Fabry-Perot antenna

Metamaterials can be successfully employed as superstrates of low-profile printed antennas in order to provide high-gain devices without the need of the feeding network. This antenna is frequently called Fabry-Perot or Leaky Wave antenna.

A Fabry-Perot antenna can be realized by placing a highly reflective frequency selective surface at a proper distance from a ground plane. The cavity, excited by a low gain antenna, converts an omnidirectional field distribution into a highly directive one.
In the case of a metallic ground plane, the cavity height equals λ/2 [2]. However, when the metallic plate is replaced by a High-Impedance Surface (HIS), the reflection phase of such ground plane can be properly designed leading to a sub-wavelength configuration.
An interesting evolution of this structure is the transformation of the antenna in an active radiator. Indeed, the employment of an active HIS in place of a static one allows the structure operating in correspondence of different frequencies. According to the leaky wave principles, a beam steering can be also obtained in correspondence of every working frequency by dynamically varying the phase response of the ground plane (it determines a fictitious variation of the cavity height) [1].  This property, other than frequency reconfigurability, is applicable for synthesizing smart antennas to Software Defined Radio (SDR) and Cognitive Radio applications [2].