Circularly Polarized Patch Antenna with a Stacked Slot-Ring

A probe-fed circularly polarized microstrip patch antenna is presented. By stacking a sot-ring a proper distance above the patch, traveling current along the periphery of the slot-ring is formed. The axial-ratio bandwidth is optimized by varying the size of the slot-ring and the size of the ground plane. Through this simple method, 3-dB axial-ratio bandwidth of 9.8% centered at 915 MHz is achieved for RFID use.