Differentially fed CP antenna with quasi-self-complementary semicircular-ring structure

This article presents a new design of a common-mode suppressed differentially fed circularly polarized (CP) antenna using a quasi-self-complementary (QSC) structure. The antenna radiating element consists of a semicircular-ring radiating patch and its complementary radiating slot counterpart. The design idea for producing CP radiation is based on field orthogonality of the QSC structure. For better impedance matching, a fan-shaped notch (stub) around the feeding position was cut from (implemented in) the semicircular radiating patch (slot). In addition, to improve the axial ratio, a QSC semielliptical structure is embedded in the central area of the antenna. For the designed differentially fed CP antenna, measurement shows that the differential-mode (DM) operating band with return loss >10 dB covers 4.76-6.38 GHz. The measured DM 3-dB axial-ratio band is 4.95-5.43 GHz. Also, very good common-mode (CM) suppression is observed with the measured CM reflection coefficient >−1.21 dB in 4.50-6.5 GHz.