US 3717877 A
Cavity backed spiral antenna apparatus having an extremely narrow depth. The spiral antenna elements are preferably equiangular and comprise less than a full turn but more than a half turn and are ferrite loaded to result in a cavity depth which is on the order of 1/80 of the wavelength of the highest frequency of interest.
Description (OCR text may contain errors)
JH-Q-GILQ DH OL-ZQ-73 OR 397179877 0 United States Patent 1 1 1111 3,717,877 Hanninen et a]. 1 Feb. 20, 1973 [s41 CAVITY BACKED SPIRAL ANTENNA 3,310,307 3/1967 Kofoid ..343/787  Inventors: Albert E. Hanninen, Londonderry; 3322 S5222; Shaun scanlon Nashua 31544920 1/1970 1401115111 1 3,555,554 1/1971 Kuo ..333/83  Assignee: ISqanPtIlers Associates, Inc., Nashua, Primary ExaminermE Lieberman Attorney-Louis Etlinger  Filed: Feb. 27, 1970 21 Appl. No.: 15,102  ABSTRACT Cavity backed spiral antenna apparatus having an ex-  U 5 Cl 343/872 343/895 tremely narrow depth. The spiral antenna elements  q 1/36 are preferably equiangular and comprise less than a full mm but more than a half mm and are ferrite 9  Field of Search 343/787 789 8 5 872 loaded to result in a cavity depth which 1s on the order  Reterences cued of 1/80 of the wavelength of the highest frequency of UNITED STATES PATENTS Russell ..343/895 11/1968 Meyers ..343/787 'IIIIIIIII YIIIIIII/A interest.
3 Claims, 4 Drawing Figures 1 CAVITY BACKED SPIRAL ANTENNA The invention herein described was made on the course of or under a contract or subcontract thereunder with the Department of the Navy.
BACKGROUND OF INVENTION This invention relates to new and improved antennas and in particular to cavity antennas which are of relatively small physical size.
Electrically small cavity antennas are useful in any application requiring compact physical dimensions, as for example in airborne, missile, space and similar environments. One type of antenna which has been used in the past for applications such as the foregoing where size and weight are critical, is the so-called spiral type antenna described, for example, in U. S. Pat. No. 2,863,145, issued Dec. 2, 1958 to E. M. Turner. Although such spiral type antennas have been adequate in many cases, the cavity depth dimension of the associated cavities have been too large for those applications in which the cavity depth must be significantly smaller than V4 wavelength at the highest frequency of interest.
BRIEF SUMMARY OF INVENTION Accordingly, it is an object of the present invention to provide a novel and improved cavity type antenna which is of relatively small physical size.
It is another object of the invention to provide a cavity backed spiral type antenna in which the cavity depth is extremely small compared tothe wavelength at the highest frequency of interest.
Briefly, the invention is embodied in antenna apparatus having a cavity with an aperture. A spiral antenna is mounted over the cavity aperture and is loaded by means of a sheet of ferrite material which is disposed within the cavity. In one embodiment, the spiral antenna elements are mounted upon the ferrite sheet. In another embodiment, the spiral elements are mounted upon a radome which is in turn mounted over the cavity aperture. A feed means is provided to couple energy to and from the spiral antenna elements. In a preferred embodiment the spiral antenna elements are'equiangw' lar and each comprises less than a full turn but greater than a half turn. With ferrite loading in this preferred embodiment, a cavity depth on the order of H100 of the wavelength of the highest frequency of interest is typical.
BRIEF DESCRIPTION OF DRAWINGS In the drawings, like reference characters denote like structural elements; and
FIG. 1 is a sectional view taken along the lines I-l of FIG. 2 of a cavity backed spiral antenna embodying the invention;
FIG. 2 is a plan view of the spiral antenna elements mounted upon a ferrite sheet in accordance with one embodiment of the invention;
FIG. 3 is a plan view of the spiral antenna elements mounted upon a radome in accordance with another embodiment of the present invention; and
FIG. 4 is a sectional view taken along'the lines 4-4 of FIG. 3 and further illustratingsaidothe'r embodiment of the invention.
DESCRIPTION OF PREFERRED EMBODIMENT Referring now to FIGS. 1 and 2, there is shown for purposes of illustration a cavity backed spiral antenna consisting of conducting antenna elements 10 and 11 having feed terminals 13 and 14, respectively. The antenna elements 10 and 11 are coiled in the configuration of a concentric Archimedian spiral. Preferably the antenna elements 10 and 11 are also equiangular. That is, the width of each element is largest at the center of the spiral and becomes progressively narrower toward the outside of the spiral. In addition, the antenna elements l0 and 11 preferably consist of less than a full turn and slightly greater than a half turn.
According to a first embodiment of the invention the antenna elements 10 and 11 are mounted upon a sheet 15 of ferrite material. The sheet 15 has a geometrical shape so as to fit the interior of a cavity 16. For the illustrated embodiment the geometrical shape of the sheet 15 and of the cavity 16 takes the form of a cylinder or disc. The sheet 15 and the cavity 18 have apertures 17 and 18, respectively, each adapted to receive a feed cable 19. The feed cable 19 comprises a center conductor 23 and an outside conductor 24 which are adapted to connect to the feed terminals 13 and 14, respectively when assembled. The ferrite sheet 15, the cavity 16 and the cable 19 are shown as separate elements in FIG. 1 in order to conveniently illustrate the features of each. When fully assembled, the ferrite sheet 15 fits within the cavity 16 as indicated by the arrow and the cable 19 fits within the apertures 17 and 18 of the sheet 15 and cavity 16. The sheet 15 may be held within the cavity 16 by any suitable fastening means such as adhesives, and the like.
The antenna elements 10 and 11 may be mounted upon the surface of the ferrite sheet 15 by any suitable technique. For example, the elements 10 and 11 may be formed by photo etching technique.
The ferrite sheet 15 has a relatively high dielectric constant compared to that of the conducting elements 10 and 11 or of the conducting cavity 16. Consequently, the ferrite sheet 15 is a substantially electromagnetically transparent material.
Referring now to FIGS. 3 and 4 there is shown another embodiment of the invention in which the spiral antenna elements 10 and 11 are mounted upon a radome 12 rather than upon the surface of the ferrite sheet 15. Radome 12 may be of any suitably electromagnetically transparent material such as epoxy, glass, and the like. In all other respects the embodiment as shown in FIGS. 3 and 4 is identical to that shown in FIGS. 1 and 2 and like structural elements bear like reference characters.
ln bothof the above described embodiments of the invention the ferrite sheet 15 serves the same purpose, namely, to provide a loading of the equiangular spiral antenna'elements 10 and 11. The particular configuration of the equiangular spiral'antenna elements 10 and 11 in somewhat less than a full turn but greater than a half turn with ferrite loading has resulted in a cavity depth w which is on the order of H of the wavelength A at the highest frequency of interest. According to one circularly polarized antenna design embodying the invention and useful in the ultrahigh frequency (UHF) communications band where the frequency of 1,000
megahertz is the highest frequency of interest, (k =1 1.8 inches) the cavity depth w =0.l5 inches. For the same design the cavity diameter d is 2.75 inches. In spite of the ferrite loading, the circularity of the polarization is maintained. It will be appreciated that the above described cavity backed spiral antenna embodiments of the present invention may be employed either for radiating purposes or for receiving purposes. Consequently, the feed cable 19 is connected at its other end to a transmitter/receiver device (not shown).
it will thus be seen that the objects as set forth above, among those made apparent from the preceding description, are efficiently attained and, certain changes may be made in the illustrated structures without departing from apparatus which embodies the invention.
It is intended that all matter contained in the above description or shown in the accompanying drawings is primarily illustrative of apparatus embodying the invention.
What is claimed is:
1. A cavity having an aperture;
a spiral antenna mounted over said aperture, said spiral antenna including a pair of conducting elements which are coiled in spiral and are equiangular, each of said elements being less than one turn and greater than one half turn,
loading means including a sheet of ferrite material mounted within said cavity whereby the cavity depth is on the order of M where A is the wavelength of the highest frequency of interest; and
feed means for coupling energy to and from the spiral antenna.
2. The invention as set forth in claim 1 wherein said spiral antenna elements are mounted upon a surface of said ferrite sheet which faces the cavity aperture.
3. The invention as set forth in claim 1 wherein said spiral antenna elements are mounted upon a radome which in turn is mounted over said cavity aperture.