Abstract
This paper presents a triple-feed RFID tag antenna whose radiation pattern is near omni-directional in three dimensions, and which has less than 3.0 dB gain deviation in all directions. The presented tag with a near omni-directional radiation pattern can be employed to overcome the null-reading problem of dipole-type RFID tag applications. A unique feature of the triple-feed design is that the tag can function effectively with three different RFID chips, whose impedances vary widely. An important advantage of this feature is that the tag does not have to be redesigned to work with different RFID chips. The antenna is comprised of an open-loop radiating element, which is excited by using inductively coupled mechanism with a small multi-feed loop that is designed to work with three different chips, namely Impinj, Alien and TI. In addition, the inductively-coupled feed makes it convenient to implement a conjugate impedance match even when the quality factor Q of the antenna is high.