Design of a circularly polarised radio-frequency identification reader antenna with high return loss

A circularly polarised (CP), radio-frequency identification (RFID) reader antenna with high return loss (RL) is presented. The antenna consists of a dual-polarised microstrip patch with high isolation and a Wilkinson power divider with a 908 phase shift between its two outputs. A novel analytical approach is developed for improving the RL of the dual-feed CP antenna and verified experimentally by fabricating and measuring a prototype antenna. The experimental results show that the RL of the prototype antenna is more than 30 dB over the frequency range of 866-936 MHz, which fully covers the bandwidth of North America (902-928 MHz) as well as South Korea (917-923.5 MHz) for a passive ultra-high-frequency RFID system. The peak antenna gain is above 8.1 dBic and the axial ratio is below 0.7 dB over the frequency range of 902-928 MHz.