Design of an annular-ring microstrip antenna for circular polarization

Several perturbation technologies have been proposed to achieve single feed circularly polarized (CP) operation for various shapes of microstrip antennas. These perturbation segments can excite two degenerate orthogonal modes with equal amplitude, and, to obtain phase quadrature, the operating frequency of the circularly polarized antennas is located between the resonant frequencies of the two modes. Notches or embedded slots are often used as the perturbation segments for rectangular, circular, and triangular microstrip antennas, and a 50 Ω input impedance can be easily found inside the patch. For an annular-ring microstrip antenna that is smaller than square or circular microstrip antennas for a given frequency, a pair of notches have been also applied to achieve the CP design (Chen, H.M. and Wong, K.L., IEEE Trans. Antennas Propagat., vol.AP-47, p.1289-92, 1999). However, an additional impedance transformer is necessary to overcome the high impedance problem of the annular-ring microstrip antenna operated at fundamental TM₁₁ mode (Kokotoff, D.M. et al., IEEE Trans. Antennas Propagat., vol.AP-47, p.384-8, 1999). We propose a new perturbation segment that involves impedance matching to achieve CP operation for the annular-ring microstrip antenna. Details of the proposed antenna designs are described and the obtained CP performances are presented and discussed.