Estimation of Axial Ratio of Single-Port Dual-Mode Circularly Polarized Antenna Using Reflection Coefficient

In this article, a simple way that can estimate the axial ratio (AR) of a circularly polarized (CP) antenna using its measured reflection coefficient is proposed. The estimation method applies to a single-port CP antenna that is composed of two resonant modes with orthogonal polarization and 90° phase difference. It is most common among CP patch antennas, cross-dipole antennas, and substrate-integrated waveguide antennas. Based on the equivalent circuit model, the analysis method builds a bridge between the reflection coefficient and excitation property. It is shown that the trace property of the reflection coefficient on the Smith Chart infers whether the antenna performs the best AR value and AR bandwidth. By locating impedance points and extracting circuit parameters, accurate circular polarization properties can be estimated. We also provide a numerical way, which is based on an optimization algorithm, to estimate the AR while considering the difference between the two equivalent antenna resonators. A verification program is compiled, and different CP antennas are simulated, fabricated, and analyzed. This study also illustrates the source of estimation errors and proves that the method reaches acceptable AR estimation without time-consuming microwave chamber measurement. Therefore, the method provides a new look to high-speed pre-test and tuning for volume production of CP antennas.