The effects of an upper dielectric layer on the radiation characteristics of a spiral antenna

An upper dielectric layer of thickness B/sub 2/ is applied to a spiral antenna printed on a dielectric substrate of thickness B/sub 1/ backed by a ground plane of infinite extent. The upper layer and the substrate are of finite extent and have the same dielectric constant. The antenna characteristics are calculated as a function of thickness B/sub 2/. As B/sub 2/ increases, the input impedance and axial ratio show oscillatory behavior with a periodicity of approximately /spl lambda//sub g//2. It is also found that the gain does not monotonically increase. An increase of 6.5 dB over the gain without the upper dielectric layer is obtained.