Different types of efficiencies are introduced for metasurface (MTS) antennas based on single point-source excitation: 1) the feed efficiency ε _feed is the power delivered by the feed to surface wave (SW) over the input power; 2) the tapering efficiency ε _tap quantifies the deviation with respect to a uniform illumination; 3) the conversion efficiency ε _conv defines the capability of the MTS to transform the SW power into leaky-wave power; and 4) the ohmic efficiency ε _Ω accounts 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 ε _feed can reach also 90% by using planar or quasiplanar circularly symmetric sources. It is also proven that the product ε _conv ε _tap cannot be larger than 58% for a uniform modulation; however, by using a nonuniform modulation, it can be increased to reach ε _conv ε _tap ≈ 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.

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