2.11 Design Guidelines for a Circularly Polarized Rectangular Microstrip Antenna

The design of a circularly polarized microstrip antenna which uses rectangular patch with an aspect ratio begins with designing a square microstrip antenna. Equations (2.8) and (2.6) may be iterated (Appendix B section B.3) to create a square patch. The resonant frequency tends to be slightly low when the iterated converged value is used. The cavity model may be used to refine the patch size and make it more accurate. The Q of a single mode, TM ₀₁ or TM ₁₀, of a square patch can be determined with measurement or computed using the cavity model to determine Q _T. The cavity model can then be used to compute a driving point location which has an input resistance at resonance of about 88 ?. Equation (2.52) is then used to compute the patch dimensions to produce circular polarization. Experimental optimization is generally required to complete the design of a circularly polarized rectangular patch antenna. The Smith Chart of Figure 2-18 illustrates the impedance trace one needs in order to achieve circular polarization. The vertex of a kink which forms a 90 ₀ angle is the point at which the optimum circular polarization occurs. The polarization sense of the antenna may be determined by consulting Figure 2-15 (I).

When a dielectric superstrate (radome) covers a microstrip antenna (Figure 2-23) which generates circular polarization, the axial ratio bandwidth will remain unchanged. ^[60]

A variety of GPS RHCP...