constant intermediatefrequency by means of an auxiliary oscillator. The_stages 1 and 2 and the input circuit of stage 3are tuned to the' frequency to be received, anda special circuit i~ the aerial stage 1 provides forthe correct phase relation of aerial and loop signals.The intermediate-frequency signals are now

first amplified in the intermediate-frequency am-plifier stages 4 and 5, and then division takesplace to the detectors 11 and 7 for visual andauditory reception respectively. 'For a u d it o r y -r e c e p t ion the intermediate-

frequency signal is detected in stage 4, and thelow-frequency signal therby obtained is amplifiedby the low-frequency amplifier stages 7 and 8 andfed to the telephone 9.For the" purpose of listening to unmodulated

signals an oscillator (la) is added, which gives an


audible beat with the intermediate frequency.Since the longwave beacon signals are always un-modulated in the position for "beacon reception"of the switch governing the kind of reception, thisoscillator is automatically connected, while in thepositions for other kinds of reception it may beconnected when necessary by means of a separateswitch.

For the visual indication in course andbeacon flying the intermediate frequency signal isrectified in stage 22. For the most satisfactoryfunctioning of the indicator part it is necessarythat the intermediate-frequency signal to be rec-tified be of a definite strength. This signal strengthmay be adjusted with the knob H on the frontplate of the apparatus, and may bc checked on a

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Fig. 6. Diagrammatic representation of the action of thevisual indicator of the direction-finder homing-device, typeV.P.K.35.

current meter III the detector circuit (F in fig. 3,and F in fig. 5; in addition, a meter 16 in serieswith this latter is installed on the dashboard III

front of the pilot.Further it is desirable that in flying a course

and in beacon reception the strength of receptionremain the same upon approaching a transmittingstation. An automatic volume control is introducedfor this purpose. It controls the conversion stage3 and the intermediate-frequency stages 4 and 5,and is set in operatien by the switch controllingthe kind of reception. The automatic volumecontrol has such a great time lag that the differencein intensity of reception between dots and dashesis not eliminated. The current pulses obtained afterdetection are conducted to a direct-current am-

plifier in stage 13 via a filter, which suppresses anyaudio-frequency modulation. In the anode circuitof this amplifier no current variations will occurwhen flying on the course line. When of the course,however, such variations will occur. In the regionof das hes the plate current will have the characterof fig. 6a, in the dot region of fig. 6c.In the secondary windings of a transformer in the

anode circuit, potentials are generated by thesecurrent variations, which have a character likethat of fig. 6a for the dash rt'gion and of fig. 6dfor the dot region. From the figs. 6b and d it maybe seen that the first impulse of the impulse com-bination in the das h region is 0 p pos i tel y di-rected to that in the dot region. Therefore, ifonly the first impulse of every impulse combinationis passed through, the indicating instrument willreact 0 p pos it e Iy for dots and dashes.This is attained in stage 14, where two valves

III bridge connection are connected to thesecondary winding of the above-mentioned trans-former. In the grid circuit there is a special arrange-ment for blocking. The valves are so adjustedthat their operating point lies in the region of thegreatest curvature of the characteristic. As soonas the anode current on one valve increases due

Fig. 7. Converter with interference suppressor for feeding thedirection-finder homing-device, type V.P.K. 35.


REVIEW OF RECENT SCIENTIFIC PUBLICATIONS OF THE N.V. PHILIPSeÓ» GLOEILAMPENFABRIEKEN

to an impulse both valves, are blocked during atleast the time period of one dot, so that impulsesoccurring in, this time are not passed through.As a result only the one valve operates for

dots and the other for dahes. The pointer of theindicator 15, which is connected between the anodesof both valves, will take up the middle positionwhen on the line of course and show a deviationfor dots and dashes respectively in opposite direc-tions.

The energy for the direction-finder is taken' fromthe accumulator on board. The filaments of theindirectly heated valves are connected directlyto the battery, while the anode potentials are

, No. 1153: J. A. M. van Liempt: Die Anät-zung von Molybdän durch alkalischeFerricyankalilösungen (Rec.Trav. chim.Pays Bas, 55, 989 - 990, Nov: .1936).

,The <etching of molybdenum by ternary mixturesof water, sodium hydroxide and potassium ferri-cyanide was studied and the results were given in adiagram. From the experiments a formula wasdeduced for a solution which is very suitable formetallographie etching and for the investigationof the so-called intermediate substance.

No. 1154: H. C. H am a k e r: A general theoryof lyophobic colloids 1. (Rec Trav.chim. Pay Bas 55, 1015 - 1025, Nov.1936).

The point of departure of this article is thecommon assumption that the reciprocal actionbetween the particles of a colloid whose hydrationmay be neglected is caused by the superpositionof an attraction according to v. d. W á a:IsandL 0 n don and an electrostatic repulsion. When thissimple conception is worked out to its logicalconclusion, it leads to esserrtially more complicatedphenomena than were hithert~ expected. With theconceptions given here a more extended range ofphenomena can be treated than could be explanedtheoretically up to the present.

obtained from a converter which runs from thebattery on board. This converter is installed in abox .with its interference suppressor and fuse(fig. 7). The suppression of interference is suchthat when the direction finder is adjusted to itsgreatest sensitivity, no disturbance, for examplefrom commutator sparking, is observable even onthe shortest waves.The apparatus is very light in construction and

weighs in total only 21.3 kg, to which the direction. finder contributes 11.4 kg and the converter 4.2 kg.The rotating loop aerial which is fitted to theoutside of the aeroplane is only 43 cm high.

Compiled by G. P. ITTMANN:

No. 1155*: W. Elenbàas Zur Frage der Be-rechtigung des Minimumprinzips inder Theorie der Bogenentladiung (Elek-troteehn. Z. 57, 1497 - 1498, Dec.1936). '

In this article objection is raised to the minimumprinciple in the theory of the electric are defended.by K e s s eIr i n g nad Kop pel m a n n, accordingto which the are will burn with a 'temperature anda diameter such that the current flows under theinfluence of the smallest possible drop in potential.El e n b a a s shows that in some cases the are isalready wholly determined before one has madeuse of the minimum principle from which fact itfollows than one no longer has the liberty of thenintroducing such a principle.'

No. 1156: W. G. 'Burgers and J. J. A. Ploosvan Amstel: "Oriented" oxidationof barium (Physica, 3, 1057 - 1063,Dec. 1936).

Filins of 'metallic barium were obtained byevaporating the metal in a vacuum and then con-densing it on a plane polished copper plate. De-pending among other factors upon the temperaturè

*) There is not a sufficient number of reprints of articlesmarked'· for distribution. The administration of theNatuurkundige Laboratorium, Kastanjelaan, Eindhoven,willon request be glad to send reprints of th,e other articles.

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