Loop-Coupled Small Antenna With Enhanced Bandwidth for On-Metal UHF RFID Tag Design

Abstract

A small antenna with a dimension of <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$36\times 39\times1.5$ </tex-math></inline-formula> mm ( <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$0.109\lambda \times 0.118\lambda \times 0.0045\lambda $ </tex-math></inline-formula> ) is proposed for wideband metal-mountable applications. An excitation loop is designed within the antenna structure for achieving good energy coupling from the loop to the outer patch. With the introduction of a narrow stub, connecting the top and bottom conductors, the 3 dB impedance bandwidth of the proposed tag can be enhanced from 16 to 50 MHz, without modifying other design parameters. The bandwidth and the impedance matching between the antenna and the microchip can be tuned by varying the stub width. The gap separation between the top patches and the L-slot on the patch can also be adjusted for bandwidth optimization. The tag is readable from at least 6 m within 896–917 MHz, regardless of the metal plate dimension (12–24 cm). The tag is also readable from at a minimum of 4 m within a wide frequency range of 887–927 MHz, covering the entire US UHF radio frequency identification (RFID) band when attached to different sizes of metallic items.