Low-cost compact circularly polarised directional antenna for universal UHF RFID handheld reader applications

A low-cost compact circularly polarised (CP) directional antenna is proposed for universal ultra-high frequency (UHF) RF identification (RFID) handheld reader applications. The antenna consists of four sequentially rotated inverted-F antennas (IFAs) fed by a compact four-feed network with closed-form design formulas. A prototype is implemented by FR-4 substrates and a thick air layer for the reduction of cost. For return loss (RL)>13dB, 3-dB gain variation, and axial ratio (AR)<2 dB, the prototype achieves the measured bandwidth of 14.9% and exhibits stable symmetrical directional radiation patterns with wide half-power beamwidth. Compared with the reported UHF RFID directional reader antennas, the proposed structure not only exhibits the largest CP bandwidth for easily covering the entire UHF RFID band but also achieves compact size of 95×100×13.6 mm3.