Compact Circularly Polarized Microstrip Antenna With Cross-Polarization Suppression at Low-Elevation Angle

Abstract

This letter presents a method of cross-polarization suppression at low-elevation angle for a compact circularly polarized (CP) microstrip antenna. The compact CP antenna adopts a circular radiation patch with a crossed slot to reduce the dimensions, and a four-feeding-point technique is utilized to excite the patch for CP operation. The height and the diameter of the compact CP antenna are $0.064\, \lambda $ and $0.264\, \lambda $ ( $\lambda $ is the free-space wavelength at 1.2 GHz). At the outside of the CP antenna, a compact meander-line ring cavity (MLRC) structure is introduced, which can reduce the cross polarization from –14.2 to –26.1 dB in horizontal direction and increase 3-dB-axial-ratio (AR) ( ${\text{axial}\; \text{ratio}} \leq {\text{3 dB}}$ ) beamwidth from 128° to 192°. The height and diameter of the cavity are just $0.064\, \lambda $ and $0.448\, \lambda.$ Through the experimental validation, the impedance bandwidth of the proposed antenna for VSWR ≤ 2 is 20.8%, and the CP bandwidth for axial ratio ≤ 3 dB is 8.5%. Because of the MLRC, the cross polarization at low-elevation angle is clearly suppressed and the level at $\theta = \pm 90^\circ $ is not more than –23.2 dB. The 3-dB AR beamwidth reaches 190°. In the operating band, the peak gain of 5.3 dBic is also obtained.