Plane-Wave Excitation of Multi-Beam Modulated Metasurface Antennas
This paper explores the design of multibeam metasurface (MTS) antennas excited by multi-directional plane-wave launchers. First, we solve the fundamental yet open problem of a plane surface wave (SW) that propagates obliquely to the modulation direction of a sinusoidally modulated MTS. Closed-form expressions are provided to accurately predict the beam pointing angles for any propagation direction of the illuminating plane SW and for any Floquet harmonic. Then, the proposed formulation is used to design a highly directive multibeam MTS antenna at K-band with linear polarization. The designed antenna combines an MTS and a pillbox quasi-optical beamformer arranged in a very compact space. A pillbox is a double-layer structure that embeds a reflector coupled to multiple primary feeds in the lower layer. The beams launched by the primary feeds are thus transformed into plane waves with different directions in the upper layer. Printing a modulated MTS in the upper layer results in a low-profile, multibeam antenna suitable for PCB fabrication. Experimental results validate the proposed formulation. The fabricated MTS antenna exhibits a maximum directivity of 30.5 dB and a 17.5% fractional -3 dB directivity bandwidth over the 19.7-21.7 GHz band. By switching feeds and modifying the operating frequency, the scanning range is [$3^o$,$35^o$] in elevation and [$-76^o$,$+76^o$] in azimuth. Additionally, different antenna designs are included that demonstrate the validity and generality of the derived formulation. The proposed multibeam concept can be exploited in satellite communications and 5G/6G networks.