Ultrathin Single Layer Transmissive Dual-Band Linear to Circular Converter for Non-Adjacent Dual Orthogonal Circularly Polarized Antenna

This paper presents a new dual-band transmissive type linear to circular polarizer (LTCP) on a single-layer ultra-thin dielectric substrate. The unit cell of the converter consists of dual circular split rings and a connected cross-grid dipole at the center, for which the metal layer is engraved on a 0.51 mm thick Taconic TLY-5 substrate. The converter rotates the polarization of the incident linearly polarized (LP) wave into single circularly polarized (CP) transmitted waves at two non-adjacent bands, X and K. The LTCP exhibits oblique angle stability up to 35° with independent controllable frequencies at 11.2 / 22.75 GHz. A prototype of the LTCP is fabricated, and the LP to CP conversion is measured to be between 10-12.54 GHz (22.5 %) and 21.82-24.57 GHz (11.8 %) with a corresponding axial ratio (AR) below 3 dB. The experimental results of the converter confer well with the simulation results at normal and oblique incidences. The size of the designed converter is
$0.2\times 0.2\times 0.019~\lambda _{L}^{3}$
, where
$\lambda _{L}$
is the wavelength at the center frequency of the lower band. The dual-feed LP stacked radiating patch is integrated with the LTCP to realize dual-band dual orthogonal (RCP/LCP) CP microstrip antenna (CPMA). The shared ground is strategically conceived in designing the dual-feed LP antenna, in which the bottom metal layer serves as a ground plane in the lower band and as a reflector in the higher band to provide broadside radiation. The simulation and measured results validate the design principle of the recommended dual-band dual CPMA.