Broadband two-arm dual-mode dual-polarized spiral antenna

A typical mode-forming network for a four-arm spiral antenna is shown. The phasing requirements for both modes of a two-arm spiral can be obtained very simply with only a single 180deg hybrid. Assuming that dual-mode operation of a two-arm spiral is feasible, the overlapping bandwidth will be narrower than for spirals with three or more arms. This is due to the nature of the higher order modes. For an N-arm spiral excited in mode M, the desired radiation occurs where the circumference of the antenna is approximately Mlambda. If the antenna is electrically large enough, radiation can occur from higher order modes where the circumference is (M+nN)lambda, where n is an integer. In this paper, the author demonstrate that two broadband modes can be obtained with a two-arm slot spiral antenna