Enhancement of circular polarization quality of single-patch two-input microstrip antennas

A novel approach to improving a circular polarization (CP) quality of single-patch microstrip antennas excited with dual-point feeds is presented. It is applicable to microstrip antennas with square as well as circular patches which are excited though two feeding inputs which have a 90° angular separation. An analytical model is formulated using a concept of superposition of far fields. This model provides novel closed-form formulas to calculate proper amplitude and phase excitation, which should be applied to the antenna inputs at the operating frequency, resulting in the enhanced axial ratio. The model processes the simulated far field of the antenna with only one input active at a time. The computed complex excitation makes the axial ratio exactly equal to one in the broadside direction at the operating frequency. Two numerical examples demonstrate this approach for microstrip patch antennas excited with through-substrate probes of coaxial connectors. It is also demonstrated through the examples that a certain excitation, referred to as the “amplitude only” excitation, may be sufficient – from practical point of view – to ensure high-quality CP for a particular patch antenna.