What is a metasurface antenna and how does it achieve beam steering without traditional phase shifters?

A metasurface antenna is a planar antenna consisting of a dense array of sub-wavelength resonant elements (meta-atoms) on a surface, where each element can be individually controlled to modulate the phase, amplitude, or polarization of the reflected or transmitted wave, enabling electronic beam steering without traditional phase shifters or a conventional beamforming network. The metasurface works by: sampling the incident wave (from a feed source or a guided wave) at each meta-atom location, applying a local phase shift determined by the meta-atom's tunable properties (varactor-loaded patches, PIN diode-loaded elements, or liquid crystal-tuned elements), and re-radiating the wave with the spatially varying phase profile needed to steer the beam (similar to how a phased array steers, but with sub-wavelength element spacing and simpler feed). Beam steering is achieved by: reconfigurable reflecting metasurface (a feed horn illuminates the metasurface, and each element reflects the wave with a programmable phase shift; similar to a flat-panel reflectarray but with real-time reconfigurability), reconfigurable transmitting metasurface (the wave passes through the metasurface, receiving a phase shift from each element), and holographic metasurface (a surface wave is launched from the edge of the metasurface, and tunable elements leak radiation at controlled phase to form a steered beam; used in Kymeta's flat-panel satellite antennas). Advantages over traditional phased arrays: no expensive T/R modules needed (much lower cost and power consumption for receive-only applications), very thin profile (< lambda/10), and massive element count (thousands of elements possible because each element is simple).