9. Metasurface Antennas

This chapter reports design and analysis methods for planar antennas based on modulated metasurfaces (MTSs). These antennas transform a surface wave (SW) into a leaky wave by means of the interaction with a MTS having a spatially modulated equivalent impedance. The basic concept is that the MTS imposes the impedance boundary conditions (BCs) seen by the SW, and therefore the MTS controls amplitude, phase, and polarization of the aperture field. Thus, MTS antennas are highly customizable in terms of their performances, by simply changing the MTS and without affecting the overall structure. Several technological solutions can be adopted to implement the MTS, from sub-wavelength patches printed on a grounded slab at microwave frequencies, to a bed of nails structure in the millimetre and sub-millimetre wave range: in any case, the resulting device has light weight and a low profile. The design of the MTS is based on a generalized form of the Floquet wave theorem adiabatically applied to curvilinear locally periodic BCs. The design defines the continuous BCs required for reproducing a desired aperture field, and it is verified by a fast full-wave solver for impedance BCs. Next, the continuous BCs are discretized and implemented by a distribution of electrically small printed metallic elements in a regular lattice, like pixels in an image. The final layout is composed of tens of thousands of pixels and it is analyzed by a full-wave solver which makes use of entire domain basis functions combined with a fast-multipole algorithm. Examples of design and realizations of MTS antennas are shown, proving the effectiveness of the concept.