Polarisation properties of small printed spiral antennas with four resistively loaded arms

A small printed four-arm spiral antenna with lumped resistive loading is investigated to assess its suitability as a compact antenna element with pure circular polarisation. The results of numerical investigations based on a simplified thin-wire model are presented, which indicate crosspolarisation suppression (XPD) improvements of ⩾14 dB in comparison to conventional spirals of the same size. The corresponding gain reductions are ~1-3 dB, which is at least equivalent to the performance of conventional cavity backed spirals. Furthermore, experimental results are discussed for a true microstrip spiral, which verify XPD improvements of 12-17 dB and show that XPD figures of 30 dB as well as a radiation pattern with acceptable symmetry can be obtained, even with simple nonoptimised feed networks. Areas for further improvements are identified and discussed