Input impedance of concentric ring microstrip antennas for dual frequency band operation including surface wave coupling

The impedance properties of a dual concentric annular ring microstrip patch antenna are described. This patch operates at two frequency bands which may be closely spaced. The TM₁₂ mode is primarily investigated but other modes are also treated. An equivalent circuit model for the input impedance is presented together with experimental verification. Surface waves are found to be important even for very thin substrates and these are included in the model. Factors which influence the ring design are discussed.