Two-arm microstrip spiral antenna with a circular aperture on the ground plane for generating a circularly polarized conical beam

A two-arm spiral antenna can have either a circularly polarized normal beam or conical beam according to the feed phase relationships. A circularly polarized conical beam can be obtained when the spiral arms are excited in-phase. The conical beam has a null in the zenith and omni-directional pattern with respect to the antenna plane. This paper presented a two-arm microstrip spiral antenna with a circular aperture on the ground plane that generates a circularly polarized conical beam. The proposed antenna is realized on a completely planar form and has a simple outer feed structure. An optimized antenna has a return loss less than 10 dB above 4.2 GHz. The angle of maximum radiation direction is about 40/spl deg/ for second mode radiation and about 58/spl deg/ for fourth mode radiation with a variation of less than 3/spl deg/.