Beam-width control using a cavity-backed elliptical Archimedean spiral antenna

This paper presents the radiation characteristics and beam-width control features of a 2-18 GHz, cavity-backed, two-arm, elliptical Archimedean spiral antenna. We first discuss a precise microwave instrumentation and measurement technique to accurately characterize the frequency dependent complex permittivity and permeability of a wide variety of microwave materials. Using the method, we have designed a lightweight, multi-layer absorbing structure to be inserted in the cavity. For optimum performance, we have used a genetic algorithm approach to derive the optimal thickness of each layer. We then varied the ratio of the major axis to the minor axis of the spiral antenna to create an elliptical structure and observed changes in the radiation pattern. Our results show that by using an elliptical configuration, it is possible to regulate the beam-width in the two orthogonal planes by compromising to some extent the purity of the circular polarization.