RADIATION CHARACTERISTICS OF CERTAIN DIELEQRIC LOADED MICROWAVE ANTENNAS

Abstract

The widespread application of paraboloidal reflector antennas in microwave communication, radar and radio astronomy has stimulated considerable interest in the development of physically small improved feed systems which will have high directivity,low sidelobes and a multiple beam facility. This work comprises of a detailed theoretical and experimental investigations into the radiation characteristics of some improved primary feeds, with greater directivity, increased on-axis gain and low sidelobes, for reflector antennas. The development of these new feed systems are based on the property of dielectric materials in improving the performance of conventional microwave antennas. The radiation behaviour of a wide angle conical horn with a homogeneous dielectric sphere infront of its aperture has been investigated on the basis of the Scattering super - position technique. Results show that the system will have patterns with greater directivity and low sidelobes as compared with conventional conical horns of the same dimensions. Analysis of the fields in the dielectric coated conical horn is presented using spherical hybrid modes. Dielectric coating is seen to increase the pattern directivity at the cost of sidelofoe level. Dielectric sphere loading in front of the radiating coated conical horn aperture is established to contribute greater directivity,higher on-axis gain and low sidelobes in addition to a variable beamwidth facility. A conical horn with a helical boundary is analysed using spherical hybrid modes and is shown to have desirable radiation characteristics to offer itself for applications in satellite communications. The technique of dielectric sphere loading is successfully applied to improve the helical horn pattern directivity with higher gain and low sidelobes. Dielectric coating on the Biconical horn walls has been treated to improve its pattern with a significant increase in its vertical pattern directivity. The effect of placing dielectric spheres in the vicinity of radiating corrugated E -plane sectoral horn and corrugated conical horn has been investigated on the basis of scattering theory approach. Sphere loading is seen to narrow the - 3 dB beamwidths. The sphere-loaded systems also possess greater on axis gain, low sidelobes and a multiple beam facility. Dielectric loaded E -plane sectoral horn is analysed and is shown to have increased directivity at the cost of slight deterioration in sidelobe levels. Dielectric spheres off-set in front of the radiating dielectric loaded E- plane sectoral horn aperture is found to be a variable beam feed system with greater on-axis gain, narrow beamwidth and low sidelobes. A dielectric loaded H -plane sectoral horn with a cylindrical aperture has been discussed to prove its ability to be an efficient reflector feed for greater directivity and low sidelobes. A multimode dielectric loaded rectangular horn operating in two orthogonal TE Q+TE/TM12 and TE ^+T'E/TK , mode sets to generate a circularly polarised elliptical beam is analysed. The antenna is seen to have fairly good directivity, on-axis gain and low sidelobes, with a nearly unity polarization axial ratio, over a wide frequency range. A technique of pattern shaping for monopulse radar antennas, by combining a controlled amount of higher order modes in dielectric loaded rectangular horn is also developed from a known throat excitation. Finally, some suggestions are made for further work in this field.