‘Sept, 11?, 1946. ‘ ‘ H. T. BUDENBOM 2,497,54g DIRECTION FINDING SYSTEM Filed July 23, 1941 ‘ 4 Sheets-Sheet l I INVENTOR H; 7? BUDENBOM A from/5y Sept 37, 3946- H. T. BUDENBOM 294N549 DIRECTION FINDING SYSTEM Filed July 25, 1941 > , F/Ci IA l 4 Sheets-Sheet 2 - ‘ I/-f\ \\_ 84 @(64 \ (,/"\~ I \_ v84’ 85 FIG/B NORTH EAST so um‘ WEST / /,‘ W INVENTIOR . By HTIBUDENBOM ATTORNEY , Sepit. 3?, 1946, . > H. T. BUDENBOM DIRECTION FINDING SYSTEM Filed July 25, 1941 4 shevets'ésheefc 5 'YIIIIIIIIYI , F O%l:_________ ~ _ BY , ATTORNEY Sept 17, 1946' H. T. BUDENBQM , 2,407,649 ‘DIRECTION FINDING SYSTEM Filed July 25, 1941 ' 4 Sheets-Sheet 4 1.1%“ INVENTOR ' H ZBUDE/VBOM BV _ ' . v ‘ A FTOWJE?’ . Patented Sept. 17, 1946 2,407,549 barren stares PATENT lorries 2,407,649 DIRECTION FINDING SYSTEM Horace T. Budenbom, Short Hills, N. 3., assignor to Bell Telephone Laboratories, Incorporated, _ New York, N. Y., a corporation of New York 1 Application July as, 1941, Serial No. 403,693’ 9 Claims. (Cl. 250-41) 2 This invention relates to direction ?nders and particularly to short wave direction ?nders. In my copending application Serial No. I741,372. the turntable shaft. Upon proper adjustment of the collar the zero point of the trace coincides with the reference line on the cathode tube, the ?led August 25, 1934, there are disclosed a method positive alternation of the trace being at the left and means for determining the path or direction of the line for the front direction and at the right of an incoming wave utilizing, a pair of spaced . . for the back direction whereby the sense as well vertical antennas mounted on a turntable and. i as the path of the incoming wave is ascertained. connected through separate receivers to a cathode In accordance with another feature of the in tube phase indicator. Brie?y, the method com vention, apparatus is added to the phase com prises rotating the turntable to a position at 10 parison direction ?nding system disclosed in my which the absorbed antenna energies are in phase _ , copending application mentioned above modi?ed agreement whereby the plane of the antennas is or not to include the improvement described perpendicular to the azimuth direction of the above, .for continuously maintaining equal gains incoming wave‘. It now appears desirable to in the two receiving circuits whereby an aura] modify the above-described system and in general 155 balance indication may be secured for checking to secure, in a phase comparison type direction the cathode tube phase comparison indication. ?nder, an instantaneous indication of the wave More specifically, each receiver is provided with direction and, in addition, to provide aural bal- ' a conventional primary automatic volume control ance means for accurately checking or con?rm~ the direction indication obtained. t is one object of the invention to determine for limiting the variation in gain and with a sec 20 accurately, and substantially instantaneously. the direction of radio waves. It is another object of this invention to obtain a continuous indication of the direction and di rectional sense of an incoming wave irrespective of changes in said direction. It is still another object of the invention to maintain equal gains in the two receiving circuits of an aural balance direction ?nder. According to one feature of the invention the turntable of the direction ?nding system disclosed in the copending application mentioned above is continuously rotated at a given low frequency rate. The in-phase component of the detected 35 audio frequency from one receiver and the quad rature phase component of the detected audio ondary volume control for further limiting the degree of gain variation. In addition, a differ ential volume control is provided so that, upon a slight change in the gain of either receiver, the gain of the other receiver is changed in the same sense and to the same degree. Preferably, the various automatic volume control circuits are ?rst calibrated by receiving a wave emitted by the local transmitting antenna and adjusting the two receivers for equal gains. After the initial ad-' justment the system may be used for obtaining an aural balance indication for a desired 'wave incoming from a distant station, the indication being obtained by rotating the turntable to the position at which signals having equal amplitudes and equal phases, as measured by head-phones or other indicating means, are obtained in the out puts of the two receivers. ' The invention will be more fully understood ‘from a perusal of the folowing speci?cation taken in the output of which a current having a fre 40 in conjunction with the drawings on which like quency equal to the rotating frequency and a reference characters denote elements of similar frequency conjugate to of athepush-pull other receiver balanced aredemodulator supplied phase related to the direction and sense of the incoming wave is obtained. The instantaneous amplitude of the rotating frequency current ob tained represents the cyclically varying phase re lation or diiference between the absorbed an function and on which: Fig. 1 illustrates tion for obtaining parison indication, ode tube indicator one embodiment of the inven an instantaneous phase com and Figs. 1A and 1B are cath traces obtained with the sys tenna energies, and this representative current tem of Fig. 1; is applied to the horizontal plates of a cathode Fig. 2 illustrates an embodiment of the inven tube indicator. The current for the horizontal tion for obtaining visual and aural indications of sweep is obtained from a circular potentiometer 50 the direction of the incoming wave; and mounted on a collar slidably associated with the Fig. 3 illustrates a remote indicating arrange turntable shaft and having a continuously ro ment which may be used with the systems of tating contact arm attached to the shaft. A low Figs. 1 and 2. I frequency elliptical trace is obtained which be Referring to Fig. 1, reference numerals I and comes a straight line upon proper adjustment of 55 2 designate non-directional vertical receiving the collar. An azimuth scale is attached to the antennas and numeral 3 denotes a local trans collar for use in determining the compass direc mitting antenna, the antennas being mounted on ' tion of the incoming wave. ‘As an alternative. a a horizontal turntable 1!. The turntable is sup sinusoidal trace may be secured by timing a linear ported by the vertical shaft 5 which is driven or sweep voltage by means of a synchronizing pulse 60 rotated by the motor 0. Reference numeral 1 obtained from a rotating contact associated with designates a switch for connecting motor 6 to 2,407,649 3 the power supply leads 8. Antenna 3 is posi tioned equally distant from, and preferably in the same plane as, antennas l and 2 and is con nected through a slip-ring assembly comprising ~ the rotating ring or circular contact 9, insulator 4 No.1 1, as shown on the drawing, one source sup~ plies a sinusoidal current of the rotation fre~ quency and comprises the circular potentiometer at, the diametrically opposite points 63, 59 of which are connected, respectively, through con tacts 5%], 5i, switch 56 and connections 52 to the opposite terminals of battery 53. The potenti ometer 41 is rigidly attached to the movable Antennas I and 2 are each connected through sleeve 54 which may be manually rotated about an antenna coupling unit I3, unbalanced line shaft 5 and locked in a given position by means conductor M and associated slip-ring assembly 10 of a conventional slot and spring plunger as to a receiver I5, the receiver I5 connected to sembly 55. Reference numeral designates a antenna I being hereafter referred to as re transformer having primary winding 51 and sec ceiver No. I and the receiver I5 associated with vondary winding 58. The primary winding is con antenna 2 being hereafter referred to as receiver nected through contact 59 to a point v6!), equally No. 2. The horizontal portion of each line con 15 spaced from points to and “is, on potentiometer ductor I4 is preferably enclosed in a shield I5 til, and to radial arm contact SI which rotates which forms with the enclosed portion of con with shaft 5 and sweep potentiometer 47. The ductor I4 an unbalanced coaxial line. The outer secondary winding 58 is connected to the input coaxial conductor I6 is connected to the ground circuit of a push-pull ampli?er comprising tubes I‘I. As explained in my copending application, 62 and 63. Reference. numeral M denotes a the local transmitter 3 is connected to receivers transformer the primary winding 65 of which is I5 by the tuning connection 58 and the local included in the output circuit of the push-pull transmitter and both receivers are simultaneously amplifier 82, 83 and the secondary winding 66 tuned by a unicontrol means at the local trans which is connected to switch 46. Numeral 61 mitter. Reference numeral I9 designates a quad 25 of designates a conventional azimuth scale assembly rature phase shifter connected to the output ter comprising the stationary azimuth scale 58 and minal of receiver I for the purpose of obtaining at the rotatable angle scale '65 attached to the sleeve terminals 28 an audio frequency (as 1090 cycles) 155. As explained below, the cathode tube trace in-phase potential Fa and at ‘terminals 2! an is either an ellipse or a straight line. audio frequency quadrature potential, Fa (90 de 30 obtained The alternative source of sweep voltage utilized grees), the potential Fa. and the potential Fb at with switch 55 in position No. 2 is essentially a the output terminals of receiver 2 being the in linear sweep circuit controlled by periodic pulses phase potentials referred to in the above-men synchronized with the turntable rotation. Refer tioned application. As described thus far the ence numeral ‘i5 designates a linear sweep shap system is, except for the addition of the slip ing tube having a cathode 25, a control electrode ring assembly, similar to that described and 27, screen grid electrode ‘II, suppressor grid elec claimed in my copending application mentioned trode ‘I2 and an anode 28 and reference numeral above. ‘I3 designates a gas-?lled trigger tube having a The output terminals 22 .of-receiver 2 and the cathode 26, control grid 27 and an anode 23, the quadrature output terminals 21 of receiver I are, tubes being connected in series. Numerals ‘Ill in accordance with the present invention, con and ‘I5 designate respectively resistances included . nected .to the rotation frequency detector 23 which in the control grid circuits of tubes ‘III and ‘I3 is similar in circuit operation to a vacuum tube and the numeral ‘I6 designates a by-pass con wattmeter. The rotation frequency detector The screen grid ‘II of tube ‘ID and the comprises tubes 24 and 25 connected in push-pull 45 denser. control grid 21 of tube ‘I3 are connected through and eachcontaining a cathode 26, a control elec resistance 11, spring contact ‘I8 and rotating con trode or grid 21 and an anode or plate 28. The tact ‘I9 to the positive terminal of an auxiliary output terminals 22 of receiver I are connected battery 80. The radial arm N! is preferably to the primary winding 29 of transformer 38, the on or embedded in the circular insulator secondary winding 3| of which is connected be 50 mounted SI which is attached to sleeve 54. The anode tween the grids 21 of tubes 24 and 25; and the circuit of tube ‘I0 includes a condenser 82, which quadrature terminals 25 of receiver I are con is connected through switch 46 to the horizontal nected to the primary winding 32 of transformer cathode tube plates 53, and a shunt path com 33, the secondary winding 34 of which is included in the common lead connecting the ‘grids 2‘! of 55 prising a resistance 3‘! and condenser 83 in series therewith. Condenser 82 is also shunted by the tubes 24 and 25 to the cathode 26. vIn other series path comprising anode resistance 38 and words, the in-phase output terminals of receiver the anode-cathode path of trigger tube ‘I3. The 2 and the quadrature phase output terminals of sweep circuit comprising tubes ‘Ill and 13 and in receiver I are connected in conjugate in the input cluding the radial contact ‘I9 is essentially the circuit of push-pull detector 23. Reference nu 60 same as that disclosed in the RCA Tube Hand merals 35 and 36 designate respectively a source book (tube 884) . of grid bias potential and asource of anode po In operation, Fig. 1, wave components incom tential and numerals 3‘! and 38 denote imped ing from the distant station and from the local ances connected respectively in the output cir cuits of tubes 24 and 25. The output terminals 65 transmitting antenna are, as explained in the or insulation Ill and the stationary spring con. tacts H, to the local phasing transmitter I2. 39 of the rotation frequency detector 23 are con nected by line llil to the horizontal plates 4| of cathode tube indicator 42. The sweep or refer ence voltage applied to cathode tube plates 43 is obtained over. line 613 from the sweep circuit 45. The sweep circuit 45 includes two distinct sources of sweep voltage either of which may be connected by the double-throwlswitch 45 to the plates 43. copending application mentioned above, inter cepted by antennas I and 2 and detected currents Fa and Pb of a constant audio frequency are obtained respectively in the output circuits of receivers I and 2, these components having a phase relation for agiven orientation of the turn table related to the direction of the incoming wave. In accordance with the present inven tion the turntable is rotated at a very low speed, for example, one to ?ve revolutions per second, Assuming switch 46 is in position 75 2,407,649 6 by motor 6 and shaft 5, and as a result the phase ' relation of the radio frequency components of the incoming distant wave intercepted by an tennas I and 2, and correspondingly the phase relation of the two detected currents, are cyc1i~ cally varied at a rate equal to the rotation fre quency. Thus, when the plane of antennas I and 2 is perpendicular to the incoming wave direction the detected currents are in phase, and when the plane is aligned with the wave direction 10 they have a maximum phase difference. In other words, during each revolution of the turntable the phase relation between the detected currents passes through a zero value twice and through a maximum or unity value twice. A portion of the Alternatively, switch 46 may be thrown‘ to position No. 2 for the purpose of obtaining both a path indication and a sense indication. As suming contacts ‘I8 and 19 are open when the switch 46 is thrown to position No. 2, the anodes of tubes 10 and ‘I3 become immediately energized with positive potential and condenser 32 begins charging. Since the flow of current from the anode 28 to the cathode 26 of ,tube ‘I3 is blocked by the relatively low positive potential on con trol grid 2‘! obtained from battery 36, the current from battery 36 flows through condenser 82 and tube ‘I0 and not through tube ‘I3. The voltage across condenser 82 is increased gradually, rela tively considered, and since this voltage is applied to plates 43 of cathode tube 42 the cathode beam output of receiver I is shifted 90 degrees by phase splitter l9 and supplied, as previously indicated, is simultaneously caused to sweep horizontally in with the in-phase output of receiver 2 to the one direction during one revolution of shaft 5. conjugate input circuit of rotation frequency de When contacts ‘I8 and ‘I9 close, a high positive tector 23. 20 potential obtained from auxiliary battery 80 is As disclosed in Patents 1,586,533 and 2,006,698 impressed through these contacts and resistances to E, Petersen, granted respectively June 1, 1926, ‘I1 and ‘I5 on the control grid 21 of tube ‘I3 where and July 2, 1935, the output voltage of the vacuum upon trigger tube ‘I3 becomes conductive and tube wattmeter or push-pull detector 23 is pro condenser 82 abruptly discharges through the portional to the product of the two input voltages anode-cathode path of tube ‘I3. At the same multiplied by the cosine of the phase angle be time the screen grid ‘II of the shaping tube 18 tween them. When the plane of the antenna is is rendered more positive so that the discharge perpendicular to the direction of the incoming of condenser 82 is accelerated, the function of the wave the phase angle between the potentials im shaping tube ‘I6 and associated circuits being to pressed on grids 21 of tubes 24 and 25 is 90 _ transform in effect the normal exponential dis degrees, the cosine of which angle is zero, and the charge characteristic of condenser 82 to a linear output potential is a minimum. When the plane characteristic and to secure an ideal saw-tooth is aligned with the wave direction the phase angle differs from 90 degrees by a maximum amount whereby the output potential is a maxi sweep. As illustrated by Fig, 1B, with the switch 46 in position No. 2, a sinusoidal trace is obtained, the position of the positive alternation 86 relative to reference lines 8? and 88 being indicative of mum. Hence, there is obtained at terminals 39 and at horizontal plates M of cathode tube 42 a sinusoidal signal voltage having a frequency the direction and sense of the incoming wave. equal to the rotation frequency, an instantaneous Thus, for example, as shown in Fig, 1B, for the amplitude representing the cyclic variation in the 40 “front” direction the positive alternation 86 is at phase relation of the above-mentioned absorbed the right of thevertical reference line 8‘! and antenna energies and an absolute phase or phase above the horizontal reference line 88 whereas for angle related to the direction and sense of the the “back” direction it is at the left of line 81 incoming wave. Upon a change in the direction and above line 88. For the side directions per of the incoming wave the phase angle of they detected signal current is altered, the frequency and amplitude remaining unchanged. Stated pendicularly related to the in-phase direction, it is symmetrically disposed relative to line 81 but is above line 88 for the right-hand side direction differently, upon a wave direction change the and below line 88 for the left-hand side direction. sinusoidal wave is shifted on its time axis, with The sinusoidal trace may be shifted and the azi .respect to a given point on said axis, an amount .50 muthal bearing ascertained by adjusting sleeve related to the angular change in the horizontal 54 and the arm ‘I9 attached thereto, and noting wave direction. the azimuth scale. Assuming switch 46 is in position No. 1, the Fig. 2 illustrates a system which includes sub potential from battery 53 supplied through con tacts 56 and 61 and winding 51 is cyclically varied, a polarity reversal being obtained in effect stantially the system of Fig. 1 and, in addition, is provided with means for maintaining equal gains in the two receivers and for securing an when arm 6| crosses point 60. This sinusoidal aural balance, Reference numerals 90 and SI reference potential is applied to the vertical designate parallel loop antennas and numerals plates 43 of tube 42 and, as illustrated by Fig. 1A, 92 designate balanced transmission lines each of a low frequency elliptical pattern 84 is obtained 60 which is connected to its associated loop antenna when the potentials have an out of phase rela through the transformer 93, The parallel loop tion other than 180 degrees and a straight line 'and balanced line arrangement which is used in trace 85 is secured when the phase angle differ place of the vertical antenna and unbalanced'line ence of the potentials is zero (or 180 degrees). arrangement included in the system of Fig. 1, If the trace is elliptical the sleeve 54 may be ad 65 functions in such a way as to eliminate the so justed until a straight line indication is secured called “night effect,” as explained on pages 221, and the angle between the Wave line direction 385, 386 and 389 of the text-book “Wireless Direc and the chosen reference line direction, as, for tion Finding," third edition, by R. Keen, since example, north-south, may be ascertained by ob the two loops present the same relative aspect to servation of the azimuth scale 67. The adjust 70 the wave at all times. Loops 98 and 9| are con ment of sleeve 54 changes for a given orientation ‘nected, respectively, to receivers I and 2 each of the turntable 4, the position of the radial arm comprising the radio frequency ampli?er 94 and 6| relative to points 48, 49 and 60 and hence intermediate frequency ampli?er 95, a portion of shifts the absolute phase of the reference 0r sweep the output of which is supplied to a conventional voltage. .75 ‘automatic volume control circuit 96 hereinafter 2,407,649 7 referred to as the "primary” automatic volume control. Considering each of receivers I and 2, the greater portion of the intermediate frequency out~ put of ampli?er 95 is supplied over line 9'! and tube I0 I- of receiver I, whenever the gain of either receiver is altered the gain of the other receiver is adjusted in the same sense and to the same degree. - Hence the gains of the two receivers are i) primary winding 98 of transformer 99 to a differ ential and secondary automatic volume control circuit IGG. Numeral ISI designates a pentode tube having its input terminals connected to the secondary winding I02 of transformer 98 and its output terminals connected to the primary wind ings I03 and H54, respectively, of transformers I65 and I95. The secondary winding IIJ'I of trans_ former I03 is connected to an individual head‘ phone jack 558 and to switch I 09, a phase splitter I9 being included as in the system of Fig. 1 in the output circuit of receiver I. In each receiver the secondary winding I I9 of transformer I93 is con_ nected to a full wave volume control recti?er com prising tubes I II and H2 and a volume control / resistance ! i3 included in the circuits of the two recti?ers. Numerals H4 and H5 designate re spectively a series resistance and a shunt resist ance connected together with resistance H3 in the grid circuit of the differential tube H6, The anode 28 of tube I IS in each receiver is connected to the suppressor grid ‘I2 of tube IllI in the other receiver. Resistance I I3 is also connected by lead II‘! to the control grid of tube I I]! of the asso ciated receiver. Hence resistance II3 constitutes a source of direct current voltage for the differ ential automatic volume control circuit and a source of direct current voltage for the secondary maintained alike and an accurate amplitude bal ance may be obtained. While the gains are maintained equal, each varies inversely as the intensity variation or fading of the incoming dis tant Wave. If desired, the two receivers may ?rst be calibrated for equal gain control operation by receiving the Wave locally emitted by antenna 3 and adjusting the contacts I I8 on resistance I I3, the transmitter I2 being adjusted so that the frequency of the locally emitted calibrating wave is'the same as the frequency of the desired in coming wave. After preliminary calibration the frequency of the local transmitter is readjusted to produce the detected frequencies Fa and Fb which are utilized in the aural null determination. The aural null indication is obtained in a man ner well known in the art. Briefly considered, with switch I 09, Fig. 2, in position No. l the bear ing of the distant station, as given by the tube indicator 42, is noted and the automatic or con tinuous rotation of the turntable is stopped. The turntable is then manually rotated as explained in my copcnding application mentioned above, to the position at which the line or axis con necting corresponding points in the two receiv ing antennas, is perpendicular to the line direc tion of the incoming wave whereby a phase bal ance is obtained. As explained above, the volume control circuits in the two receivers are then the two receivers are connected to the reference generator or horizontal sweep circuit 45 and to adjusted to obtain an amplitude balance. With the system adjusted for both a phase balance and an amplitude balance the audible indications ob tained in the ear pieces connected to jacks I08 the rotation frequency detector 23 for the purpose are balanced. automatic volume control circuit, In operation, with switch I89 in position No. l, Fig. 3 illustrates apparatus which may be indication, as explained in connection with Fig. 1. 40 added to the systems of Figs. 1 and 2 for obtain ing at a cathode tube indicator located many With the switch I99 thrown, to position No. 2 miles from the turntable either the low frequency the receivers are connected, as explained in my elliptical directional pattern, Fig. 1A, or the si copending application mentioned above to ap nusoidal pattern, Fig. 1B. For remote indication paratus for obtaining a high frequency ellipse in operation, the apparatus of Fig. 3 is inserted, as dication or a red-green visual indication. An , indicated by the dot-dash lines X-—X and Y-Y, aural balance may, of course, be obtained by Figs. 1, 2 and 3, in transmission line 40 conveying means of head-phones connected to jacks I98, the signal current frequency Ft and transmission with switch 509 in either position. It should be line 44 conveying the reference or horizontal noted that the systems of Figs. 1 and 2 are phase of obtaining an instantaneous or snap bearing comparison systems and that in so far as the 50 sweep current Fr or F5 to the cathode tube indi cathode tube indications are concerned, the pri mary difference between the two arrangements is that in the system of Fig. 1 the outputs of two single non-directional antennas‘ are compared as to phase, whereas in the system of Fig. 2 the total outputs of two loop antennas are compared as to phase. cator. Referring to Fig. 3, the reference line 44 is connected through a frequency doubler I20 to the input terminal I2I of the long two-wire tele phone line I22 and the line 40 is connected to terminals I2I through the two parallel paths, one including the frequency tripler I23 and the voice frequency blocking ?lter I24 and the other Referring particularly to the volume control including the voice frequency path ?lter I25. As circuit I00 the recti?ed voltage drop across re sistance [I3 in receiver I functions, as is well known, to vary inversely the gain of tube IIlI is apparent, line I22 conveys a sweep reference current of double frequency (ZFR or 2Fs) and a in receiver I with variable signal, the gain being decreased as the input signal intensity increases put terminals I 26 of line I22 are connected to the horizontal de?ecting plates 43 of the indi cator 42 through the ?lter I27, which blocks the and vice versa. Preferably, the primary auto matic volume control circuit 96 limits the varia tion in gain to approximately 3 decibels and the signal current of triple frequency 31%. The out passage of voice frequency current and the triple signal current 3FT, and through the 2 to 1 fre quency demultiplier I28. Terminals I26 are also secondary automatic volume control II'I further connected through the ?lter I29, which blocks the limits the variation in gain to approximately 1/2 voice frequency current and the reference double ‘decibel or less. Also, since the voltage drop across resistance H3 connected to the full Wave 70 frequency (ZFR or 2Fs), and through the 3 to l demultiplier I30, to the vertical de?ecting plates recti?er III, II2 of receiver I is impressed 4| of the cathode tube 42. The demultipliers I28 through tube II6 on the suppressor grid of tube and I30 function, respectively, to reduce the “II of receiver 2, and similarly the drop across sweep and signal currents to their original fre resistance II3 included in receiver 2 is impressed through the tube II6 on the suppressor grid of 75 quency so that the frequency applied to the oath 2,407,649 9 10 ode tube is the same as that used in a local indi 5. A direction ?nder comprising a pair of spaced independent antennas mounted on a turn table and positioned to receive a particular in coming wave, separate detecting means connected to each antenna for obtaining currents represent ing the antenna energies, modulating means con cating system of Figs. 1 and 2. The demulti pliers may be of any conventional type and pref erably are of the type described in the article by R. L. Miller entitled “Fractional-frequency generators” published in the Proceedings of the I. R. E., July 1939, page 446. A voice frequency pass ?lter I3! is included between the line ter minals I26 and the output terminals I32. The voice frequency pass ?lters I25 and l3l and as nected to said separate detecting means for ob taining from said currents a third current having a frequency equal to the turntable speed and a 10 phase angle related to the direction of said wave, generating means for obtaining a fourth current having a phase angle related to a reference com sociated shunt paths permit transmission of the voice frequencies through the system of Fig. 3 without frequency change. Although the invention has been described in connection with certain speci?c embodiments 15 pass direction, and means for determining the phase angle relation between the third and fourth currents, whereby the direction and sense of the incoming wave may be ascertained. 6. A direction ?nder comprising a pair of an apparatus may be successfully used in practicing tennas spaced for receiving equal intensity com ponents of an incoming wave, substantially, sepa the invention without exceeding the scope of the invention. 20 rate receivers connected to said antennas and having independent primary automatic gain con What is claimed is: 1. A direction ?nder comprising a pair of elec trol circuits, an indicator connected to the out trically independent spaced antennas for obtain put of said receivers, and means for compensat ing separate radio frequency energies from an ing for the difference in the gain of said receivers incoming wave, said antennas being positioned 25 comprising a pair of secondary gain control cir cuits each having its input terminals connected on a turntable, means for rotating the turntable, to a different one of said receivers and its output and producing a cyclically varying phase differ terminal connected only to the other receiver. ence between the absorbed antenna energies, 7. A direction ?nder comprising two antennas means connected to said antennas for obtaining mounted on a turntable and spaced for receiving a current representing the amount and sense of from an incoming wave components having sim said phase di?‘erence, and means for comparing the phase angle of said current with that of a ilar fading characteristics, said antennas being current representing a predetermined reference positioned along a horizontal line perpendicularly related to the vertical plane containing the de direction. 2. A direction ?nder comprising a pair of re sired incoming wave, separate receivers each in cluding an ampli?er and an independent auto ceiving channels, each including an independent antenna, for obtaining a pair of currents rep matic gain control circuit therefor, an aural indi resenting components of the received wave, means cator connected to said antennas, and a differ ential gain control means for changing .the gain for cyclically moving said antennas at a given of either ampli?er upon a change in the gain of frequency in opposite directions with respect to the other ampli?er, said means comprising a pair a distant transmitter, means for obtaining from of recti?ers each of which is connected between said pair of currents a sinusoidal current having the output circuit of a different ampli?er and a frequency equal to said frequency, an ampli thereof, it is to be understood that it is not to be limited to these embodiments and that other only the input circuit of the other ampli?er. tude representing the phase angle relation of said 8. A method of determining the direction of an pair of currents and a phase angle representing 45 incoming wave which comprises receiving sepa the direction and sense of said wave, and means rate components of said wave, producing in the for comparing the phase angle of said current phase-angle relation of the received components with that of a current representing a predeter a cyclic variation related to the wave direction, mined reference direction. intermodulating said components to obtain a 3. A direction ?nder comprising a pair of inde sinusoidal current having a phase angle repre pendent spaced antennas for receiving a given senting said variation and said direction, obtain wave, means common to said antennas for cycli ing a second current having a phase angle repre cally and oppositely moving said antennas along the path of said waves, means comprising a mod senting a reference direction and comparing said ulator connected through separate receivers to 55 > phase angles. said antennas for obtaining a current having a phase angle representing the direction and sense of said waves and representing the phase rela tion between the antenna energies, means for ob taining a current having a phase angle related to a reference direction, and means for deter mining the phase angle relation of said currents. 4. A direction ?nder comprising a pair of spaced vertical loop antennas for receiving a given wave, said antennas being positioned in parallel planes and mounted on a turntable for rotation at a given rate in a horizontal plane, means comprising separate receivers and con nected to said antennas for obtaining a current from the antenna energies having a frequency dependent upon the turntable speed and a phase related to the direction of the incoming wave, means for obtaining another current having a phase related to a reference direction and means for comparing the phases of said currents. 9. A method of determining the compass point direction of an incoming wave, utilizing a pair of spaced antennas, which comprises receiving on said antennas a pair of separate components of said wave, oppositely moving said antennas at a given cyclic rate relative to said direction to pro duce a cyclic variation in the phase-angle rela tion of said received components, obtaining from the received antenna energies a pair of currents “ having the same cyclic variation in their phase— angle relation, modulating said currents to pro~ duce a sinusoidal current having a frequency equal to said rate and a phase angle representing said direction, obtaining a reference current hav ing a frequency equal to said frequency and a phase angle representing the reference direction and comparing the phase angles of said last mentioned currents. HORACE T. BUDENBOM.