Annular Slot Antenna

This invention relates to an annular slot antenna and, more particularly, to a directional, annular slot antenna with broad bandwidth and high gain using a corporate feed and adaptable for circular polarization.

Slot array antennas have been disclosed in a number of prior patents. U.S. Pat. No. 2,433,924, for example, discloses an antenna adapted to provide non-directional radiation in a horizontal plane.

U.S. Pat. No. 2,570,824 discloses a slot antenna intended to be flat for airborne use and have a band width of several percent through the provision of a plurality of slots fed by a resonant cavity. U.S Pat. No. 2,589,664 also discloses a wide band airborne antenna having a plurality of slots and designed to be incorporated into an aircraft without protruding surfaces. Thus, a structural member of the aircraft, such as a vertical stabilizer, is provided with slots on opposite sides of the stabilizer, covered with dielectric material, and fed from a single T-shaped cavity so that the radiated patterns of each of the slots are in phase in the fore and aft directions of the aircraft and radiate horizontally polarized energy.

U.S. Pat. No. 2,628,311 discloses a broadband, multiple-slot antenna system having a plurality of slots spaced apart by a distance that is small with respect to the wavelength and fed by resonant chambers to provide a substantially uniform current distribution over the outer surface of the antenna structure. The multi-slot antenna can be either a planar or cylindrical array of slots.

U.S. Pat. No. 2,981,949 discloses an antenna intended primarily for airborne application provided with a plurality of center-fed, radially expanding, waveguide portions to project energy radially outwardly from the center so that tee energy may leak through annular slots in the walls of each of the radially expanding waveguide sections to provide an omnidirectional or toroidal beam expanding in the horizontal direction. By progressively feeding adjacent sectoral waveguides, a sectoral beam may be created and swept or scanned about in the horizontal plane about the vertical axis of the antenna.

U.S. Pat. No. 4,647,940 discloses a parallel waveguide, microwave antenna that may be inexpensively manufactured and reliably used even though exposed to the elements. The antenna is comprised of a pair of plates of dielectric material, preferably glass, spaced apart and separated by air, inert gas or vacuum, preferably air, with one of the plates having a metallized surface to provide a ground plane and the other plate having a metallized surface defining a series of waveguide slots or apertures arranged and configured to provide a radiated beam having desired polarization beam, with beam characteristics and parameters as desired. The metallized portions of the two plates are arranged to face each other and define the enclosed air space, and the two plates hermetically are sealed at the edges and fed by a central coaxial cable so that energy introduced to the antenna structure from the central waveguide propagates outwardly in the enclosed air dielectric as expanding circles and escapes to free space by radiation at the plurality of slots or apertures.

U.S. Pat. No. 4,633,262 discloses a TV receive-only antenna of the type disclosed in U.S. Pat. No. 4,647,940 that may be inexpensively manufactured and reliably used outdoors. The TV receive-only antenna is comprised of a first glass plate having a metallized surface and a second glass plate having a metallized circuit pattern designed to receive a planar wave as, for example, from a geostationary equatorial satellite. The glass plates are arranged with their metallized surfaces facing each other and spaced from each other to define an air space between the circuit pattern and ground plane and sealed at the edge to protect the metallized surfaces from the environment.

U.S. Pat. No. 4,825,221 discloses a dielectric transmission line for transmitting electromagnetic waves radiated from one end portion thereof into surrounding space by providing an end portion of the dielectric line contoured to a configuration required for emitting electromagnetic waves in the form of predetermined wave front. In accordance with this patent, the dielectric line may have a plurality of end configurations, including a convex face, a concave face, a conical end, and a flat end; and the end portion of the dielectric line may be provided with varying dielectric constants to shape the wave emitted from the end of the dielectric.

Notwithstanding the prior development efforts represented by the patents above, a need still exists for an efficient, broadband antenna with unidirectional sensitivity, especially an antenna having a single-feed means, that may be inexpensively manufactured and adapted to receive communications from satellite transponders.

This invention provides an inexpensive, efficient, broadband, slot-type antenna with unidirectional sensitivity. In the antenna, a slot-forming means defines a plurality of substantially concentric and generally coplanar annular slots; and a non-resonant antenna connection means, or antenna feed means, transmits electromagnetic energy to and from the plurality of annular slots. The antenna feed means can have a "corporate feed" form. The antenna connection means forms a plurality of non-resonant radial-extending cavities that are adapted to combine electromagnetic energy received at the plurality of concentric, annular slots substantially in phase and to divide electromagnetic energy between the plurality of concentric, annular slots for transmission from the slots generally in phase and along the central slot axis that lies perpendicular to the plurality of concentric, annular slots. The cavity-forming means of the antenna connection means interconnects the plurality of annular slots with a connector for electromagnetic energy.

In preferred embodiments of the antenna of this invention, a plurality of polarizing antenna elements is carried by the slot-forming means adjacent at least one or two of the substantially concentric, annular slots to enhance uniformity of polarization and the unidirectional sensitivity of the antenna. Such a plurality of polarizers may be carried by the slot-forming means in a plurality of locations spaced above and over at least one or more of the concentric annular slots and distributed around their peripheries at locations to suppress cross polarization to and from the antenna. Such antenna elements may be a plurality of short elongated conductors having lengths less than about one-half wavelength of the center frequency of operation of the antenna and carried over the one or more slots at a distance less than about one-quarter of the wavelength of the center frequency of operation of the antenna. To provide consistent polarization of the electromagnetic energy at the slots, the polarizers may cross the slots at an acute angle. The antenna and antenna connection means may be adapted to send and receive electromagnetic radiation with circular polarization.

FIG. 1 is a perspective view of an antenna of this invention broken away to show a cross section at a plane through the geometric center of the antenna;

FIG. 1A is a cross-sectional view of another embodiment of the antenna of FIG. 1;

FIG. 2 is a upper plane view of another antenna of this invention;

FIG. 3 is a cross sectional view of the antenna of FIG. 2 at a plane through the geometric center or axis of rotation of the antenna;

FIG. 4 is an H-plane, linear pattern of the propagation characteristic of the antenna of FIGS. 2 and 3;

FIG. 5 is an E-plane linear pattern of the propagation characteristic of the antenna of FIGS. 2 and 3;

FIG. 6 is an illustration of another antenna of this invention having a plurality of polarizers to suppress cross polarization and enhance the unidirectional propagation of the antenna; and

FIG. 7 is a spinning linear pattern of a circular, polarized array of the antenna of FIGS. 2 and 3.

FIG. 1 illustrates a simple embodiment of an antenna 10 of this invention. As shown in FIG. 1, the antenna of this invention includes a slot-forming means 11, defining a plurality (e.g., two) of concentric, generally coplanar, annular slots 12, 13. The width of slots 12, 13 is not critical and is generally less than one-quarter of the wavelength of the frequency at the center of the operating band width of the antenna. The slot-forming means comprising portions 11a, 11b, and 11c is generally coplanar, although it is not necessary that portions 11a, 11b, and 11c lie in exactly the same plane. The radial distance between the concentric annular slots 12 and 13 in the embodiment of FIG. 1 equals the width of portion 11b of slot-forming means 11. Preferably, the radial distance between slots 12 and 13 is between one-half wavelength and one wavelength of the frequency at the center of the bandwidth of operating frequencies of antenna 10 to suppress grating lobes. The maximum distance "d" between slots for grating lobe suppression is given by the formula: ##EQU1## where n=the number of slots;

.theta.=the beam angle from broadside; and

.lambda.=wavelength at desired frequency.

For example, for a four-slot antenna with the beam steered to broadside (i.e., .theta.=0), ##EQU2## or 0.875 wavelengths. Larger spacings should not affect the impedence match of the antenna; however, grating lobes will occur in the radiation pattern near the horizon. Hereafter, where reference is made to wavelengths and frequencies, it is to be understood that such a reference is to the frequency at the center of the operating bandwidth of the antennas of this invention. It should be noted that antennas of the invention have effective bandwidths on the order of one octave or more.

Antenna 10 also includes an antenna connection means 20 for transmitting electromagnetic energy to and from the plurality of concentric, annular slots. As shown in FIG. 1, connection means 20 defines a plurality of non-resonant radially extending cavities 21 and 22 that are adapted to combine electromagnetic energy received from concentric, annular slots 12 and 13 and to divide electromagnetic energy supplied to antenna 10 by connection means 23 between concentric, annular slots 12 and 13. As shown and described, antenna connection means 20 is adapted to combine electromagnetic energy from slots 12 and 13 generally in phase for reception by connection means 23 and divides electromagnetic energy provided from connections means 23 so that it is propagated in phase, as indicated in FIG. 1. Such antenna feed means as are shown in FIGS. 1 (and in FIGS. 1A and 3) have a form that may be referred to as a "corporate feed".

Thus, antenna connection means 20 provides a non-resonant cavity-forming means interconnecting slots 12 and 13 with connection 23. As shown in FIG. 1, antenna connections means 20 forms a lower, circular cavity 21 extending radially from connection 23 to a peripheral annular opening 24 An upper cavity 22 is annular and expands radially outwardly from a peripheral, annular opening 24 to terminate at outer annular slot 12. Upper annular cavity 22 also contracts radially inward from the peripheral, annular opening 24 and terminates at innermost annular slot 13 as shown in FIG. 1. An annular power divider 25 may be carried by slot-forming means 11 (see portion 11b of slot-forming means 11) within upper annular cavity 22 adjacent peripheral, annular opening 24 between upper annular cavity 22 and lower circular cavity 21.

In the embodiment of FIG. 1, the height of the lower cavity is about one-half wavelength; and the height of the upper cavity is about one-quarter wavelength. It should be noted, however, that the height of an inner, annular cavity portion 22a and the height of an outer annular cavity portion 22b may be different as shown in FIG. 1A. For example, by making the height of the inner annular cavity portion 22a between peripheral, annular opening 24 and innermost annular slot 13 less than the height of outer cavity portion 22b between the peripheral annular opening 24 and outer annular slot 12, as is shown in FIG. 1A, the electromagnetic energy may be divided by the antenna connection means to provide a uniform power density both around the periphery of innermost slot 13 and around the longer periphery of outermost annular slot 12.

It should be understood that connection means 23 may be any connection means known in the art; for example, connection means 23 may be a waveguide that opens into lower cavity 21, preferably coaxially at the center of antenna 10 as shown in FIG. 1. Connection means 23 may be, as shown in FIG. 3, a plurality of phased stub feeders located centrally in antenna connection means 20. Connection means 23 may be and is preferably, adapted to transmit and receive an electromagnetic energy with circular polarization. The antenna connection means 20 of antenna 10 is also preferably operated in the TEM mode.

FIGS. 2 and 3 show another embodiment 30 of an antenna of this invention. Antenna 30 of FIGS. 2 and 3 provides slot-forming means 31 that defines four slots 32, 33, 34, and 35. In the embodiment of FIGS. 2 and 3, each of slots 32-35 can be separated from the adjacent slot by a radial distance calculated as set forth above. As shown in FIGS. 2 and 3, for example each of the sections 31a, 31b, and 31c has a radial width equal to about one-half wavelength; and the diameter of portion 31d of slot-forming means 31 is equal to about one-half wavelength.

An antenna connection means 40 of antenna 30 defines a plurality of cavities 41, 42, 43, and 44. Each of the cavities 41-44 extends radially within the antenna connection means and is adapted to combine electromagnetic energy received at the plurality of concentric annular slots substantially in phase within the antenna connection means and to divide outgoing electromagnetic energy between the plurality of annular slots in such a manner that it is propagated from the plurality of annular slots generally in phase along the central axis perpendicular to the plane of the plurality of annular slots.

As shown in FIG. 3, the plurality of radially extending cavities includes a lower circular cavity 41 extending radially from connection means 47 and terminating in a peripheral, annular opening 48 which communicates with annular cavity 42. As shown in FIG. 3, annular cavity 42 includes an inner, annular cavity portion 42a extending from peripheral, annular opening 48 and terminating at an inner, annular opening 49. Annular cavity 42 also includes an outer, annular cavity portion 42b extending from peripheral, annular opening 48 to an annular, outer opening 50. Inner, annular opening 49 communicates with inner, annular cavity 44; and outer, annular opening 50 communicates with outer, annular cavity 43 as shown in FIG. 3. Electromagnetic energy thus flows between connection means 47 and the plurality of annular slots 32, 33, 34, and 35 by travelling through the intervening cavity portions. In its travel between the plurality of concentric, annular slots 32, 33, 34, and 35 and connection means 47, electromagnetic energy to or from slots 32 and 33 travels through outer, annular cavity 43 and is divided or combined in phase at the outer, annular opening 50. Electromagnetic energy to or from concentric, annular slots 34 and 35 travels through inner, annular cavity 44 and is divided or combined in phase at inner, annular opening 49. The combined energies to or from annular slots 32 and 33 travel through outer, annular cavity portion 42b to peripheral, annular opening 48; and the combined energies to or from slots 34 and 35 travel through inner, annular cavity portion 42a to peripheral, annular opening 48. The electromagnetic energies to or from slots 32, 33, 34, and 35 are divided, or combined, in phase at peripheral, annular opening 48 and travel through cavity 41 to connection 47. Cavities 41-44 are non-resonant.

As shown in FIG. 3, the antenna connection means may be provided with a plurality of annular power splitters 51, 52, and 53 located, respectively, adjacent peripheral, annular opening 48; inner, annular opening 49; and outer, annular opening 50 to assist the division of electromagnetic energy at openings 48, 49, and 50 within cavities 42, 43, and 44, respectively.

In some embodiments, the height of the lower circular cavity 41 is about one-half wavelength. The height of annular cavity 42 is about one-quarter wavelength; and the height of outer, annular cavity 43 and inner, annular cavity 44 are about one-eighth wavelength. As set forth above, the heights of the inner and outer annular portions of each of annular cavities 42, 43, and 44 may be adjusted to distribute the power among slots 32, 33, 34, and 35 in such a manner that the power density around the periphery of all of the slots is substantially equal. The heights of the respective cavities may be adjusted to achieve other desired power amplitude distributions between and around the annular slots, for example, a distribution to provide low side lobes.

As shown in FIG. 3, connection means 47 comprises a plurality of coaxial connectors located centrally within chamber 41. The plurality of connectors 47a and 47b comprising connection 47 may be driven in a phase relationship to provide electromagnetic energy at the periphery of slots 32, 33, 34, and 35 which is generally in phase. In addition, connection means 47 may be driven to provide circular polarization to the electromagnetic energy radiated from the antenna and may receive circularly polarized electromagnetic energy.

The antenna of FIGS. 2 and 3 provides an efficient, substantially unidirectional antenna. FIG. 4 shows the H-plane, linear pattern that is typical of the antenna of FIGS. 2 and 3 driven in the TEM mode from connection 47; and FIG. 5 shows the corresponding typical E-plane linear pattern of the antenna As noted from FIGS. 4 and 5, the antenna has substantial unidirectional characteristics. The zero degree axes of FIGS. 4 and 5 corresponds to an axis through the center of the antenna (that is, the central axis of the concentric, annular slots 32, 33, 34, and 35) perpendicular to the plane in which they generally lie.

While the antennas shown in FIGS. 1-3 are capable of transmitting electromagnetic energy which is generally in phase at the periphery of each of the plurality of concentric annular slots and are capable of efficiently combining received energy generally in phase within the antenna connection means, it is preferable to provide the antennas with a plurality of antenna elements carried by the slot-forming means adjacent one or more of the plurality of concentric, annular slots to correct for small polarity differences around the periphery of the plurality of annular slots to suppress cross-polarized energy and to enhance the unidirectional sensitivity of the antenna As shown by FIG. 6, the plurality of antenna elements 60 is carried by the slot-forming means 61 in a plurality of locations at least above and over, for example, two concentric, annular slots 62 and 63. The plurality of antenna elements is distributed around the peripheries of the two concentric, annular slots to correct for deviations in polarity of the energy about the periphery of the slots and to suppress cross polarization. Such antenna elements may be short, elongated conductors having a length less than one-half of a wavelength. Such antenna elements may be carried above the slots a distance less than about one-quarter wavelength. As shown in FIG. 6, the antenna elements 60 may be located to lie across the concentric, annular slots 62, 63 at various acute angles to effect correction of the polarization of the electromagnetic energy at those portions of the concentric annular slots.

Antennas of this invention may be inexpensively manufactured by a number of means. For example, the slot-forming means may be formed from inexpensive, printed circuit board material, such as a dielectric substrate, copper clad on both surfaces, which has been photoetched to define a plurality of concentric annular slots on one surface and a plurality of antenna elements on the other surface located to correct polarization of energy from the plurality of concentric, annular slots and to suppress cross polarization and increase the unidirectional sensitivity of the antenna. Such a substrate may or may not be punched to define the slots. The antenna connection means may also be manufactured by microstrip techniques to provide a durable antenna that can be inexpensively manufactured and capable of efficient reception of electromagnetic energy from satellites and other household and commercial applications where expense is a factor.

In addition, the antenna and antenna connection means may be stamped from thin sheet metal, may be cast, or may be metallized molded plastic, or other such inexpensive manufacturing methods. Such manufacturing methods may be used to make a broad band, slot-type antenna with unidirectional sensitivity, comprising slot-forming means defining one or more annular slots and an annular corporate feed for transmitting electromagnetic energy to and from the one or more annular slots.

For example, the antenna of FIG. 1 can be made with a plurality of conductive plates, which may be inexpensive sheet metal such as tinplate. As shown in FIG. 1, such an embodiment of the antenna may include a circular, metallic, ground plane 26 having a base 26a and an extension, including portion 11a of slot-forming means 11, a terrace 26b, and sloping sidewall portions 26c and 26d. A first circular, metallic plate 27 may be disposed parallel to and spaced from 26a of the ground plane to provide peripheral, annular opening 24 as an annular feeding slot between the periphery of first circular plate 27 and the extension portion 11a. First circular plate 27 can have a raised section disposed centrally thereon to define portion 11c of slot-forming means 11. A second annular, metallic plate 11b can be disposed parallel to and spaced from both first circular plate 27 and terrace portion 26b of the circular ground plane. The inner peripheral edge of second annular plate 11b and raised portion 11c of first circular plate 27, as shown in FIG. 1, can provide inner annular slot 13 and the outer peripheral edge of second annular slot 11b, and extension 11a can provide an outer annular slot 12.

While presently preferred embodiments are shown and described above, it should be apparent to those skilled in the art that other embodiments may be devised without departing from the spirit and scope of the following claims.