# The Leaky-Wave Perspective for Array-Fed Fabry-Perot Cavity and Bull's-Eye Antennas
## 2 Fabry-Perot Cavity Antennas
### Fabry-Perot Cavities for Far-Field Polarization Reconfiguration  
Fabry-Perot cavity antennas find application in polarization reconfigurability and beam scanning. The demand for circular polarization is rapidly growing in the civil, medical, and military sectors, due to a number of advantages, such as the absence of orientation matching, resilience to multipath fading, and possibility to overcome obstacles on the propagation path [36]. Linear to circular conversion is a solution that may avoid complex geometries, to produce conditions of phase quadrature and magnitude equality on both horizontal and vertical field components. Once again, the 2-D LWAs and low-complexity standards required by relevant feeders may effectively simplify the design.  
In [37], by independently exciting the fundamental TE-TM leaky mode pair, a simple polarization-reconfigurable elevation scan is obtained. The relevant feeding network consists of a coaxial cable inserted through the ground plane to obtain a VED-like source, whereas, by etching in the ground plane radial slits in a circular configuration, a source equivalent to a vertical magnetic dipole (VMD) is obtained (refer to [37] for an exhaustive geometrical description and parameters). Cavity optimization [38] gives a large frequency span (approximately from 12 to 19 GHz in the case considered in [37]), in which only a couple of TE and TM leaky modes with zeroth azimuthal order are independently excited by relevant sources.  
In a homogenized regime where the PRS has a simple scalar description [39; 40], a dispersion analysis may readily be carried out via the transverse resonance technique [1] to obtain and verify the equalization between TE and TM wavenumbers \(k_{\rho}^{\text{TE}}\) and \(k_{\rho}^{\text{TM}}\). While attenuation-constant equalization is obtained through PRS design, the phase constants of the two modes are equalized by acting on the effective dielectric constant of the cavity: a wire medium (WM) layer can be inserted in a second low-permittivity dielectric, thus slowing down the phase velocity of the quasi-TEM mode. A sketch of the structure's geometry can be found in [37].  
A single frequency optimization provided at 16 GHz [37] produces a noticeable broadband beam stability, in a range between 12 and 19 GHz (see Figure 6). Complex amplitudes for both VEDs and VMDs are chosen to obtain equal peak amplitudes and similar phase constants. Further, by simply imposing adequate complex amplitudes on feeders, it is shown that a swift polarization conversion is possible: a frequency-driven right-hand circularly polarized (RHCP) beam scan from an elevation angle of 30\({}^{\circ}\) to 70\({}^{\circ}\) was performed, with a low level of cross-polarized circular component, i.e., left-hand circular polarization (LHCP) component (see Figure 7).  
Figure 6: Frequency sweep and related beam scanning for produced TE and TM modes, over a frequency span between 12 and 19 GHz. Only a slight deviation from the equalization condition was observed around 12 and 19 GHz. Reproduced from [37].  
The overall system, while not being able to scan too close to broadside due to constraints on far-field patterns generated by azimuthally invariant sources, demonstrates a versatile behavior in performing angular scan, both with linear and circular polarization.