Design of circular patch antennas on ferrite substrates

The design and performance of circular microstrip patch antennas on biased ferrites are described. Fundamental and higher-order modes are discussed and measurements presented for two different ferrites. The high permittivity of the substrate reduces the dimensions of the patches. The cavity model is extended to include multilayer ferrite-dielectric substrates with electromagnetic feedline coupling to the patch. This provides an accurate and fast CAD tool for predicting the resonant frequency over a wide bias range. Applying a DC magnetic bias field normal to the ferrite allows tuning of the resonant frequency by about 15%. Two hands of circular polarisation are generated when the magnetic bias is applied. The resulting radiation performance of the patches, in terms of the gain and quality of circular polarisation, is presented. It is shown that the gain is not dependent on the applied bias field and that good axial ratios are measured over a wide range of bias fields, both above and below saturation magnetisation