Design of a Circuit-Free Filtering Metasurface Antenna Using Characteristic Mode Analysis

This communication presents a method of designing filtering metasurface (MTS) antennas, which is to excite the candidate in-band modes and suppress the out-of-band interference ones. The candidate modes are selected depending on the expected radiation patterns and operating band and excited by choosing a reasonable feeding structure. The suppression of interference modes is achieved by controlling the modal currents with the aid of a characteristic mode analysis (CMA). As a result, the MTS works as a radiator and a filter simultaneously. A $3\times3$ array MTS antenna is implemented to demonstrate the concept. Targeting the broadside radiation pattern and 5G operating band, two candidate MTS modes are selected. A ground slot is used to excite them and introduce an extra slot mode. The out-of-band interference modes are eliminated by removing specific patches and etching a split slot on the central patch of the MTS. Moreover, to improve the selectivity at the lower band edge, a probe is added at the center of the antenna. The antenna has a simulated −10 dB impedance bandwidth of 25%, covering the 5G n77 band (3.3–4.25 GHz). The out-of-band suppression level exceeds 27 dB. Thin-band peak gain is 10.9 dBi, while the peak efficiency reaches 95%.