Compact Magnetic Loop Antenna for Omnidirectional On-Metal UHF Tag Design

For the first time, a planar and low-profile magnetic loop antenna is proposed for designing an omnidirectional metalmountable radio frequency identification (RFID) tag antenna. The proposed tag antenna, which consists of an annular slot exciter, a shorted patch, two shorting stubs, and four notches, can be easily made on one side of a flexible polyimide film. It has a dimension of 38 mm × 38 mm × 1.6 mm (0.116λ x 0.116λ x 0.0049λ). The tag antenna is able to generate stable omnidirectional read patterns when placed on metal. The notches are not only employed for bringing down the tag's resonant frequency to the ultrahigh (UHF) RFID band, but they can also be used for effectively reducing the gain fluctuation in the azimuth plane. Good maximum power transmission of ~90% is attainable. Measurements are conducted with reference to a transmitting power of 4W EIRP, and it has been shown that the proposed tag antenna is able to generate omnidirectional radiation patterns with a stable read distance of 5.29-5.65 m on metal in all directions in the azimuth plane. The tag's resonant frequency is also very stable with a maximum fluctuation of only 0.22%.