Patch-Loaded Semicircular Dipolar Antenna for Metal-Mountable UHF RFID Tag Design

A compact tag antenna, which has a geometrical dimension of 40 mm x 40 mm x 3.048 mm (0.1224λx 0.1224λ x 0.0093λ), is proposed for metal-mountable applications in the UHF band. The antenna structure comprises a pair of semicircular microstrip dipolar patches which are orthogonally loaded with another pair of similar patches on the top. Rotating the orientation of the top-loading patch is found to be able to effectively tune the tag antenna's resonant frequency and its impedance. The capacitive and inductive effects introduced by the top-loading patches are crucial for bringing the resonant frequency of the tag antenna down to the useful UHF band. Conjugate impedance matching is achieved between the antenna and the microchip. An equivalent circuit has been constructed to study the impedance characteristics. Measurements show that the proposed tag antenna is able to achieve a read distance of 12.25 m at the Effective Isotropic Radiated Power of 4 W. The operating frequency of the patch-loaded dipolar patches is found to be insensitive to changes in the size of the backing metal plate, which is very useful for designing a stable radio frequency identification (RFID) tag.