Design of compact low-loss Rotman lenses

The design of compact low-loss Rotman lenses is discussed with particular emphasis on minimising the level of scattered energy in the parallel-plate waveguide. Of central importance is the design of the elements used to excite and to extract energy from the waveguide so as to maximise the absorption of incident energy; a problem very similar to that of minimising the active reflection coefficient of a phased array with its intimate involvement with mutual coupling. A method of designing the elements is described for the case where the elements are monopoles and the parallel-plate guide is terminated by shorting planes. Constraints on the lens geometry imposed by the minimisation of scattered energy are discussed and an approach to the design of the lens geometry which takes account of these constraints is described. Results of measurements on an experimental lens are presented and analysed.