Quadrifilar Helix antenna for UHF RFID

Radio Frequency Identification (RFID) technology is widely used for automatically identifying and tracking objects. Specifically Ultra High Frequency (UHF) (900 MHz) RFID technology is rapidly expanding because it offers the best tradeoffs between read range, cost, and size. UHF RFID has been focused on supply chain management, serving the needs of manufacturing, distribution and shipping. The low cost of the RFID tags has made the systems very attractive. While UHF RFID tags cost tens of cents, RFID reader systems cost hundreds to thousands of dollars. Much effort has been concentrated on increasing tag read distance so that a reader can cover a larger area. Increasing the coverage area reduces the number of readers, and antennas necessary. Commercial UHF RFID reader antennas are based upon the microstrip patch antenna. The radiation pattern of a patch antenna has peak gain at boresight (directly in front of the antenna) as seen in Fig. 1. A radiation pattern with peak gain at boresight is not always ideal. There are many applications where an RFID reader antenna is centered above the coverage area and the path loss at boresight is lowest. This paper describes the design of a Quadrifilar Helix Antenna (QFHA) with a uniquely shaped radiation pattern to efficiently address this situation and increase the coverage area.