Design of Dual-Band UHF RFID Tag Antenna Using Even and Odd Modes of Asymmetric Split Ring for Anti-Metal Applications

For the first time, an asymmetric split-ring resonator (SRR) is employed for designing a dual-band anti-metal tag antenna that can cover both the lower European (LEu) and North American (NA) UHF radio frequency identification (RFID) frequency ranges. It is intended to be applied on small metallic objects. The two frequency ranges are generated by concurrently exciting the even and odd modes of the asymmetric SRR, which has a shorting post at the middle of the split ring. Each operating frequency can be tuned independently without affecting the other much by adjusting the asymmetricity of the SRR as well as the lengths of the loading open stubs. Design equations have been derived for calculating the two operating frequencies. The proposed tag has a miniature size (40 x 30 x 1.653 mm or 0.1224 lambda x 0.0916 lambda x 0.00504 lambda at lambda(o) = 916 MHz), and it can be effectively read in the range of similar to 5 m (4 W effective isotropic radiated power-EIRP) on metal and similar to 2.5 m in air and on small metallic objects in the boresight direction (theta = 0 degrees) for both the frequency ranges, showing a stable frequency performance.