Performance of concentric annular patches as a dual frequency band microstrip array element

The concentric ring microstrip antenna is a useful dual frequency band geometry offering good bandwidth with a wide range of band spacing ratios. A disadvantage is that excitation in the TM₁₂ mode leads to larger physical dimensions which may limit beam scanning in some applications. The theoretical model presented for the input impedance gave good agreement with measured values. This powerful model allows unwanted higher order modes to be identified which often cause radiation pattern distortion and high crosspolarisation. A novel method of mode suppression using circumferential slots is discussed which significantly reduces crosspolarisation. Further work needs to be carried out to optimise the slots. There is some experimental evidence that surface waves may be propagating and this must be investigated further