Compact Circularly Polarized Antenna for UHF RFID Handheld Reader

RFID is to paste tags on goods, animals, or people for the purpose of identification and tracking by electromagnetic waves. In recent years, RFID technology has developed rapidly and appeared in many applications, such as logistics transportation, traffic toll collection, asset tracking, employee attendance record, and so on. With the advantages of rapid identification, long-range read-write, high memory capability and high reliability, the ultra-high frequency (UHF) passive RFID is now a preferred technology among all the RFID technologies.

The reader antenna is one of the key components in RFID systems and has been designed with circularly polarized (CP) operation, because CP waves can avoid severe polarization mismatch and ensure the reliability of communications between readers and tags. Another issue researchers need to consider is that each country has a different frequency for UHF RFID applications and the universal UHF RFID frequency ranges from 840 to 955 MHz (bandwidth of 12.8%). Therefore, a reader antenna covering the entire UHF RFID frequency would contribute to the development of cost-effective RFID system for worldwide application. In addition, RFID handheld readers should have compact size and directional radiation pattern.

To achieve CP radiation across the universal UHF RFID band, structure of stacked corner-truncated patches fed by suspended microstrip feed line through four probes was investigated in, and then the multi-feed structure was improved by using a horizontally meandered strip feed technique. To simplify the structure and design process of the universal UHF RFID reader antenna, the directly-fed single-layer patch with a coplanar parasitic patch and an L-shaped ground was presented in. For the purpose of miniaturization, crossed dipole, tortuous monopole, fractal monopole, and square slot structure are employed to CP RFID reader antennas. Slot-loaded circular and square patches fed by L-shaped probe and proximity-fed line are adopted for designing compact directional CP antenna. Nevertheless, the CP bandwidths of these antennas are still narrow.

Actually, the multi-feed antenna based on a feed network and sequentially rotated elements can achieve larger bandwidth and higher CP performance compared with single-feed technique. A small reader antenna consists of four monopole fed by a series feed network is presented in. Methods of meandered line are applied to both radiating elements and feed network for miniaturization at the expense of small CP bandwidth. Recently, the two-feed antenna using shorted inverted-L radiating elements and shorted inverted-T strip is studied in. But the asymmetric radiating structure deteriorates the axial-ratio (AR) of the antenna. To the best knowledge of the authors, there were no report and literature about the compact CP directional universal UHF RFID antenna for handheld reader applications at present.

Tags : RFID handheld readers universal UHF RFID antenna UHF RFID reader antenna

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