Low‐profile CP RFID hard tag antenna with short‐circuited microstrip‐fed line mounted on metallic plane

Abstract

In this study, a circularly polarized (CP) radio frequency identification (RFID) tag antenna with a capacitive short-circuited microstrip feed line was mounted on a metallic plane. This tag antenna comprised a low-profile substrate and a square-ring patch radiator with two pairs of T-shaped slots that produced CP radiation. A conjugate match to an Alien Higgs-4 chip with an impedance of 8-j149 Ω at 925 MHz was obtained by connecting two short-circuited microstrip lines in series. This article describes a new method for matching CP impendences. The chip impedance can be easily matched by adjusting the length of microstrip lines and by using coupling gaps. Tests of an RFID reader with a linearly polarized (LP) antenna indicated that the CP tag antenna had a low 3-dB axial ratio bandwidth (921-930 MHz) and an omnidirectional reading range. In contrast, the LP tag had double null cancelation in the reading region. The proposed CP tag antenna had a similar reading range but a different attached orientation. This antenna tag (and other RFID hard tags) can also be affixed to surfaces with screws, pop rivets, and industrial adhesives. The proposed antenna can be attached to goods transported by freight trucks.