Investigation into the cross-polarization of loaded microstrip ring antennas

Among many microstrip antenna configurations, the closed-ring microstrip antenna can be considered as an intermediate structure between patch and printed loop antennas. As the ring width becomes narrower, the resonant frequency shifts downward, its TM₁₁ mode resistance increases to a few thousands of ohms and becomes impossible to match to a 50-Ω impedance (Bafrooei, P.M. and Shafai, L., 1999). One way to feed a ring antenna is to use a loading mechanism. Stub loading, investigated by Garg, R. and Reddy, V.S., (see IEEE Trans. Antennas Propag., vol.51, no.8, p.1941-6, 2003), obtained good impedance matching using a 50-Ω line. Other possible loading techniques however, such as notch, gap or shorting pins, have not yet been studied to our knowledge. Garg and Reddy observed that the stub loading mechanism increases the cross-polarization, as it disturbs the current distribution and generates another mode. However, our investigations by simulation and measurement have shown that some loading techniques could achieve low cross polarization, which is the subject of our investigation. For one stub loading, the cross polarization level was -10.8 dB, and decreased to -18.0 dB for two stubs. The simulated cross-polarizations for a gap and a shorting pin were -46.5 and -34.4 dB, respectively, but measured at -30.7 and -28.5 dB, because of limited measurement ability.