Design of a ridged waveguide feed network for a wideband Rotman lens antenna array

Planar electromagnetic lenses like the Rotman lens are attractive candidates for beambeam forming networks forming networks because of their high bandwidth of operation and their capability to form multiple beams at lower cost as comparable electronic phase shifting networks. The principles of Rotman lenses have been studied for several decades and examples have been successfully built using different technologies, including parallel plate cavities fed by rectangular waveguides or microstrip lines on dielectric substrate. Due to the true time delay nature of Rotman lenses they theoretically exhibit extremely large bandwidth. However, lenses with rectangular waveguides as ports suffer from propagation of higher order modes inside the waveguide which limits the bandwidth to less than one octave in practical applications. Higher order waveguides, e.g. single or double ridged waveguides, allow for the propagation of a transversal electromagnetic (EM) mode and thus incorporate a larger bandwidth. But the symmetric nature of the TEM mode and the presence of the ridge complicate integration in the lens. The proposed paper outlines the design of a novel feed network based on higher order waveguides suited for multi- octave operation in Radar or Electronic Support Measures (ESM) applications. Each port includes coaxial feeding, a step transformer and a tapered section leading into the parallel plate section of the lens. An overview over the system will be given and numerical as well as experimental results on matching, insertion loss, mutual coupling and expected bandwidth obtained for a laboratory sample will be shown.