Slotted Circular Patch With Multiple Loading Stubs for Platform Insensitive Tag

A low profile and metal-mountable ultrahigh frequency (UHF) tag antenna, which has a geometrical dimension of 47 mm x 47 mm x 0.8128 mm (0.1431 lambda x 0.143 lambda x 0.0025 lambda), is proposed. It has four identical open stubs attached to the circumference of a circular patch for performing coarse- and fine-tunings on the tag's resonant frequency without affecting the read performances. At the center of the patch, a rectangular slot of 0.121 lambda x 0.009 lambda x 0.0025 lambda is etched for accommodating a microchip. Good impedance matching between the tag antenna and the microchip can be easily achieved by changing the dimensions of the center slot. It is also found that the introduction of the slot is crucial for bringing down the tag's resonant frequency to the UHF band. In this paper, an equivalent circuit is also derived for studying the impedance characteristics of the proposed tag antenna. Measurements have been carried out with the use of an effective isotropic radiated power of 4 W, and maximum read distances of 3.45 m and similar to 1.7 m are attainable, respectively, when the tag is placed on metal plate and dielectrics. It is found that the resonant frequency of the proposed tag antenna is stable and not affected much by its backing objects.