Spiral Cavity Backing Effects on Pattern Symmetry and Modal Contamination

The majority of reported results on lossless, cavity-backed planar spiral antennas do not provide a good indication of higher-order modal contamination or azimuthal pattern symmetry. These parameters are crucial for determining the purity of the radiated far field and, thus, the actual bandwidth. Effects of the spiral backing on these parameters are studied in this letter. It is demonstrated that the best backing for the printed spiral remains the conventional absorber loaded cavity. The addition of either perfect magnetic or perfect electric (PEC/PMC) backing contributes to the overall pattern asymmetry of the antenna, even if some other far-field parameters such as gain are improved. However, we show that for a simple and inexpensive horizontally fed slot spiral, the far-field modal purity and gain can be improved with various counter- or cowound slot spiral backings.