A Defocused Rotman Lens With Reduced Conjugate Port Coupling

The Roman lens is suitable as an antenna beam-forming network, but performance suffers from high coupling between symmetrically opposed (or conjugate) ports. A practical technique to reduce this coupling is presented and experimentally validated. It is proposed that the curvature of the lens contour is modified to be sub-optimal in terms of phase errors, but is done in such a way that the defocusing spreads the power and leads to reduced conjugate port coupling. The phase errors thus generated are shown to be of the same order as those found in conventional Rotman lenses. This new, defocused lens is compared to the conventional lens showing improved coupling performance without degradation of the phase performance. A lens is implemented in waveguide and measured results agree well with simulations.