Miniature 3-D-Dipole Antenna for UHF RFID Tag Mounted on Conductive Materials

A novel compact 3-D-dipole antenna for a radio frequency identification (RFID) tag is presented in this article. The proposed antenna enables the tag to be read when mounted on surfaces composed of electrically conductive materials. A close match between the input resistance and reactance of the antenna to the desired conjugate impedance of an Alien Higgs-4 chip (<inline-formula> <tex-math notation="LaTeX">$8.2- j 148.5\,\,\Omega $ </tex-math></inline-formula> at 925 MHz) can be achieved by controlling the C-arm length and the width of the 3-D dipole. Experiments demonstrated that when an RFID tag antenna is attached to a conductive surface, a 0.2 mm gap between the antenna and the conductive plane enhances the antenna’s gain by increasing reflection from nearby surfaces (e.g., a metal object, a container of water, and a human wrist) within range of the antenna. Maximum read ranges are measured for the tag antenna fabricated with dimensions of 32 <inline-formula> <tex-math notation="LaTeX">$\times 25\times3.2$ </tex-math></inline-formula> mm3 (<inline-formula> <tex-math notation="LaTeX">$0.10\,\,\lambda _{0} \times 0.08\,\,\lambda _{0} \,\,\times0.01\,\,\lambda _{0}$ </tex-math></inline-formula>) and mounted on various materials and objects. In an outdoor environment, an effective isotropic radiated power (EIRP) transmitted by the RFID reader of 4.0 W achieves the maximum reading distances of 5.1, 3.3, and 3.9 m when the proposed tag antenna is placed on a metallic plate, a container of water, and a human wrist, respectively. The experiment and simulation results have a good agreement with each other.