Investigation on the circularly polarized ferrite antenna in different designs

A coplanar waveguide (CPW) feeding circularly polarized (CP) ferrite antenna with rectangular slot and a microstrip feeding CP annular-ring antenna with a pair of identical meandered strips (U-shaped strip) are proposed in this paper They use the same ferrite substrate with ε_r = 7.5 (tanδ_ε = 0.01) and μ_r = 7.2 (tanδ_μ = 0.2). Ferrite can efficiently reduce the dimension of the antenna with respect to the dielectric substrate. Not only that, ferrite can also make the bandwidth of the antenna become wider and energy easier radiate. The rectangular slot antenna and annular-ring antenna can achieve CP easily by embedding arc-shaped strip and a pair of diagonal U-shaped strips, respectively. Electromagnetic simulation software Ansoft HFSS based on the finite element method has been used to make simulation and optimization for the proposed two designs. We contain the simulated results of two different designs in multiple parameters which include return loss, 3 dB axial ratio, radiation patterns and the gain. We found that the rectangular slot antenna has a wider 3 dB axis ratio bandwidth (from 865 MHz to 982 MHz, 12.7% @ 920 MHz) and larger peak gain (2.42 dBi) than annular-ring ferrite antenna (from 901MHz to 940MHz, 4% @ 920MHz; -23.5dBi). The annular-ring ferrite antenna has a wider return loss bandwidth (from 815 MHz to 1.29 GHz) than rectangular slot ferrite antenna (from 825 MHz to 1.0GHz, 18.9% @ 925MHz). The dimensions of the rectangular slot antenna and annular-ring antenna are 106*94.5 mm² and 60*36 mm² respectively, so annular-ring antenna has a smaller size. They both can be used for UHF RFID applications.