US 3795005 A
Antenna elements for substantially circularly-polarized electromagnetic energy are shown. The disclosed elements, formed on a tapered form having an elliptical cross section, are particularly well suited for use as elements in an antenna array because such a cross-sectional shape permits individual elements to be more closely positioned with respect to each other, thereby raising the frequency of the electromagnetic energy at which grating lobes occur.
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Description (OCR text may contain errors)
United States Patent Monser et al.
[4 1 Feb. 26, 1974  BROAD BAND SPIRAL ANTENNA  Inventors: George J. Monser; John R.
Ehrhardt, both of Santa Barbara,
 Assignee: Raytheon Company, Lexington,
 Filed: Oct. 12, 1972 ] Appl. No.: 297,112
Related U.S. Application Data  Continuation-impart of Ser. No. 142,223, May 11,
 US. Cl. 343/895, 343/854  Int. Cl. H01q l/36  Field of Search 343/895, 908, 854
 References Cited UNITED STATES PATENTS 3,587,106 6/1971 Crooks et al. 343/895 3,454,951 7/1969 Patterson et al 343/895 3,019,439 l/l962 Reis et al..... 343/895 3,508,269 4/1970 Snyder 343/895 2,919,442 12/1959 Nussbaum 343/895 AMPLIFIER AMPLIFIER PARALLEL PLATE LENS FOREIGN PATENTS OR APPLlCATlONS 411,888 7/1945 ltaly 343/895 Primary Examiner-E1i Lieberman Attorney, Agent, or Firm-Philip J. McFarland; Joseph D. Pannone ABSTRACT Antenna elements for substantially circularlypolarized electromagnetic energy are shown. The disclosed elements, formed on a tapered form having an elliptical cross section, are particularly well suited for use as elements in an antenna array because such a cross-sectional shape permits individual elements to be more closely positioned with respect to each other, thereby raising the frequency of the electromagnetic energy at which grating lobes occur.
In a second embodiment the eccentricity of the tapered form is relatively large, so the greater part of each antenna turn lies in a plane. Such a configuration radiates substantially linearly polarized radio frequency energy.
5 Claims, 4 Drawing Figures ANTENNA PATENT'ED FEB26 I974 SHEU .1 [If 2 AMPLIFIER AMPLIFIER F/GJ ANTENNA nvvewrms GEORGE .1. a/ JOHN R. EHRHARDT PATENTED FEBEB I974 SHEET 2 [IF 2 BROAD BANDSPIRAL ANTENNA This application is a Continuation-ln-Part of the copending application entitled Broad Band Antenna, Ser. No. 142,223, now abandoned, filed May I l, 1971 by George J. Monser and John R. Ehrhardt and assigned to the same assignee as this application.
BACKGROUND OF THE INVENTION This invention pertains generally to antenna arrays for radio frequency energy and particularly to antenna arrays for circularly or linearly polarized radio frequency energy.
It is known in the art that multi-beam antenna arrays may be arranged to produce simultaneously existing beams of electromagnetic energy having circular polarization. Thus, it is known that a circular polarizer assembly may be disposed in the path of the electromagnetic energy passing to or from an array having antenna elements which form either plane-polarized electromagnetic energy for transmission or are sensitive to plane-polarized energy on reception. It is also known that circularly-polarized electromagnetic energy may be transmitted from, or received by, a multi-beam array without a polarizer assembly if antenna elements are conventional helices.
Unfortunately, known circular or linear polarizer assemblies limit the bandwidth and scan capabilities of multibeam antennas. That. is, because such polarizers must be designed and fabricated to operate on electromagnetic energy of a particular frequency or several discrete frequencies and because they must, of necessity, absorb some of the radiated energy, their effectiveness decreases with changes in frequency from the design frequency. With increased scan angles the reflections of the external polarizers greatly reduce the effectiveness, particularly with regard to power transmission.
The use of conventional bifilar helical antenna elements avoids, to a large extent, the power limitations imposed by wide scan angles and, to a lesser extent, the limitations imposed by frequency changes on multibeam array antennas sensitive to circularly-polarized energy. However, in a multi-beam antenna array which is desired to be broad-band (meaning an antenna of such type which is useful over, say, an octave of frequency change) the use of conventional cylindrical bifilar helical antenna elements presents difficulty. Thus, it has been found in such applications that grating lobes appear (at the higher end of the band) in the antenna pattern of an array using conventional bifilar helical antenna elements. Such lobes, of course, may not be tolerated in operational equipments because of their adverse effect on angular accuracy and where maximum directive gain is required.
SUMMARY OF THE INVENTION AND DESCRIPTION OF THE DRAWINGS Therefore, it is the main object of this invention to provide an improved helical antenna element for use primarily in a multi-beam antenna array, such improved element being adapted substantially to circularly polarized electromagnetic energy and, at the same time, avoid the formation of grating lobes in the antenna pattern of such array, under wide scan operation on the order of 1 60 from the array normal.
Another object of this invention is to provide improved helical antenna elements which, when incorporated in a multi-beam antenna array, increase the bandwidth of such array.
Another object of this invention is to provide improved helical antenna elements which, when incorporated in a multi-beam antenna array, cause linearly polarized energy to be radiated.
These and other objects of this invention are attained generally by providing a tapered helical antenna element which is supported on a form having a cross section conical near the apex and tapering to substantially an elliptical shape. In a second embodiment, the form is, in cross-section, an ellipse with a large eccentricity. For a more complete understanding of this invention, reference is now made to the following description of a preferred embodiment as illustrated in the accompanying drawing in which:
FIG. 1 is a representation, partially in block diagram form and partially in the form of a sketch of the contemplated antenna element used as an array element in a multi-beam antenna array of a transponder using this invention;
FIG. 2 is an isometric view of a single one of the antenna elements shown in FIG. 1; and
FIG. 3 is an isometric view of an alternative embodiment of this invention to radiate substantially linearly polarized radio frequency energy;
F IG. 4 is an isometric view of an alternative embodiment of this invention.
DESCRIPTION OF THE PREFERRED EMBODIMENT Before referring to the drawings, it will be advantageous to note that our inventive concepts are applicable either to phased antenna arrays (meaning arrays having a separate adjustable phase shifter in the path of electromagnetic energy between a single focal point and each one of a matrix of antenna elements, the adjustment of each such phase shifter determining the direction of a single beam relative to the matrix of antenna elements) or to multi-beam antenna arrays (meaning arrays having an electromagnetic lens arrangement associated with a matrix of antenna elements, such lens arrangement determining the direction of each one of a plurality of simultaneously existing beams relative to the matrix of antenna elements). For expository purposes, however, we have chosen to show a preferred embodiment of our invention in a multi-beam antenna array.
With the foregoing in mind it may be seen in FIG. 1 that a transponder according to our invention includes a multi-beam receiving antenna 11 and a similar multibeam transmitting antenna 13 (the former being described in detail hereinafter), individual ones of the antenna elements 15a through 15n being connected through transmission lines 17a through 17n to a corresponding coupling point 19a through 19n of a parallelplate lens 2lr. It is known, for example as shown by W. Rotman and RF. Turner in their paper entitled Wide Angle Microwave Lens for Line Source Applications in the Transactions of the Institute of Electrical and Electronic Engineers, Nov., 1963, pp. 623-632 published by the Institute of Electrical and Electronic Engineers, Inc., New York, N.Y., that an array of antenna elements connected through transmission lines of chosen lengths to a parallel-plate lens may be arranged to focus a planar wave of electromagnetic energy at a point along an arc of best'focus, the particular point on such are at which such energy is focused being determined by the direction of the origin of the planar wavefront. Thus, by connecting each one of a group of amplifiers 23a through 23x to a different point 25a through 25):, an output signal from any one of such amplifiers is indicative of a radio frequency signal from a particular direction. it is noted here that the number of different points 25a through 25x (and, therefore, the number of amplifiers 23a through 23x) may be varied as desired to change the number of beams. in any event, the output terminal of each one of the amplifiers 23a through 23x is connected to a corresponding point 27a through 27x on the arc of best focus of a parallelplate lens 291. The latter is similar to the parallel plate lens 21r. in like manner, transmission lines 31a through 3ln, each of which has the same length as a corresponding one of the transmission lines 17a through 1711, is connected between a point 33a through 33m to an antenna element 35a through 35n of the multi-beam transmitting antenna array 13.
Referring now to FIGS. 1 and 2 it may be seen that each one of the antenna elements 35a through 35n is a modified conical-helical antenna with a single winding, the base 37 of each such winding being mounted on a metallic ground plate (not numbered) in any convenient manner. As is known, a single conical-helical antenna mounted on a metallic ground plate has a relatively broad antenna pattern, say in the order of 70 to 80 between half-power points, with nulls orthogonal to the axis of the helix. it is also known that, as a transmitting antenna, a single conical-helical antenna radiates circularly-polarized radio frequency energy and that, as a receiving antenna, an antenna of such configuration is responsive to circularly polarized radio frequency energy. The sense of polarization, i.e. whether left-hand or right-hand, to which a conical-helical antenna element is responsive depends on the direction of the active winding of the element. It is also known that a single conical-helical antenna element may be made to be operative over a relatively wide band of frequencies, meaning more than an octave. Thus, it is known that a single conical-helical antenna element substantially maintains its efficiency from a lower frequency limit determined by the diameter of its base to an upper frequency limit determined by the pitch of its active winding. When, however, conical-helical antenna elements are disposed together in an array, they are subject to the same strictures as apply to any antenna elements so used. That is, in order to avoid the formation of grating lobes, it is necessary that antenna elements in an array be disposed so that the distance between phase centers of adjacent elements be less than onehalf wavelength of the radio frequency energy passing to or from such elements. Such a limitation obviously determines the upper end of the band of frequencies within which any array of antenna elements may be used.
With the foregoing in mind it may be seen that the antenna element we contemplate, as illustrated and clearly shown in FIG. 2, comprises a base 37 having a generally elliptical shape on which notched spacers 39a, 39b, 39c, 39d are mounted as shown to form a generally conical winding support (not numbered) for a wire winding 41. The pitch of the wire winding 41 preferably varies as shown. It is noted here in passing that although the pitch of the wire winding 41 may vary, the locus of such winding on the generally conical winding support is substantially helical. The feed for the wire winding 41 is effected, through a conventional matching circuit 43 and connector 45 mounted in the center of the element, from a transmission line, said transmission line 17a (FIG. 1).
When a number of the antenna elements through 15n are disposed along a metallic ground plate, as in FIG. 1, to form a linear array, such elements are arranged so that the major axes of successive ones thereof are parallel to each other as shown. Thus, the phase center of successive antenna elements 15a through 15m are, in the plane of the elements, as close together as possible. The exact spacing between successive antenna elements 15a through 15n is, of course, dependent upon the length of the minor axes of such elements. In any event, it will be clear that the spacing between successive antenna elements 15a through l5n will be smaller than would be possible with conventional conical-helical antenna elements having diameters equal to the major axes of the disclosed antenna element. Consequently, the upper limit of the frequency band in which an array according to our inventive concepts will be higher than a conventional conical-helical antenna array having the same lower limit. It will be recognized that the polarization of the radio frequency energy to which our antenna array is best fitted is elliptical rather than circular, the ellipticity of such polarization corresponding to the eccentricity of the base 37 of each antenna element 15a through l5n. We have found, however, that the extension of the upper limit of the frequency band to which our array is responsive more than compensates for any degradation of polarity in many practical systems.
Referring now to FIG. 3, it may be seen that the eccentricity of the base 37 (FIG. 2) may be increased so as to make the cross-section of the wire windings appear almost rectangular. The polarization of the energy radiated by such a modified antenna element is, then, substantially linear. Thus, the base 37' of FIG. 3 is an extremely eccentric ellipse, causing a tapered support 50 to be similarly eccentric. Consequently, the greater part of each one of the helical turns of the bifilar wire windings 41, 41' about the tapered support 50 is substantially flat. Wire winding 41 here is an extension of the center conductor of a coaxial line 52 helically wound around the tapered support 50 as shown and wire winding 41 here is a wire connected in any convenient manner to the shield of the coaxial line 52 and interlaced with wire winding 41. Alternatively, wire windings 41, 41 could be wires coupled to the arms of a hybrid junction, such junction being fed by a transmission line.
The dimensions of the base 37 and of the tapered support 50 and the number of helical turns of the wire winding 41 may be varied within wide limits. it is necessary only that the length of each turn of the wire winding 41 be equal to or greater than the wavelength of the radio frequency energy to be radiated.
Having described two embodiments of our invention, it will be apparent to one of skill in the art that many changes and modifications may be made thereto without departing from our inventive concepts. For example, each antenna element could incorporate a conventional bifilar winding in place of the single winding as shown in FIG. 2. in addition, the shape of the base could be changed from an ellipse to a hexagonal shape, as shown in FIG. 4, if a planar array is desired; such change would make the distance between the phase centers of adjacent antenna elements as small as possible in'orthogonal planes to prevent grating lobes from forming in either the azimuthal or elevation planes. It is felt, therefore, that this invention should not be restricted to its disclosed embodiments but rather should be limited only by the spirit and scope of the appended claims.
What is claimed is: 1. An antenna array for substantially circularly polarized radio frequency energy, such array comprising:
a. a plurality of like antenna elements, each one thereof including a phase center and a wire radiators, the active portion thereof being helically wound around the frustum of a conic-like supporting member having an elliptical cross section; and
b. means for mounting each one of the plurality of like antenna elements in juxtaposition one to another to form an antenna array wherein the major axes of the elliptical cross sections of the conic-like supporting members are parallel one to another, the phase centers of adjacent elements being separated by less than one-half the wavelength of the radio frequency energy.
2. An antenna array as in claim 1 wherein the wire radiator comprises bifilar windings.
3. An antenna array as in claim 2 wherein the end of each one of the bifilar windings adjacent to the top of the frustum is passed through the center thereof.
4. A planar antenna array for substantially circularlypolarized radio frequency energy, such array comprismg:
a. a plurality of like antenna elements, each one thereof having a phase center and being helically wound around the frustum of a pyramid having a hexagonal base; and
b. means for mounting each one of the plurality of like antenna elements to abut one to another to form an antenna array, the phase centers of adjacent elements being separated by less than one-half the wavelength of the radio frequency energy.
5. An antenna array for radio frequency energy comprising:
a. a plurality of like antenna elements, each one thereof including a phase center and a wire radiator having its active portion helically wound around a tapered supporting member, any cross section of such member being a highly eccentric ellipse to cause radio frequency energy propagated from the active portion of the wire radiator to be substantially plane polarized; and
b. means for mounting each one of the plurality of like antenna elements in juxtaposition one to another to form the antenna array, the phase centers of adjacent elements being separated by less than one-half the wavelength of the radio frequency energy.