Two shorted inverted-L radiating elements are located on the orthogonal corner of the square substrate and connected to each output port of a Wilkinson power divider. A shorted inverted-T strip is placed on the other corner of the square for a perturbing resonator to generate circularly-polarised (CP) radiation in the UHF band. Proper adjusting of the horizontal and vertical lengths of the inverted-T strip will differentiate two orthogonal linearly modes with 90°phase difference for a good CP radiation. Simulated and measured results indicate that the proposed structure can achieve circular-degree polarisation operation. A measured impedance bandwidth (VSWR ≤ 2) of about 16.4% (810-960 MHz) and a 3 dB axial-ratio (AR) bandwidth of about 4.4% (890-930 MHz) at 915 MHz were obtained.

A highly compact (25 × 25 × 0.8 mm 3) dual-band folded square-loop antenna is proposed for reading near-field RF identification (RFID) tags. The antenna has a strong and uniform magnetic field in the nearfield zone of the square loop. The side length of the square loop (∼λ/16) generates resonant frequencies in two conventional modes (0.5λ and 1.0λ). Experimental interrogation of a small RFID button tag attached to a blood sample collection tube showed that the antenna has a small interrogation zone (15 × 15 mm 2), which avoids unnecessary interrogation. Experiments also showed that with an input power of 25 dBm, the antenna can tag at a distance of up to 40 mm, which confirmed its suitability for reading button-type RFID tags attached to blood sample collection tubes and other specimen containers.