Impedance, axial-ratio, and receive-power bandwidths of microstrip antennas

Bandwidth is a key concern in the design of microstrip antennas since the high-Q nature of microstrip antennas makes them inherently lower bandwidth structures. An analytical expression for the impedance bandwidth of a linearly polarized (LP) microstrip antenna is well known. Approximate numerical results for the axial-ratio (AR) and impedance bandwidth of a single-feed circularly polarized microstrip antenna have also been obtained previously and reported in the literature. However, closed-form expressions for these bandwidths have evidently not been available previously. This work presents simple, analytical expressions for the axial ratio and impedance bandwidth of a single-feed circularly polarized (CP) microstrip antenna of arbitrary shape, in terms of Q. In addition, the receive-power bandwidth for microstrip antennas that are either linearly or circularly polarized is derived, and closed-form expressions are obtained.