Single-Fed Low-Profile Broadband Circularly Polarized Multi-Mode Metasurface Antenna Array Using Characteristic Mode Analysis

A single-fed low-profile metasurface (MTS) antenna array is proposed for broadband circularly-polarized (CP) radiation. Different from conventional aperture-fed broadband MTS antennas using a single-mode slot feeder, the proposed antenna and array use a dual-mode slot feeder so that two groups (instead of two) of orthogonally-polarized modes of a multi-mode MTS are excited for a significantly improved CP bandwidth. Evaluated in the $5-\mathrm{GHz}$ bands, a four-element array is designed with an overall size of $1.96 \lambda_{0} \times 1.96 \lambda_{0} \times 0.068 \lambda_{0}$, ($\lambda_{0}$ is the freespace wavelength at 5.5 GHz), achieving a measured $3-\mathrm{dB}$ axialratio bandwidth of $\mathbf{3 0 \%}$ ($\mathbf{4. 6} \mathbf{- 6. 3} \mathbf{~ G H z}$) with $\left|\mathrm{S}_{11}\right|\lt 10 \mathrm{~dB}$ and gain from 10.0 to $\mathbf{1 3. 2}$ dBic.