Abstract
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 <formula formulatype="inline" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex Notation="TeX">${\rm TE}_{11}$</tex></formula> mode inside the waveguide and the higher order <formula formulatype="inline" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex Notation="TeX">${\rm TE}_{21}$</tex></formula> 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 <formula formulatype="inline" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex Notation="TeX">${\rm TE}_{21}$</tex> </formula> 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.
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