Low Profile Quad-beam Circularly Polarized Antenna Using Transmissive Metasurface

This paper presents a high efficiency and low profile quad-beam circularly polarised transmitarray antenna using metasurface. In order to improve the aperture efficiency, the small unit cell is employed to obtain the beam forming since the gain of multi-beam is higher and the feed antenna suitable for low profile transmitarray is selected. To further improve the aperture efficiency, the cells with high transmittance are located at the centre of metasurface to increase the transmission efficiency since the electric fields of the low profile transmitarray are more concentrated on the centre of metasurface than those of the high profile transmitarray. The unit cell of metasurface is stacked by optimising with equivalent circuit analysis and generalised sheet transition conditions that can confirm a multi-layer performance easily. The principle of superposition was used to easily calculate the phase set required for quad-beam forming. The designed metasurface is 17 × 17 array of four-layer unit cells of 0.25λ 0 × 0.25λ 0 size at 5.8 GHz. The finally designed antenna with circularly polarised quad-beam has the peak gain of 12.1 dBic at 5.72 GHz, and the aperture efficiency and 1 dB gain bandwidth were found to be 30.7% and 3.5%∼5.5% at four beams, respectively.