The thin-slot and thin-arm planar spiral antenna operated with and without a ground-plane

Most planar spiral antennas are made as complementary structures, i.e. with equal slot and arm widths. This paper is a study of the properties of the thin-slot and thin-arm spiral where the slot is respectively narrower or greater than the spiral arm, and includes results for free space and in the presence of a ground plane. A numerical study is conducted for a range of arm-to-slot ratios, from 5% to 95% where 50% is the complementary case. The results show that changing the arm-to-slot ratio can be used to vary the input impedance, but that the thin-arm case is of limited practical value. Results are compared to theoretical expectations and independent numerical results. As an introduction, a discussion on the band theory and a physical insight into the power flow near the antenna is presented