Characteristic Modes Analysis for Circularly Polarized 1-Bit Dual-Layer Transmitarray Design

A novel approach for designing a circularly polarized (CP) transmitarray (TA) by resorting to the characteristic modes analysis (CMA) is presented. Two miniaturized dual-layer polarization-insensitive unit cells are tailored to provide the two copolar transmission coefficient phase states that act as a 1-bit spatial phase shifter in the TA. The exploitation of a pair of characteristic modes (CMs), radiating either right-handed CP (RHCP) or left-handed CP (LHCP) scattered field, is at the basis of the TA operation. The TA provides a forward CP scattered field ensuring a low copolar insertion loss (IL) as well as a copolar transmission phase difference of 0 degrees or 180 degrees. To validate the proposed CMA-inspired theoretical study, a CP 1-bit TA has been designed, simulated, fabricated and tested. Measurements assess a good agreement with simulations, showing a maximum TA gain of 21.8 dBic with a remarkable 3 dB gain bandwidth of 15.22 % (7.77 GHz - 9.05 GHz), a peak aperture efficiency of 28.5 % and a very low profile.