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Low-profile broadband RFID tag antennas mountable on metallic objects

Radio Frequency Identification (RFID) systems in the UHF (Ultra High Frequency) have become more and more popular in different service industries. In many applications, the tag antenna needs to be placed on a metallic object. In this scenario the performance of most popular dipole-like antennas will be seriously decreased because of the variation of the antenna impedance. Many planar inverted-F antenna (PIFA) configurations used for metallic objects can be found in scientific papers [1]-[3], but the bandwidths achieved are narrow. In [4], Li Xu proposed a broadband RFID Tag antenna with two shorted patches, but its bandwidth became narrow as placed on a metal plate. On the other hand the antenna in [4] has a thickness of 4.6 mm, which is not suitable for some applications. In [5], L. Mo developed a broadband UHF RFID tag antenna by embedding a pair of U slots on the radiating patch to excite a new resonant mode, but at the price of lowered antenna efficiency, which leads to a shorter reading range. It has been reported that by using additional patches directly coupled or gap coupled to the radiating edges of a rectangular patch, a broadband antenna can be achieved [6]. In this paper, we will demonstrate that adding gap-coupled parasitic patches to the driven patch can excite additional resonant modes for RFID Tag antennas thus the bandwidth can be greatly enhanced.