  One of the long-standing problems in antenna engineering is the realization
of highly-directive beams using low-profile devices. In this paper we provide a
solution to this problem by means of Huygens' metasurfaces (HMSs), based on the
equivalence principle. This principle states that a given excitation can be
transformed to a desirable aperture field by inducing suitable electric and
magnetic surface currents. Building on this concept, we propose and demonstrate
cavity-excited HMS antennas, where the single-source cavity excitation is
designed to optimize aperture illumination, while the HMS facilitates the
current distribution that ensures phase purity of aperture fields. The HMS
breaks the coupling between the excitation and radiation spectrum typical to
standard partially-reflecting surfaces, allowing tailoring of the aperture
properties to produce a desirable radiation pattern. As shown, a single
semianalytical formalism can be followed to achieve control of a variety of
radiation features, such as the direction of the main beam or the side lobe
level, by proper modification of the HMS and the source position. Relying on a
cavity excitation, this can be achieved without incurring edge-taper losses and
without any degradation of the aperture illumination for arbitrarily-large
apertures. With the recent demonstrations of Huygens' metasurfaces at
microwave, terahertz, and optical frequencies, the proposed low-profile design
may find its use in a myriad of applications across the electromagnetic
spectrum, from highly-directive antennas to highly-efficient quantum-dot
emitters, reaching near-unity aperture efficiencies.
