Dual-frequency circularly polarised annular-ring microstrip antenna

The design of the circular polarisation (CP) for an annular-ring antenna operated at its TM₁₁ and TM₁₂ modes is presented. The CP radiations of the proposed design are achieved by placing an L-shaped strip connected to two orthogonal sides of the inner boundary of the annular patch. The dual-frequency CP operations are excited by a single probe-feed, and no additional 90° phase shifter and impedance-transformer circuit are required. A prototype of the proposed design was implemented and measured. Measured results show that broadside radiation patterns with good CP characteristics are obtained at the two operating resonant modes. Agreement between measured and numerical results is satisfactory.