Circularly Polarized Ring-Patch Antenna

A method is presented to enhance the flat-gain, the return-loss, and the axial-ratio bandwidths of a probe-fed circularly polarized (CP) microstrip patch antenna. The antenna occupies a volume of 124 × 117 × 33.2 mm3(the ground plane is excluded). Bandwidths are widened by making use of an opposite shift on the inner and outer contours of a stacked ring, at an equal distance relative to the patch's boundary. The antenna is implemented in the ultrahigh frequency band for the radio frequency identification (RFID) system. The measured 10-dB return-loss bandwidth is 16.4% (835-975 MHz), and the 3-dB axial-ratio bandwidth is 7.7% (870-940 MHz). The gain is about 7 dBic within the overlapped bandwidth.