Scanning antenna with fixed dipole in a rotating cup-shaped reflector

Abstract: A scanning cup-dipole antenna (10) comprises a fixed dipole (11) and a dipole feed (17) coupled to the fixed dipole (11). A rotatable antenna cup (22) is disposed around the fixed dipole (11). An antenna rotating apparatus (26) is coupled to the antenna cup (22) and is adapted to rotate the antenna cup (22) relative to the fixed dipole (11).

Description: The present invention relates to a cup-dipole antenna comprising at least one dipole, a dipole feed coupled to the dipole, and an antenna cup disposed around the dipole. Such a cup-dipole antenna is known from US-A-3 740 754. Conventional cup-dipole antennas have been used extensively to provide high aperture efficiency for small antenna apertures that span approximately one wavelength. The cup is formed from a cylindrical conductor shorted at its base with a conducting plate. A dipole is recessed within the cup and has a coaxial transmission line penetrating the base of the cup. A conventional method for achieving a scanned beam is to rotate the dipole and cup assembly as a single unit, necessitating the use of an RF joint such as a flexible coaxial cable or a rotary joint. However, conventional RF joints, particularly rotary joints, are very expensive to design and manufacture. RF joints present a reliability concern for long-life spacecraft, and are susceptible to passive intermodulation (PIM) generation and multipaction for space applications. RF joints are generally massive and clumsy to package, and produce undesirable Ohmic loss and reflections. Thus, conventional antennas do not employ rotation of the cup while the dipole/feed assembly remains fixed. As a consequence, an RF joint has been required with its inherent disadvantages mentioned above. A better understanding of Conventional cup-dipole antennas may be had from a reading of a book entitled "Microwave Cavity Antennas", by A. Kunar and H. D. Hristov, published by Artech House, Boston (1989). Specific reference is made to Chapter 5 which discusses various conventional cup-dipole antennas. A directive antenna system with a paraboloidal reflector is known from US-A-2 759 182. The paraboloidal main reflector has an aperture at its vertex and a coaxial line connected to a radar transceiver extends through this aperture. A dipole is positioned in front of the main reflector and connected to both conductors of the coaxial line. Further, a cylindrical reflector is positioned in front of the dipole and attached to the outer conductor of the coaxial line. A passive linear antenna element is included between the main reflector and the dipole and supported by the outer coaxial line conductor. The main parabolic reflector is supported in an excentric cup-shaped portion of a gear plate which meshes with a gear portion that is driven by a motor. The coaxial line is stationary. Further a conically scanning radar antenna is known from FR-A-2 581 257. The antenna comprises a principal parabolic reflector and a primary rear-feed source fed by a circular waveguide and displaced with respect to the focus of the parabolic reflector. Conical scanning is produced by the rotary movement of the parabolic reflector about the rear-feed source, said source and the circular waveguide remaining fixed. From "Introduction to Radar Systems" by Merrill Skolnik, 2nd edition, page 155, it is known that one of the simplest conical-scan antennas is a parabola with an offset rear feed rotated about the axis of the reflector. If the antenna is small, it may be easier to rotate the dish, which is offset, rather than the feed, thus avoiding the problem of a rotary or flexible RF joint in the feed. It is, in view of the above prior art, an object of the present invention to provide for a scanning cup-dipole antenna that is inexpensive to design and manufacture. The above object is achieved by a cup-dipole antenna, wherein said antenna is a scanning antenna, said dipole is fixed and said antenna cup is rotatable, and wherein an antenna rotating apparatus is coupled to the antenna cup and is adapted to rotate the antenna cup relative to the fixed dipole. The present invention generally provides for improved scanning cup-dipole antennas having a fixed dipole, or dipoles, and a rotating cup. The cup is formed from a cylindrical conductor shorted at its base to a conducting plate. A dipole is recessed within the cup and has a coaxial transmission line that penetrates through the base of the cup and is coupled to the dipole. The present invention achieves beam scanning in a novel way by mechanically rotating only the cup, and wherein the dipole and feed assembly remain fixed. A plurality of dipoles may be disposed within the cup in a symmetrical array, and wherein the dipoles are scaled for any desired frequency. The present antennas support transmission of linear or circular polarized energy. By using a hybrid coupler and symmetrical dipole arms, circular polarized energy may be radiated. Also, circularly polarized energy may be radiate without the use of the hybrid coupler, by employing asymmetrical dipole arms. More specifically, the present invention is a scanning cup-dipole antenna comprising a fixed dipole, a dipole feed coupled to the fixed dipole, a rotatable antenna cup disposed around the fixed dipole, and a gimbal coupled to the antenna cup that is adapted to rotate the antenna cup relative to the fixed dipole. The antenna may further comprise a second fixed dipole oriented orthogonal to the fixed dipole. In one embodiment, the dipole feed may be comprised of a hybrid coupler network coupled by way of a plurality of coaxial transmission line feeds and a four-post balun to the fixed dipoles. A short-circuit ring is disposed around the periphery of the four-post balun, and is disposed in a axially-located opening in a cup base plate. The antenna cup is comprised of the conducting cup base plate and a cylindrical cup rim coupled thereto. The first and second crossed dipoles lie in a plane that is generally orthogonal to a central axis of the antenna. In an alternative embodiment, the dipole feed may be comprised of a turnstile, crossed-dipole feed. In another embodiment, the dipole feed may be coupled by way of a coaxial transmission line feed to single fixed linearly polarized dipole. Because the rotating cup is detached from the dipole and feed assembly, a radio frequency (RF) joint (e.g., rotary joint or flexible transmission line) is not required. For high-power applications, the present invention is therefore less expensive to design and manufacture than conventional antennas, it is more reliable, it is not susceptible to passive intermodulation (PIM) generation and multipaction in space applications, and it does not produce undesirable Ohmic loss or reflections. The present invention may be adapted for use as a high-power transmit antenna for a satellite, for example. The present invention provides beam scanning from a device that is aperture efficient, light weight, reliable, and inexpensive to manufacture. The various features and advantages of the present invention may be more readily understood with reference to the following detailed description taken in conjunction with the accompanying drawings, wherein like reference numerals designate like structural elements, and in which: Fig. 1 is a cross sectional view illustrating several embodiments of a scanning cup-dipole antenna having a fixed dipole and a rotating cup in accordance with the principles of the present invention; Fig. 2 shows an end view of the antenna of Fig. 1 and Fig. 3 shows an embodiment of the present antenna comprising an array of dipoles. Referring to the drawing figures, Fig. 1 is a cross sectional view illustrating several embodiments of a scanning cup-dipole antenna 10 in accordance with the principles of the present invention. The scanning cup-dipole antenna 10 has a fixed dipole 11 (or dipoles 11) and a rotating antenna cup 22. In one embodiment, the scanning cup-dipole antenna 10 is comprised of a (3 dB) hybrid coupler network 12 that includes electrically isolated right-hand and left-hand circular polarization ports 13, 14 and first and second hybrid output ports 15, 16. The first and second hybrid output ports 15, 16 of the hybrid coupler network 12 are coupled to a dipole feed 17. The dipole feed 17 is comprised of a plurality of coaxial transmission line feeds 18 and a four-post balun 19. The plurality of coaxial transmission line feeds 18 are coupled between the first and second hybrid output ports 15, 16 and the four-post balun 19. A short-circuit ring 21 is disposed around the periphery of a portion of the four-post balun 19. The four-post balun 19 is coupled to first and second crossed dipoles 11 that lie in a plane that is orthogonal to a central axis of the antenna 10. However, it is to be understood that a single dipole 11 may be employed in the antenna 10 that is used for generating a single polarization. The antenna cup 22 is comprised of a conducting cup base plate 23 and a cylindrical cup rim 24. The short-circuit ring 21 is disposed in a axially-located opening 25 in the cup base plate 23. The cup 22 (shown in solid outline) is concentric to a feed axis of the dipoles 11. An antenna rotating mechanism 26 is coupled to the antenna cup 24 and is adapted to rotate the antenna cup 24 along a selected axis or set of axes, that is generally orthogonal to the axis of the antenna 10. A non-scanning cup axis 27 of the antenna 10 is designated by the solid arrow. A first dashed arrow shows a scanning axis 28 of the cup 24 when the antenna 10 is scanned. Also, a second dashed arrow shows a direction of the peak gain 29 of the antenna 10. The antenna cup 24 is also shown disposed in a second orientation illustrated by the dashed cup 24 shown in Fig. 1. Fig. 2 shows an end view of the antenna 10 of Fig. 1 and shows the short-circuit ring 21, the four-post balun 19, the first and second crossed dipoles 11, the opening 25 in the cup base plate 23, and the cup rim 24 with more clarity. A first plane of rotation 31 is shown in Fig. 2 that is generally along a line parallel to a first crossed dipole 11. The antenna 10 may also be rotated along a second direction that is generally orthogonal to the first plane of rotation 31 and that is along a line parallel to the second crossed dipole 11. The use of the crossed dipoles 11 and the hybrid coupler 12, for example, permit dual circular polarizations to be radiated by the antenna 10 by feeding the two electrically isolated right-hand and left-hand circular polarization ports 13, 14. If so desired, and in the alternative, a single dipole 11 fed by a single coaxial transmission line feed 18 may be disposed in the rotating cup 22 to achieve a scanned, linearly polarized beam. The cup 22 shown in solid outline in Fig. 1 is concentric with the axis of the dipole feed 17, which produces a far-field antenna pattern having peak gain 29 in the direction of the feed axis of the dipoles 11. The cup 22 shown in phantom (dashed outline) is rotated, leaving the dipole feed 17 and hybrid coupler network 12 fixed in space. Mechanical rotation of the cup 22 results in scanning of the antenna beam pattern. The hybrid coupler network 12 is not required in all configurations of the scanning cup-dipole antenna 10, which is illustrated by the dashed box surrounding it. Thus the transmission line feeds 18 are directly coupled from the input ports to the four-post balun 19. Elimination of the hybrid coupler network 12 produces a second embodiment of the scanning cup-dipole antenna 10. Furthermore, and as is illustrated with reference to the elongated dipole 11 having the dashed outline, the single dipole 11 may be disposed in the rotating cup 22 that is may be fed by a single coaxial transmission line feed 18 to achieve a scanned, linearly polarized beam. This produces a third embodiment of the scanning cup-dipole antenna 10. It is to be understood that the dipoles 11 employed in any of the disclosed embodiments may be scaled for any desired frequency. The present invention may be implemented to generate circular polarization without using the hybrid coupler network 12 by using a dipole feed 17 comprising a turnstile, crossed-dipole feed 17. The turnstile, crossed-dipole feed 17 replaces the hybrid coupler network 12 and the crossed dipole feed 17 of Fig. 1. For the purposes of completeness, Fig. 3 shows an embodiment of the present antenna comprising an array of dipoles. A plurality of dipoles 11 are disposed within the cup 22 in a symmetrical array. A breadboard antenna 10 was built and tested to demonstrate the scanning capabilities of the present invention. The breadboard antenna 10 used the embodiment of Fig. 1 comprising two crossed dipoles 11 and the hybrid coupler network 12 to generate circular polarization. It was found that the antenna pattern scanned in the direction of the axis of the rotated cup 22 with minimal degradation in pattern gain 29 and axial ratio.

Claims: A cup-dipole antenna (10) comprising: at least one dipole (11); a dipole feed (17) coupled to the dipole (11); and an antenna cup (22) disposed around the dipole (11), characterized in that said antenna is a scanning antenna (10), said dipole (11) is fixed and said antenna cup (22) is rotatable; and an antenna rotating apparatus (26) is coupled to the antenna cup (22) and is adapted to rotate the antenna cup (22) relative to the fixed dipole (11). The antenna (10) of claim 1, characterized by a second fixed dipole (11) oriented substantially orthogonal to the fixed dipole (11). The antenna (10) of claim 1 or 2, characterized in that the dipole feed (17) comprises a hybrid coupler network (12) coupled by way of a plurality of coaxial transmission line feeds (18) and a four-post balun (19) to the fixed dipole. The antenna (10) of claim 3, characterized in that the hybrid coupler network (12) comprises electrically isolated right-hand and left-hand circular polarization input ports (13, 14) and first and second hybrid output ports (15, 16) coupled to the coaxial transmission line feeds (18). The antenna (10) of claim 1 or 2, characterized in that the dipole feed (17) comprises a turnstile, crossed-dipole feed. The antenna (10) of claim 1 or 2, characterized by an array of dipoles (11) disposed in the antenna cup (22). The antenna (10) of claim 6, characterized in that the array of dipoles (11) are symmetrically disposed in the antenna cup (22). The antenna (10) of claim 6, characterized in that the array of dipoles (11) are asymmetrically disposed in the antenna cup (22). The antenna (10) of claim 1 or claim 3, characterized by: a fixed plurality of crossed dipoles (11); said dipole feed (17) having first and second input ports (13, 14), and having first and second output ports (15, 16) coupled to the fixed plurality of crossed dipoles (11); said rotatable antenna cup (22) being disposed around the fixed plurality of crossed dipoles (11); said antenna rotating apparatus (26) being adapted to rotate the antenna cup (22) along a selected axis relative to the fixed plurality of crossed dipoles (11). The antenna (10) of claim 9, characterized in that the dipole feed (17) is comprised of a hybrid coupler network (12), a four-post balun (19), and a plurality of coaxial transmission line feeds (18) coupled between the hybrid coupler network (12) and the four-post balun (19).

Inventors: BOLDISSAR FRANK, CAULFIELD MICHAEL F, FORMAN BARRY J, SCHALIT MARK A, VIRKLER ROY J, BOLDISSAR, FRANK, CAULFIELD, MICHAEL F., FORMAN, BARRY J., SCHALIT, MARK A., VIRKLER, ROY J.
Applicants: HUGHES ELECTRONICS CORP, HUGHES ELECTRONICS CORPORATION
Application year: 1995
Main classification: H01Q3
IPC codes: H01Q9, H01Q21, H01Q1, H01P1, H01Q19, H01Q3