Investigation of Circularly Polarized Microstrip Antenna With Tri-Band Polarization Conversion Metamaterials

A method for circularly polarized (CP) microstrip antenna based on reflective polarization conversion metamaterials (PCM) is proposed in this letter. On the one hand, the proposed metamaterials can convert the linearly polarized (LP) waves to the cross direction in tri-band 4.58–4.73, 5.68–6.12, and 6.3–6.77 GHz. On the other hand, the LP patches with center frequencies of 4.75, 5.8, and 6.6 GHz are placed in the Fabry–Perot (FP) cavity formed by PCM, which can realize the linear-to-circular polarization conversion of the three conventional microstrip antennas. The theoretical analysis reveals that the function of polarization conversion results from dipole oscillation on the connected asymmetrically split ring of the metamaterials. Simulated and measured results show that the axial ratio bandwidth of the three PCM-loaded microstrip antennas is greater than 200 MHz. And due to the FP cavity, the gain of the PCM-loaded antennas is also improved. This design for the CP antenna satisfies the reciprocity principle and is applicable to more scenarios.