A Simplified Feed Model for Investigating the Cross Polarization Reduction in Circular- and Elliptical-Rim Offset Reflector Antennas

Abstract

An analytic model for the primary feed of offset reflector antennas is presented that facilitates a more general investigation of the cross polarization reduction in both asymmetric and inter-cardinal planes. It includes both first and second order azimuthal modes, primarily the and type modes, and allows selection of symmetric and asymmetrical patterns, with different field tapers in the principal planes. Using this model first the influence of the second order mode on the cross polarization of an offset reflector with a circular rim is investigated. Then, the problem for an offset reflector with an elliptic rim is also studied. It is shown that minimizing the cross polarization in the asymmetric plane does not necessarily reduce it globally, as in the inter-cardinal plane it remains high. The procedure for reduction of the cross polarization globally and its effects on the sidelobe levels of the reflector antenna are also investigated. With the proposed feed model, the aperture efficiency of the reflector antenna is improved compared with a standard Gaussian feed with the same edge taper. Based on this study, a dual-mode circular waveguide horn is also proposed and designed. Its performance on an offset reflector antenna is studied and shown that the cross polarization levels are reduced quite satisfactorily within a respectable frequency band.Index Terms-Cross polarization, offset reflector antenna, tapered dual-mode primary feed.

Improved Cross-Polarization Performance of a Multi-Phase-Center Parabolic Reflector Antenna

“…In order to electronically displace the phase-center locations of these aperture antennas, each is illuminated with an over-moded primary feed, which is capable of generating up to the first three modes of the circular waveguides, namely the , and modes. A simplified analytical version of these modes, as introduced by the authors in [10] and [12], is assumed throughout this letter. Thus, the primary radiation patterns of the feed can be expressed in a closed-form by the following equations [10]:…”

“…However, the cross-polarization performance deteriorates, which can effectively be reduced if an appropriate feed is used to conjugately match the focal plane field. The classical trimode matched feed, proposed by Rudge and Adatia [14], is an excellent example, whose mode content factors are strictly dependent on the offset reflector geometry as fully investigated by the authors in [12].…”

Some novel properties of asymmetric adaptive medium to large aperture antennas

“…The tri-mode matched feed, first proposed in 1975 by Rudge and Adatia [7], are the best example. Recently, authors proposed tapered dual-mode feeds [8] as matched feeds. Several examples [9][10] will be provided and their effectiveness in cross polarization reduction of offset reflector antennas will be presented over a respectable frequency range.…”

Exploring further potentials of over-moded primary feeds for parabolic reflector antennas

“…The exponential terms n 1 and n 2 define the tapering function in the E-and H-planes of the dominant mode. More detail of this model is found in [4]. In the next two sections, the feed will be placed at the focal point of the offset reflector under study.…”