Archimedean spiral-mode microstrip antenna with improved axial ratio

This paper presents a technique to reduce the axial ratio of two-arm Archimedean spiral-mode microstrip (SMM) antennas without incurring large gain loss, like those exhibited in cavity-loaded spiral (CLS) designs. This was achieved through the use of a circular cavity with conical bottom under the spiral. When free space fills the cavity (/spl epsiv//sub r/=1), the proposed antenna produces a better axial ratio than the SMM antenna without the gain loss associated with the CLS. Furthermore, an angle /spl alpha/=60/spl deg/ produces a relatively constant gain with frequency and better axial ratio than the CLS. When /spl alpha/=30/spl deg/, the cavity /spl epsiv//sub r/ can be increased to five to reduce the antenna dimensions without significant performance degradation.