A Low-Profile Broadband Circularly Polarized Metasurface Antenna Aperture Coupled by Shorted Annular-Ring Slot

A low-profile (LP) metasurface antenna is proposed for broadband circularly polarized (CP) operations. The metasurface radiator, formed by a 5 x 5 array of square patches, has a microstrip line fed at its bottom through a shorted annular-ring slot centered on the ground plane. First, the characteristic mode analysis (CMA) methodology is employed to examine the intrinsic modes of the metasurface. Subsequently, a pair of mutually orthogonal modes is selected and excited by the feeding structure with a 90 degrees phase difference to produce CP radiation. For the purpose of verification, a prototype antenna is developed, and the obtained data reveal that the antenna exhibits an impedance bandwidth of 43.9% ranging from 4.8 to 7.5 GHz (S11<-10 dB). The 3-dB axial ratio bandwidth is 22.2% in the range of 5.6-7 GHz, among which a satisfactory left-handed CP radiation is achieved, and the maximum gain in the broadside direction reaches 9.3 dBic at 6.9 GHz.