Shorted Four-Element Patch Antenna with High Directivity and Wideband for Small Metal-Tag

This paper presents a miniaturized tag for UHF RFID applications which provided a simple mechanism to tune the impedance match at the desired frequency and extended the bandwidth of the proposed electrically small tag antenna on a metallic object. The compact tag antenna is configured by the four-element radiating patchs and a separated ground plane, which was connected through the four via pins and excited by a feed in the middle of the two 100 Ω microstrip lines. To match the complex input impedance of a UCODE 8/8m chip, the tag antennas’ impedance was controlled by the four radiating edges of the rectangular patches. Besides, dividing a ground plane into two small ground plates significantly enhanced the directivity pattern and radiation efficiency of the small tag. The optimized prototype dimensions of 30.01 × 30.01 × 3.12 mm
³
(0.092 λ
0
× 0.092 λ
0
× 0.0098 λ
0
) yielded a reading distance of 8.5 m, the high radiation efficiency of 38%, and a wide bandwidth of 40 MHz, which covers a spectrum of 26 MHz of North America band (902 – 928 MHz).