Low-Profile PIFA Array Antennas for UHF Band RFID Tags Mountable on Metallic Objects

Two PIFA array antennas, designed for UHF band RFID tags mountable on metallic objects, are presented in this paper. The proposed array antennas are fabricated on a very thin substrate with a thickness of 0.8 mm. The first array that has shorting pins placed at the outer edges of PIFAs is fed by two quarter-wavelength microstrip lines. This array has a simple feeding structure, which is its main advantage. Meanwhile, the second array has shorting pins located at the inner edges of PIFAs, and incorporating additional matching stubs into the quarter-wavelength microstrip lines is needed to achieve complex impedance matching, which can offer a broad bandwidth. By properly selecting the spacing between the PIFAs, an enhanced antenna gain and a good reading-range performance can be obtained for both array antennas. In addition, the two array antennas proposed are studied mounted on differently sized metallic plates. It is found that there is a negligible influence of the metal-plate size on the reading range is found for the second array antenna, which is superior to the first one.