US2689305A - Antenna - Google Patents

Description

P 14, 1954 H. J. RIBLET ETAL 2,689,305

ANTENNA Filed July 9, 1945 2 Sheets-Sheet 1 FIG. I I0 PI/WW5 HENRY .RlBLET ROY C. SPENCER p 1954 H. J. RIBLET m-AL 2,689,305

ANTENNA Filed July 9, 1945 2 Sheets-Sheet 2 AMPLITUDE AMPLITUDE INVENTORS HENRY J. RIBLET ROY C. SPENCER -.a plane intersecting said axis.

Patented Sept. 14, 1954 ANTENNA Henry J. Riblet, Cambridge, and Roy 0.

Arlington, Mass., assignors, ments, to the United States Spencer, by mesne assignof America as represented by the Secretary of War Application July 9, 1945, Serial No. 604,025

20 Claims. 1

This invention relates broadly to the formation of unidirectional electromagnetic wave radiation and morepa rticularly to devices for obtaining particularly desirable radiation patterns and characteristics therein. In wave propagation it frequently becomes desirable to project in one direction all or substantially all the energy emanating from a given antenna or antenna array. Directional radiation characteristics of this nature are particularly favorable for use in radio echo apparatus in order to indicate the bearing or elevation or both of the objects producing the reflections.

Since the advent of the so-called microwave region of the radio frequency spectrum, radiators or antennas resembling point sources have been available. Antennas of this type readily lend themselves to use with parabolic reflectors in facilitating the synthesis of unidirectional diffraction patterns. For instance, in airborne radio object-locating equipment it is generally desired to provide a pattern which will enable a continual and simultaneous search of both near and far objects remotely disposed on the earths surface or at equal elevations relative to the antenna. A radiation pattern applicable for this purpose can be attained by a mechanical arrangement which nods or tilts the axis of the parabolic reflector in a vertical plane which inevitably involves the use of intricate mechanical and electrical features including rotary joints in both the antenna feed line and reflector support member. A more favorable scheme for producing an equivalent vertical distribution of the radiation pattern may be attained through the synthesis of the desired pattern by means of a suitably chosen antenna array and excitation system.

It is here to be understood that even though the invention will be largely shown and described as applied to airborne equipment it can also be applied toother apparatus, such as for example, a beacon system.

It is an object of this invention to construct an antenna arrangement particularly adaptable for airborne radio object-locating use in providing a continual search of near and far objects.

It is another object of this invention to provide an antenna arrangement for use in synthesizing a desired radiation pattern.

It is another object of this invention to provide an antenna arrangement for use in connection with a parabolic reflector for producing a csc radiation pattern, wherein the angle 0 is measured with respect to the axis of directivity of the parabolic reflector and in one quadrant of Other objects and features of the present invention will become apparent upon a careful consideration of the following detailed description when taken together with the accompanying drawings, the figures of which are not to be construed as a definition of the limits of the invention.

In the drawings:

Fig. 1 is a pictorial representation of a typical use for the present invention.

Fig. 2 is a diagrammatic side elevation view illustrating the pattern produced by projection of a linear array of dipoles adjusted for equal intensity according to the present invention and also illustrating the amplitude of the diffraction pattern.

Fig. 3 is a diagrammatic side view similar to Fig. 2 in which the pattern is produced by a linear array of dipoles with tapered intensity according to the invention.

Fig. 4 is a view partially in cross-section of a typical antenna arrangement constructed in accordance with the teaching of this invention.

Fig. 5 is a plot of the power in the radiation pattern at a given point versus the angle relative to the axis of directivity of a typical antenna system constructed in accordance with the teachings of the present invention.

Fig. 6 is a cross-sectional view of a rectangular wave guide feed system; and

Fig. 7 is a perspective view of a dipole radiator such as may be used in the present invention and as shown in cross-section in Fig. 6.

Reference is now had to Fig. 1 wherein there is shown a sketch illustrating the principles and practicability of flaring a radiation pattern in the vertical plane. Herein is shown an airplane Hi from which the desired radiation pattern II is projected for use in detecting objects remotely disposed on the earths surface, such as the ship l2. As shown in the figure, the amount of energy reaching objects on the surface of the earth is normally inversely proportional to 1' But since h2 sin 0 the actual amount of energy reaching the targets will be directly proportional to the sin 0. Hence, in order to provide uniform illumination along the earths surface and within a given range of 0, the vertical distribution pattern must contain a corrective factor= or csc a .3 grees measured relative to the axis of the paraboloid. Also, the axis of the paraboloid should be tilted downward at an angle of about 20 from the horizontal in order to provide adequate illumination forward or laterally from the plane depending upon the orientation of the antenna. Thus, the angular distribution of energy will be confined between the limits of =20 and 0=50 or 60 depending upon the degree of flare and. tilt.

Since in radio object-locating use the same antenna is used in transmitting and receiving, the directive characteristic of the antenna will enter into both transmission and reception. Thus, the antenna pattern herein set forth will compensate for the relative weakness of echoes from similar reflecting objects disposed over the earths surface.

Special fanning or flaring of the diffraction or radiation pattern, such as a csc 0 pattern as referred to above is particularly useful in connection with radio object-locating systems such as are used for long range search, blind bombing and beacons. Itis also desirable to have a shipborne antenna which will produce a square diffraction or radiation pattern which will continue to illuminate the target with a constant intensity as the ship rolls.

The present invention is concerned with means for flaring out the diffraction or radiation pattern so as to simulate any desired pattern of greater width including a square pattern and more particularl a 050 0 pattern. In essence, the teachings of the invention are that if a series of radiating elements are placed in the focal plane of a parabolic reflector and the current distribution. along the ii element varies as f(i) then the diffraction pattern resulting will beapproximately the projected image of the current distribution, or f(i) also. A dipole array, for example, may be used to illuminate a, parabolic reflector and the currents feeding the dipoles adjusted to produce a beam pattern willbe approximately 050 0. In general this invention comprises the projection method of modifying the antenna so that its diffraction pattern will approximate the desired polar diagram and employs; a modified feed placed substantially in the focalplane of areflector.

Fig. 2 shows. a paraboloidal reflector l3. illuminated by a radiating source M. Source l4 comprises a linear. array of, radiating, elements such as dipoles Pl-P5 fed in a conventional manner and locatedv in aplane substantially perpendicular to the axis of reflector l3 at thefocal point thereof as more fully explained hereinafter.

It is well known that when a luminous point source is projected by an optical system the-angular width of the diffraction pattern image is of the order of where A is the wavelength of the illuminating energy and D is the diameter of the reflector. On the other hand when a collection of point sources such as Pi-P5 of Fig. 2 is projected, the diffraction pattern Qi-Qs is, according to the superposition theorem, the sum of the individual diffraction patterns. This results in a blurred and inverted image of the source. According to this, thediffraction pattern of the paraboloidal reflector I3 is essentially the projected image of-the current distribution in the focal plane of the reflector l3, and both electron .currents and displacement currents are included.

In the projection of a light source, intensity patterns are added since the phases of the individual radiating atoms are random. However, in the case of microwaves, amplitude patterns are added because the relative phases-.of'the antenna feed currents are fixed with respect to the phase of the generator. Two embodiments of the projection method according to this invention are shown in Figs. 2 and 3.

Fig. 2 shows how a square or pie-shaped pattern can be produced. Each dipole of the array PiePsplaced at the focus of parabolic reflector I3 is adjusted" so as, to radiate with approximately the same intensity, and this results in a diffraction pattern Qr-Qs in which each of the individual point image patterns are of equal intensity and hence of equal amplitude. Thus a substantially square pattern of uniform amplitude is produced as indicated in the amplitude diagramof Fig; 2.

Fig. 3 shows the projection method" as applied to obtaininga fan-type beamsuch as a csc 0 beam. In this modification alinear'array type feed l5. such. as a plurality of; dipoles Pl-Pn (nbeing an integer) are placed in the foca1 plane of' parabolic reflector I6. Thedntensityof-dipoles Pi-Pn is adjusted to give tapered illumination and to produce the patternQr-Qa asindicated bythe relative size of the points Pl-Pn and the relative shading of the patterns-Qr-Q The reflector I6 is preferably tilted in order'to minimize the distortion which occurs for dlpolesplacedoff the reflector axis. It is noted that if two adjacent dipoles are made 180 out of phase a zero would appear in the diffraction. pattern. It is therefore preferable thatthe phase difference between adjacent dipoles should" not exceed 60'to Referring now to Fig. 4 thereis shown a typical embodiment of the present invention comprising a parabolic reflector iTanda plurality of equally spaceddipole radiators l8; each projected; from the same. broad side of a. suitable wave guide transmission line l9 and so arranged as to constitute a linear antenna array. To facilitate further discussion, the dipole radiator nearest the excitation source will" hereinafter be referred to as the initial.dipolejwhereas the dipole farthest disposed from said". sourcewill be, referred to as the terminating. dipolefi In this arrangement the outer diameter of the pa-raboloid is preferably chosen as1a.functionofthe number andspacings of, the dipoles. A. rough approximation of the preferred, diameter may be had by. setting the angle as, described; by apair of lines drawn through the extreme end dipoles and convergent on. the paraboloid vertex,,equalto a minor. fraction, say /2 theanglie, s, between the axis and a line drawn betweenthe vertex, and the outer, edge of the paraboloidj. For example, an antenna array comprising 6 dipoles having A spacing excited at, approximately 10. cm. have been used in cooperation witha,10 fo.ot.naraboloid.

In ,synthesizingdiffraction patterns, suchas the csc 6 pattern,,-by means, of, a ,paraboloid reflector andrlinear. antenna array, it. is, usually desirable to position. allthe radiationlsources inlthe, focal plane of the paraboloid.,that is aplane which is situatedperpendicular to the axis ofjthe paraboloid and passes through the focal point thereof as referredto hereinabove with reference to Figs. 2 and 3. Due, however; to certain unavoidable reflections set up by the wave guide itself; an apparent radiation source will be established which will lieat some point between the dipole plane and the commonside; of-thewave-=guide from which the dipoles project. Hence, in order to secure optimum results the lineararray of dipoles will actually be positioned in a plane parallel to, but closer to the vertex of the paraboloid than the focal plane. In view of the foregoing analysis it appears that there exists an optimum spacing between the dipoles and the broad surface of the wave guide from which they project. This spacing is usually so adjusted that the reflected wave from the wave guide will reinforce the direct radiation into the paraboloid. Spacings in the order of one-quarter wavelength or an odd multiple thereof, where the wavelength is that measured in space, have been found to provide adequate reinforcement of the direct radiation.

As previously mentioned, the dipoles l8, constituting the linear array, are in general fixed at equal intervals along the wave guide l9 and in such a manner as to cause a predetermined degree of flare in the diffraction pattern. An increase in the overall length of the array will broaden the total flare angle. Variations in spacing of the dipoles affect their relative phases which cause minor changes in the smoothness of the flared beams. For instance two equally excited adjacent dipoles 180 out of phase will have a zero or null between their peaks. Here the flare of the pattern is confined to the vertical plane below the axis of the paraboloid-since the antenna array is situated entirely above the paraboloid axis. mate equivalent of x, where x and A, are the wavelengths measured inv air and in the guide respectively, have been used to produce a flare of about 30. spacings of this order will cause adjacent dipoles to be excited in near phase opposition thereby producing a null in the diffraction pattern, which effect is highly undesirable and may easily be remedied by alternately reversing the electrical orientation of the dipoles l8.

To provide excitation of the dipoles, radio fre quency energy derived from a suitable source such as the magnetron 2B and magnet 2| 7 is transmitted to the wave guide l9 by way of a suitable coupling link such as line 22 which contains an outer conductor secured to the wave guide and an inner conductor penetrating the magnetron field as a pick-up loop. a In order to excite oscillations of the desired mode such as the TEdr mode in wave guide 19 the inner conductor 23 of the coupling line 22 extends through the wave guide-l9 parallel tothe shorter dimensionsides thereof and is terminated at the opposite sidev by a suitable terminal plug 24.

'In the antenna array each dipole I8 is provided with a variable pick-up loop 25 adapted to penetrate an adjustable depth into the wave guide l9 so as to provide a means for controlling intensity of radiation or-power from'each dipole. As will be understood the power radiated is proportional to the square of the current. The pickup probes 25 may be adjusted to produce the same or different amounts of radiation from each dipole as indicated'with reference to Figs. 2 and 3. Here, for example, wherein consideration is primarily given the csc e pattern, the pick-up loops 25 are adjusted to produce a different amount of radiation from each dipole. An excellent csc pattern has been obtained when the terminating dipole adjacent the end 26 of the wave guide 19 has been adjusted so that its current is a certain value I which can be set forth as a standard or a unit amount. The next dipole up the guide toward the source and adjacent the Spacings of A or the approxi- 6. terminating dipole is adjusted to receive a unit, the-next a unit and so on up the array. Generalizing, the current, which is representative of the intensity of radiation, in each dipole can be represented by the formula I an where n is the position of the dipole relative to the terminating dipole and I is the unit current heretofore defined. Therefore, since intensity of radiation or power is proportional to the square of the current, power is inversely proportional to n squared. A radiation pattern simulating the curve shown in Fig. 5 has been obtained with this arrangement where the angle 0 is measured in a downward vertical direction from the axis of the paraboloid. Here the angle 0 is confined to the space below the axis, since the array itself is situated entirely above the axis with the terminating dipole on the axis. v

Energy proceeding along the guide [9 toward end 26 diminishesby amounts depending upon the proportions abstracted therefrom by vthe dipoles l8. Usually, however, a small amount, perhaps 5 to 10%, of the total energy will not be picked up by the dipoles. residual energy, even though it is a comparatively small amount of the total energy should not be allowed to remain in the guide and thereby set up standing waves. Therefore, a suitable piece of absorbent material, such as cotton gauze impregnated with graphite, is usually inserted into the end of the guide as indicated by the dotted lines 21.

Thus far consideration has been given the antenna only in a fixed orientation. It may be desired, however, to apply the present antenna system to either a sector scan or a P. P. I. (plan position indication) type of radio object-locating equipment wherein the antenna is used to scan a circular expanse about a given reference point. Hence, the paraboloid i1 is then rotated about a vertical axis by way of a suitable rotating mechanism coupled to a supporting yoke such as yoke 28. Guide l9 and the associated antenna array may then be held in the propenplane by any suitable meanssuch as the brace member 29 secured to the paraboloid l1. With this op: crating condition it may become desirable to rotate, the transmitter and certain other components of the system in unison with the antenna. Sucha system can then utilize a platform or support 30' secured to the back of the paraboloid by way of any suitable brace scheme 3| capable of containing the transmitter as here represented by the magnetron 20 and magnet 2|. Thereceiver may then be coupled to the antenna through wave guide l9, a rotatable joint and a suitable T-R switch of known form for decoupling the receiver from theantenna duringpulse transmission. The latter two features are not shown here since they do not constitute a part of the applicant's invention. a

The particular type dipole radiator, feeding and supporting members used in abstracting energy from the wave guide I9 is not critical and may, for example, be similar to that shown in Figs. 6 and 7. The dipole arms 32 and 33 are both supported in a tubular member 34 which may be suitably secured to the wave guide I9 as indicated. Member 34 is provided with a pair of diametrically oppositelongitudinal slots 35 and 36, disposed from the plane of the dipole arms 32 and 33and closed at one end by plug 31 which also serves as a support for an inner aesasoe tubular member 38. Slot. 36 is indicated diagrammatically in. Figure. 7.. The inner tubular member 38 is secured: to the dipole: arm 32 by avpin 39 which cooperates: with plugv 31 in coaxially supporting the inner member 38. within member 34. Snugly but slidably engaged within the inner member 38 isan energy pick-up rod 25 extending an adjustable depth into the guide l9.

The longitudinal slots 35' and 36 each have a totalleng-th of A so that the outer'tubularmember1'34- is effectively divided into upper and lower segments, the central portion of which may be excited by'the: energy picked up by probe 25-so as to cause an R. F. voltage to appear across the dipole arms llz and 33. Hence by adjusting the depth of penetration of probe 25 into wave guide Hi the intensity oi radiation emanating from the dipole may be regulated: as desired.

Although we have shown and described only a certain andspecificembodiment off the present invention, we are fully aware of" the many modifications possible thereof. Therefore it is desired thatthis invention not to-be limited to the precise details set forth.

Whatis claimed is:

1 antenna system for synthesizing a desired; radiation pattern comprising a rectangular wave guide, a plurality of dipole radiators, including an initial and a last dipole radiator, projectedfi'omacommon-side of said guide at nearly one-half wavelength intervals soas to constitute a co-linearantennaarray, a par-aboloidal reflecting means, means positioning said array in a plane perpendicular to theaxis of said reflecting meanswith the last dipole of said arraylying in the horizontal axial plane of said reflecting means, the diameter of the reflecting means being such that the angle between the axis and a=-linedrawn from the vertextothe' initial dipole is but a minor'fracti'on oftheanglebetween the axis and a line drawnfrom the vertex to" the edge of the reflecting means, means for energizingall dipole radiators simultaneously, and a means' for adjusting the radiationintensity emanating from each of said dipoles;

2: An antenna as claimed in claim I wherein the adjusting means adjusts the radiation in:- tensityfrom the dipoles to produce a radiation pattern from the reflecting means which varies substantially asthesquare ofthecoescant of the radiation angle measured from the axis" of. directiv'ity" of the paraboloidal' reflecting means;

3; Anantenna'system'iorsynthesizinga desired radiation pattern'comprising'a paraboloid reflecting-means, arectangul'ar-waveguide; a plurality of dipole radiators projected from a common side of said guide at one-half wavelength intervals to form a linear antenna array; means positioning said array in aplane-perpendicularto the axis of said reflecting means and having the last dipole thereof lying inan axial plane of said reflecting means, and means for adjusting the radiation intensity from said dipoles to various predetermined values:

4. An antenna system for synthesizing a. desired radiation pattern comprising, a parabolic reflecting means, a plurality. of. coplanar radiation sourceslpositionedin a plane perpendicular to the axis of said parabolic reflecting. means and havingthe last source thereof lying in the horizontalplane intersecting saidaxis, means for adjusting the intensity, of each of said sources to'conform tothe product of: an arbitrary intensity" unit times the: reciprocal of the relative position number of the. source counted from. the last source 5'. An antenna system for synthesizing a. desired radiation pattern comprising, a parabolic reflecting means, a rectangular wave guide section, a plurality ofv equally spaced dipole radiators projected from a common side of said wave guide to form a linear antenna array, means positioning said array in. a plane perpendicular to the axis of said parabolic reflecting means with thelast dipol'e thereof lying in a planethat intersects the axis of said reflecting means, means: for simultaneously energizing allot said dipole radiators, and means for adjusting the radiation intensity from each of said dipoles.

6. An antenna system for synthesizing a desired radiation. pattern comprising, a parabolic reflecting means, a series of spaced coplanar radiation: sources positioned ina plane perpendicular to the axis of said parabolic reflecting means and having the last source of said series lying in a plane intersecting said axis, and means for'adjusting theintensity of radiation from each of said sources to a different value.

'7'. Anantenna system for synthesizing a desired radiation pattern comprising, a paraboloid reflecting means, a rectangular waveguide, a plurality of dipole radiators projected from a common side of said guide at onehalf wavelength intervals toform a linear array, means positioning said" array in a plane perpendicular to the axisof said reflecting means and having an end dipole thereof lying in the horizontal axial plane of said reflecting means, and'meansfor adjusting the radiation intensity of each of said dipoles so that the intensity of radiation of said end dipole is some arbitrary value R and the intensity of radiation of each of the other dipole radiators is equal to 12/11. where n is theposition number of the dipole radiator relative to the end dipole radiator where said end dipole radiator has the position number 1.

8. Apparatus for synthesizing adirectional radiation pattern ,f(i) having predetermined characteristics comprising a series of equally spaced coplanar energy radiation sources, a parabolic reflecting means for directing energy radiated from said" radiation sources; and means for adjusting the radiation intensity of each of said sourcesrelativeto the radiation intensity of an end source so that the radiation intensities of said sources, as progressively measured from said end-source, follow the curve f(i).

9; Apparatus for synthesizin a directional radiation pattern f(i) comprising a series of coplanar radiators, a parabolic reflector fordirecting energy radiated fromsaid radiators; said radiators being disposed substantially along a line through the focus'of" said reflector; means for adjusting the intensity of radiation of said radiators relative to the radiation in tensityof one of said radiators so that the radiation intensities of said radiators, as progressively measured from said one radiator, follow the function f(i), and means for simultaneously energizing all of said radiators.

10. The apparatus defined in claim 9- wherein the'phase difference between'the currents in adjacent radiators isless than 11. The apparatus defined in claim 9 wherein the plane of said radiators is substantially perpendicular to the axis of said parabolic reflector.

12. The apparatus defined in claim 9' wherein saidmeans for adjusting the currents of said radiators causes said currents to vary inversely as the distance of each radiator from the focus of said parabolic reflector.

13. An antenna system for synthesizing a desired radiation pattern comprising, a paraboloid reflecting means, a rectangular wave guide, a plurality of dipole radiators projected from a common side of said guide at one-half wavelength intervals to form a linear antenna array, means positioning said array in a plane perpendicular to the axis of said reflecting means and having the last dipole thereof lying in the horizontal axial plane of said reflecting means, and means for adjusting the current in each of said dipoles so that the current in said last dipole is some arbitrary value R and the current in each of the other dipole radiators is equal to R/N, where N is the position number of the dipole radiator relative to the last radiator when said last dipole radiator has the position number 1.

14. The antenna system of claim 1, wherein the angle between the axis of said paraboloidal reflecting means and the line drawn from its vertex to the edge thereof is at least four times as great as the angle between said axis and a line drawn from said vertex to said initial dipole.

15. The antenna system of claim 1, wherein the electrical orientation of alternate dipole radiators is reversed relative to their adjacent radiators so as to avoid excitation in phase opposition.

16. The antenna system of claim 1, wherein the plane in which said dipole radiators are disposed is displaced from the focal plane of said paraboloidal reflecting means toward the vertex thereof, the displacement from the focal plane being sufiicient to place the apparent radiation source, between the common side of said guide and the plane of said radiators, within said focal plane.

17. The antenna system of claim 16, wherein said dipole radiators are spaced from said common side of said guide by an amount equal to a quarter of a Wavelength of the operating frequency as measured in space.

18. The antenna system of claim 17, wherein each of said dipole radiators is respectively mounted upon a wave guide projecting from a common wall of said rectangular wave guide, the two poles of each dipole being disposed apart and projecting from the circumference of their supporting wave guide, each supporting wave guide having a pair of longitudinal slots, each slot having a length of one-half wavelength, said slots being 180 apart and at right angles to the dipoles.

19. The antenna system of claim 18, wherein said rectangular wave guide is energized at its end remote from said last dipole, and further including energy dissipation means terminating the end of said rectangular wave guide adjacent said last dipole.

20. The antenna system of claim 19, wherein the axis of said paraboloidal reflecting means is tilted at an angle with the horizontal.

References Cited in the file of this patent UNITED STATES PATENTS Number Name Date 1,301,473 Marconi Apr. 22, 1919 2,106,771 Southworth Feb. 1, 1938 2,436,380 Cutler Feb. 24, 1948 2,458,885 Warren Jan. 11, 1949 2,480,208 Alvarez Aug. 30, 1949 2,605,413 Alvarez July 29, 1952

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