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Patent Number: 2484824 Filing Date: Sep 12, 1945
Application Number: Issue Date: Oct 18, 1949
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Oct. 18, 1949. P. G. HANSEL 2t4849824 RADIO DIRECTION FINDING METHOD AND APPARATUS Filed Sept. 12, 1945 f 2 Shoots-Shoot 2 47 w I OLE Cr 4_7 0 - LU - j L6 cc 0 U. 0 X CL U 0 5 W tn w ir cr:D w X tA.2 ELI cr 4: LL. z L w W w :3 It O!qo- a 0 < a - i z a -a -J < Luy fx IL. tn @o_ INVE NTO P- 0 PATJL G. HANSEL BY

Pjitented, OeL 1$o 1%9 2p484l824 UNITED STATES PATENT OFFICE 2,494,824 ]RADIO DMECTION FINDING METHOD AND APPARATUS Paul G. Hauml, Red ]3ank, N. J. AppUcation September 12,1945, Serial No. 615,905 16 Claims. (CL 343-113) (Granted under the act of March 3,, 1993, as amended April 30, 1928; 370 0. G. 757) 2 The invention described berein may be manufactured and used by or for the Governinent for govermnental purposes, without the payment to me of any roYaltY thereon. My present invention relates to, radio direction 5 finding inethods and appgratus, and particiflarly .to radio direction finders of the spaced-aerial type. One of the objects of this invention is to provide a method for altering the effective electrical 10 spacing between two or more aerials without altering the actual physical spacing. A further object of this invention is to provide a method for reducing bearing errors due to, space- diversity effects. 1 15 A still further object of this invention is to provide practical spaced-aerial direction finders having less than the usual physical spacing be- tween the individual aerials, thus making them more suitable for shipboard or aircraft use. 20 It is well known to workers in the fleld of short- wave radio direction finding that rnany signals received on spaced-aerial antenna systems exhibit almost continual changes in the apparent direc- tion of arrival. Frequently, the bearing indica- 25 tion of a direction finder having a cathode-ray tube indicator will spin rapidly around the entire 360-degree range of the azimuth scale, thus ren- dering accurate reading of the bearing almost impossible. It has been customary to attribute 30 such fluctuations of the apparerit direction of arrival almost entirely to polarization changes and scattering. It has also been quite widely believed that no itistrumental mea-sures can be taken at the direction finder to permit the taking 35 of accurate bearings under such conditions. However, recent experimental studies indicate that the arnount of bearing fluctuation which exists is often closely related to the physical spac- 40 ing between the individual aerials of the antenna system. In one test two direction finders were operated side-by-side. Both direction finders employed fpur-element, :ftxed Adcock antenna systems, one with a 35-foot diagonal spacing and 45 the other with a 16-foot spacing. In many in- stances a signal received on the widely-spaced antenna system exhibited violent fluctuations In apparent direction while the same signal received on the closely-spaced antenna exhibited either 50 a steady bearing or a bearing with only minor fluctuations. These and further Investigations have shown that in the operation of a weu-de- signed spaced-aerial direction finder, space-di- cause of bearing lnst@birty than either polarization changes or scattering. As is well known, most of the spaced-aerial direction flnders of prior art require equal field strengths at all of the aerials. Therefore, when the spacing is an appreciable part of a wavelength, for example one-quarter wavelength, space diversity effects which produce signifleant and constantly varying fleld-strength inequalities among the individual aerials will cause the bearing indication to- fluctuate. The foregoing discussion suggests that the obvious solution to the problem of direction finding when space-diversity effects exist is to employ a conventional spaced-aerial antenna system with a very smau physical spacing. However, although it is true that the use of a very small physical spacing results in substantial fulffllment of the equal field strength requirement, it Is found in practice that a direction finding antenna system with a very small spacing is extremely critical to coristruct and adjust. The reason for this is that slight electrical unbalances among the aerials and their interdonnecting circuits will override the intended differential effects due to the @mall phase differences between ihe signals induced in the individual aerials. In accordance with this inventlon I overcome this difficulty by employing a spaced- aerial antenna system with a small physical spacing and then I expand the effective electrical spacing by frequency multiplication to emphasize the smau direction- dependent phase differences. The use of a small physical spacing results in greatly improved operatiqn when diversity effects are present, and expansion of the effective electrical spaciiig makes the phase difference between the signals induced in the individual aerials large enough to overcome the effects of unavoidable circuit unbalances. For a better understanding of the invention, together with other and further objects thereof, reference is had to the following description taken in connection with the accompanying drawings, wherein like parts are indicated by like reference numerals and Wherein: Flgure I is a schematic circuit diagram of my invention which illustrates the basic method by which the electrical spacing between two spaced aerials is increased; Figure 2 Is a schematic circuit diagram which shows a practical direction :dnder with a twinchannel receiver embodying the invention; . and versity effects often constitute a more serious 55, - Figure 3 is a schematic circuit diagram of a

3 single-channel direction finder embodying the invention. Referring now to Fig. 1, there are shown a pair of spaced aerials IO and If, at points A clnd B, respectively. Assume that a wave arrives from the right along a line of direction passing through both antennas. If -the fleld at the midpoint c is expressed bylan equation of the form: Ec=sin wet then the signal voltages eA and eB induced in the antennas at points A and B, respectively, may be expressed by equations of the form, + -d ea'=sin (2) eb:=sin (3) The electrical spacing o between the antennas at A and B is: 2-d (4) To multiply this electrical spacing by any -arbitrary factor N the operation of frequency multiplication is performed by frequency multipliers 12 and 03 before the signals are combined in the receiving and indicating means 14, as illustrated in Mg. 2. After multiplicatiori the signal due to antenna 9 0 at point A becomes: rd e'A=ksin(N.@t+NX) (5) and the signal due to antenna II at point B becomes: ird e'B=k sin (6) Therefore, w r itliln the receiving and indicating means I & the apparent or eff'ective electrical spacing is: 2,rq o@=N (7) x or, in other words,'the effective spacing is multiplied by t]4e order of the frequency multipliei's 12 and 03. The basic method which has been described thus far, while valid in principle, is difficult to carry out with the frequency multipliers interposed directly between the antennas and the receiving and indicating means. Figure 2 illustrates a direction finder which employs the method of this invention in a more realizable manner. In Fig. 2, two spaced antennas 20 and 21 are shown. It is assumed that these antennas are movabie and can be oriented with respect to the wavefront of the received signal. Antennas 20 and 2 0. each derive a signal of the carrier frequency fc and these signals are applied to frbquency converters 22 and 23, respectively. A local oscillator 29 supplies a common signal of frequency fo to the two frequency converters. A signal having the frequency (f.-fe) is obtained from each converter. These signals are amplified independently in intermediate-frequency amplifiers 25 and 26 and are then applied to frequency multipliers 27 and 28 to obtain two signals having the frequency N(f.-fe) and, of course, correspondingly expanded phase differences. The two frequency-multiplied or phase-expanded signals are applied to an indicator 29 which is a cathoderay tube 30, utilized as a phase comparator. In operation, the relative phase and gain character- 2,484,824 4 lsties of the L-P. amplifiers 25 and 26 are so adjusted that the indicating display 31 is a straight line making an angle of 45- degrees with the axes of deflection when equiphase signals are induced in antennas 20 and 2 1. The display will then be a line when antennas 20 and 21 both lie in the wavefront and up into an ellipse when the antennas are not in the wavefront. For monitoring purposes an audio channel is 10 provided which consists of a detector and audio amplifier 32 and headphones 34. This channel is connected ahead of the multiplier 28 to avoid distortion. In some practic@l cases it will be inconvei.ient 15 to employ a iiiulti-channel receiver as shown in Fig 2. Figure 3 shows one way in which this ln;;ntion can be employed with a single-channel receiver. An oscillator 42 supplies a signal of frequency fl, and a second oscillator 43 supplies 20 a signal of a second frequency f2. The outputs of these oscillators are applied to a modulator 44. At the output of said modulator a filter 45 selects the frequency (fi+f2), @nd a second filter 46 selects the frequency (fi-f2). Suitable values for fi and /2 are 400 and 100 cycles per second 25 _respectively. The frequency (fl+f2) is used in a modulator 47 to modulate the signal derived by antenna 40. Similarly, the frequency (fl-f2), is used in rnodulator 48 to modulate the signal derived by a second antenna 41. Modulators 47 30 and 48 are preferably of the single side-band or asymmetric side-band type. The carrier and sideband outputs of modulators 47 and 48 are applied to the input of receiver 50, in this case 35 of the superheterodyne type comprising an R.-F. amplifier 51, a frequency converter 52, an I.-F. amplifier 53, a detector and audio amplifier 54, and;a sound reproducer 55. Part of the output signal from the intermediate frequency-amplifier 53 of said receiver is impressed upon a frequency 40 multiplier 56, wherein the frequency is multiplied by a factor N. The output of said multiplier is rectified by a detector 57 to produce two signals having the frequencies N(fi+f2) and N(fi-f2),. respectively. A filter 60 selects the frequency 45 N(fi+f2) which is impressed upon modulator 62. Another filter 61 selects the frequency N(fi-f2) which is impressed upon modulator 63. A signal of the frequency f2 is derived from oscillator 43 and multiplied N times in multiplier r'O 66 to produce a signal of the frequency Nf2 which is in turn impressed upon modulators 62 and 63. Filters 64 and 65 select signais of the frequency Nfi from the outputs of modulators 62 and 63, respectively. The signal from filter 64 is related in phase to the carrier signal induced in antenna 40 and, in a like fashion, the signal from filter 65 is related to the carrier signal induced in antenna 41. The two signals of the frequency Nfl, are.then applied to an indicator 64 which 60 may consist of a cathode-ray tube utilized in the manner shown in Fig. 2. The relative phase and amplitude characteristics of filters 64 hnd 65 should be mgde adjustable to facilitate balancing or calibration before operation. 65 Of the many modified embodiments of this invention which are possible, a few will be referred to briefiy. A direct reading direction finder could be constructed along the lines shown in Fig. 2 by employing four aerials with separate receiving 70 channels and frequency multipliers associated with each aerial. In this manner, four signals having expanded phase differences could be obtained. These four signals could then be applied directly to the respective deflection plates of a 75 cathode-ray tube to present directional informa-

29484,824 r3 tion, or inechanical or electronic goniometric of deriving a signal from said wave energy at each techniques could be used. of a plurauty of physicauy-displaced points, mul- In the direction ftder shown in Fig. 2 It may, ti plying the frequency of each signal so derived under certain circumstances. be preferable to em- a n equal integral number of times, and trans- ploy a common beat-frequency oscillator for I.- F. 5 la ting the phase differences among the fre- ampliflers 25 and 26, then to rectify the output of q uency-multipued signals into directional ln- @ach I.-P. amplifier and apply the rectified signals te higence. to individual synchronized multivibrators or re- 7. The method of deterniining th6 direction of laxation-oscillator frequency multipliers. a rrival of a received wave, which comprises re- If a presentation method is used which re- 10 c eiving said wave at a plurauty of spaced points, quires substantial equality among the phase- lo caby generating a pair of altemating potentish expanded signals, amputude limiting or automatic h aving difrerent frequencies heterodyning said gain dontrol can be applied advantageously to p otentials to produce sum and difference fre- each signal. q uency potentials, modulating the wave received The methods illustrated In Fig. 3 can be utilized 15 at one point with said sum frequency potential, readily In the design of direct reading direction in odulating the wave received at another point finders employing four or more spaced aerials. w ith said difference frequency potential, com- In such designs a third oscillator providing a bi ning both of the modulated waves to derive a frequency f3 could be associated with oscjllators r esultant wave, frequency multiplying said re- 42 and 43 and modulator 44 in Fig. 3. 20 s ultant wave an integral number of times, de- From modulator 44 low-frequency, phase-inter- te cting the frequency multiplled wave to derive a locked signals of the frequencies (fi+f2), p air of sigiqals having frequencies equal to said (fi-f2) , (fl+f2+f3) , (fl+f2-f3) , (fi-f2+f3) and s um and difference frequencies multiplied by said (fi-f2-f3) would then be available to modulate in tegral number, separating said signals, fre- the signals derived by the spaced aerials. 25 q uency multiplying one of the locally generated Although this invention has been described In p otentials the same number of times to derive a relation to direction finding techniques it will of r esultant potential, separately heterodyning said course be apparent that the methods disclosed si gnals with said resultant potential to derive two here will have ai:)Plieation to other flelds involv- r esultant signals having the same frequency, and ing measurement, comparison, or utilization of 30 d etermining the phase relationship between said small phase differences. r esultant signals to determine said direction of While there have been described what are at a rrival. present considered pr embodiments of the 8. A direction finding system comprising at invention, it will be obvious to those skuled in least a pair of physicauy spaced aerials, means the art that various changes and modific 35 for locauy generating a pair of altemating po- may be made therein without departing from tentials having different frequencies f, and f2, Invention, and it is aimed in the appended cla means for heterodyning said potentials to pro- to cover aU such changes and modificatiozis as duce a sum frequency potential and a difference fall within the true spirit and scope of the In- frequency potential, means for separately modu- vention. 40 lating the respective waves reeeived by said aeri- What is claimed is: als by said sum and difference frequency poten- 1. The method of determining the direction of tials, respectively, means for combb2ing all of the arrival of wave energy which comPriSes the SteP modulated -waves to derive a resultant wave. of deriving signals from said wave energy at more nieans for frequency multiplying said resultant than one point in space, multiplying the lDh9,se 45 wave by a factor N, means for detecting said fre- difference between at least one pair of said de- quency multiphed waves to derive signals having rived signals, and indicating said direction as w frequencie@ N(fi+f2) and N(fi-f2) cycles, means function of the multipued Phase difference. for separating said signals, means for frequency 2. The method of changing the electrical spac- niultiplyirig one of the locally genemted poten- ing between physicaUY-displaced antennas of a 50 tials by the factor N to derive a resultant poten- direction-sensitive receiver, which comprises the tial, means for separately heterodyning said sig- step of frequency multiplying the output of each , nals with said resultant potential to derive two of said antennas an equal number of times. 3. The method of changing the electrical spac-ing between physicauy-displaced antennas of a direction-sensitive receiver, comprisi4ig the steps of frequency converting the output of eadh of said antennas and then frequency multiPlYing selected frequency conversion products. 4. A direction finder comprising means for deriving a signal at each of 9. plurauty of physicall,j-displaced points, means for expanding the phase differerices among the signals so derived, and direction indicating means responsive to the resulting signals. 5. The method of determiwng the direction of arrival of wave energy which comprises the steps of deriving a signal from said wave energy at each of at least two physieauy-displaced points, electricary expanding the phase differences be. tween the signals so derived, and translating the expanded phase differences Into directioral information. 6. The method of determining the direction of arrival of wave energywhich comprism the steps resultant signals having the same,frequency, and means for measuring the phase relation of said 55 resultant signals to determine the direction of arrival of the received wave. 9. In combination in a direction flnding sys.. tem, a plurality of physically- displaced CoUeetor elements,. means for multiplying the- effective 60- electrical displacement between said coueetor elements, and direction- indicating means responsive to said effective electrical displacement. 10. A direction finder comi3rising a plumlity of Ph ysically-displaced wave couectors; a plurality 65 of equal-order frequency multipU6rs, one associated with each of said corectors; and direction indicating means receptive of the outputs of said frequency multipliers, and responsive to the phase relation of the outputs of said frequency 70 multipliers. 11. A direction finder comprising a plurauty of Phy sicaUy-displaced wave collectors; a piurg@ity of receiving channels, each receptive of the output of at least one of said wave couectors; a plu. 75 rality of equal-order frequency multiphers, one

7 associated with each of said receiving channels; and direction-indicating means receptive of the outputs of said channels, and responsive to the phase relation of the outputs of said frequency multipliers. 12. A direction finder comprising means for deriving a plurality of waves from a plurality of points; means for modulating each of said waves at a different frequency and for combining the resulting modulation products In a common receiving channel; frequency-multiplying means In said channel for expanding the phase differences among said modulation products; and means for translating said phase differences into directional intelligence. 13. The method of increasing the directional, sensitivity of a spaced-aerial direction finder in ,which the spacing of the aerials is reduced to minimize space- diversity eff ects, which comprises compensating for said reduced spacing by multiplying by an equal amount the frequency of the waves received by each antenna. 14. The method of increasing the directional sensitivity of a spaced-aerial direction finder in which the spacing of the aerials is reduced tO minimize space- diversity effects, which comprises compensating for said reduced spacing by separately multiplying by an equal amount the frequency of the waves received by each antenna. . 15. A direction finder comprising means for deriving from a radiated viave a plurality of waves at a plurality of physically-spaced points, a plurality of heterodyne frequency converters for sep- 8 arately converting the frequency of said derived waves to like lower frequency waves, a like plurality of frequency multipliers for separately mwtiplying the frequencies of said lower fre- quency waves by equal amounts, and means for comparing the phase relation of the outputs of said frequency mwtipliers to determine the directlon bf arrivai of said radlated wave. 16. A direction finder as set forth in claim 15, 10 wherein all of said frequency converters are coupled to a common local oscillator. PAUL G. HANSEL. REFERENCES CITED 15 The following references are of record in the fUe of tWs patent: UNITED STATES PATENTS Number Name Date 20 1,510,792 Merritt Oct. 7, 1924 1,952,879 Martin 27, 1934 2,035,759 Pool -------- ------- Yar. 31, 1936 2,133,303 Mirick -------------- Oct. 18, 1938 2,144,203 Shanklin ------------ Jan. 17, 1939 25 2,198,113 Holmes -------------- Apr. 23, 1940 2,234,654 Runge -------------- Mar. 11, 1941 2,282,402 Hefele --------------- May 12, 1942 2,387,569 Eggers -------------- Oct. 23, 1945 2,403,500 Carlson -------------- July 9, 1946 so 21423,103 Koechlin ------------ July 11 1947 2,423,437 Budenbom ------------ July 8, 1947 2,437,695 Jansky ------------ Mar. 16, 1948

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