Large Metal Objects As Near Field UHF RFID Antennas

This paper demonstrates that a long thin metal structure can act as a near-field antenna for a passive UHF RFID system. It is demonstrated that a conventional metal mountable tag can be detected up to 30 m along a metal bar from the feed point far exceeding the maximum far field free space range of the tag when mounted on metal. Further system improvements are achieved by altering the physical length of the metal structure or by changing the carrier frequency used. It is demonstrated that the fields generated along the structure (e.g., E-field or H-field) dictate the antenna type which will be best suited for coupling to it. A conventional passive tag utilizing a dipole antenna and oriented normal to the metal surface efficiently harvests the E-field. It is deduced that the tag can be continuously detected up to at least 50 m along a bar from the antenna feed.