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

Closed-form expressions for the impedance and axial-ratio bandwidths of a single-feed circularly polarized microstrip antenna have been derived. Receive-power bandwidths for both linearly and circularly polarized microstrip antennas have also been derived, which quantify the bandwidths when the microstrip antennas are used as receivers. The microstrip antennas are assumed to be probe fed and are modeled as RLC circuits (neglecting the probe inductances). For circular polarization, the probe feed is assumed to be along a 45° line across the microstrip antenna such that two orthogonal modes are identical in magnitude and in phase quadrature at the center frequency. Although derived for microstrip antennas, the expressions for impedance, axial-ratio, and receive-power bandwidth are also valid for other high-Q resonant antenna structures having an impedance behavior that is approximated by the same type of RLC circuits.