The Dielectric Cavity Antenna - An Alternative to the Choke Ring Antenna

Frank N. Bauregger, Todd Walter and Per Enge

Abstract:	Antennas for airborne GPS applications would ideally have a radiation pattern which is uniform for elevation angles greater than 5 °, and which has zero gain for angles below. Such an antenna would be immune to radio frequency interference (RFI) originating from below the aircraft, but would still provide coverage for low elevation GPS satellites. Though not ideal, choke ring antennas reject multipath and interference very well. Unfortunately, choke ring antennas cannot be used for airborne applications due to their large size. This paper describes an interesting type of interference resistant antenna which is a variation of the choke ring. It resembles a choke ring antenna, but instead of a corrugated surface, its ground plane is a single cavity filled with dielectric material. The cavity is sized such that the transverse magnetic field at the surface vanishes, resulting in an artificially soft surface for TM waves. This novel ground plane and its associated patch antenna source are collectively called the dielectric cavity antenna (DCA). The DCA is analyzed using a 2.5-dimensional finite difference time domain (FD-TD) technique. In this method, all field components are assumed to obey a cos ¢ or sin ¢ variation, thus obviating the need for a full three-dimensional analysis. The radiating source for the DCA is a nearly-square microstrip patch anterma, which is approximately modeled by a circumferential magnetic line current, obeying a cos ¢ variation. A prototype DCA has been built and tested. During the fabrication process, the dielectric material used for the prototype DCA became filled with a small amount of trapped air, unexpectedly lowering the effective permittivity of the dielectric. This effect was observed as a deviation from the expected radiation pattern, but was subsequently predicted by the FD-TD algorithm by artificially reducing the permittivity of the cavity. While the prototype DCA exhibits high zenith gain, it attenuates signals near the horizonseverely. When the proper dielectric is used, the performance of the DCA is nearly identical to that of a choke ring antenna. Smaller, airworthy versions of the DCA may be built if suitable, high-permittivity materials are used in the cavity.
Published in:	Proceedings of the 2001 National Technical Meeting of The Institute of Navigation January 22 - 24, 2001 Westin Long Beach Hotel Long Beach, CA
Pages:	344 - 352
Cite this article:	Bauregger, Frank N., Walter, Todd, Enge, Per, "The Dielectric Cavity Antenna - An Alternative to the Choke Ring Antenna," Proceedings of the 2001 National Technical Meeting of The Institute of Navigation, Long Beach, CA, January 2001, pp. 344-352.
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