A Ring Choke Excited Compact Dual-Mode Circular Waveguide Feed for Offset Reflector Antennas

A simple ring choke excited compact dual-mode circular waveguide feed, for offset reflector antennas, is proposed. It consists of a central round waveguide with a single longitudinal slot opening to a surrounding single choke of circular cross-section. It generates the TE₁₁ mode inside the waveguide and the higher order TE₂₁ mode inside the ring choke, with a quadrature phase difference. The proposed design, therefore, separates the two modes spatially, and isolates the input waveguide from the detrimental effects of the higher order TE₂₁ mode. It thus provides a wide impedance bandwidth, without the need for a matching network. The measured results of the return loss and radiation patterns on a fabricated prototype show excellent agreement with the numerical data. The antenna structure is very compact and when used as a primary feed in offset reflectors it reduces the cross polarization.