Efficiency of Metasurface Antennas

Different types of efficiencies are introduced for metasurface (MTS) antennas based on single point-source excitation: 1) the feed efficiency εfeedis the power delivered by the feed to surface wave (SW) over the input power; 2) the tapering efficiency εtapquantifies the deviation with respect to a uniform illumination; 3) the conversion efficiency εconvdefines the capability of the MTS to transform the SW power into leaky-wave power; and 4) the ohmic efficiency εΩaccounts for the antenna losses. The product of all of these factors provides the overall efficiency of the antenna. New, quite general analytical formulas are suggested to estimate any individual efficiency. In particular, it is seen that εfeedcan reach also 90% by using planar or quasiplanar circularly symmetric sources. It is also proven that the product εconvεtapcannot be larger than 58% for a uniform modulation; however, by using a nonuniform modulation, it can be increased to reach εconvεtap≈ a/λ/(a/λ + 2), where a/λ is the antenna radius in free-space wavelengths. A new closedform formula for estimating the losses is given. In accordance with the prescription given here, one can reach 75% of global efficiency, including losses, until a radius of 20 wavelengths with commercially available substrates.