# The Leaky-Wave Perspective for Array-Fed Fabry-Perot Cavity and Bull's-Eye Antennas
## 2 Fabry-Perot Cavity Antennas
### Fabry-Perot Cavities for Beam Scanning in Elevation and Azimuth  
Frequency scanning in elevation is a property of LWAs of considerable interest, albeit a full-beam control requires losing azimuth symmetry, and in turn the conical radiation pattern typical of FPCAs. Recently, a novel solution was proposed embedding a dipole array into the cavity, which allows us to vary the azimuth angle at which the radiation maximum is observed. Limitations of free-space arrays are well-known, often requiring a very large number of elements. To avoid radiation through unwanted grating lobes, the spacing between adjacent elements should be smaller than half a free-space wavelength [41]. Meeting the above limitation may prove challenging, especially when the operating frequency is raised up to tens of GHz and beyond. The elevation selectivity of radiated leaky-wave beams allows us to significantly relax such a limit, given that an elevation-directive element pattern washes away lobes in the array factor. This latter aspect was proposed in [42] to find a profitable solution in array thinning to enlarge inter-element distance and reduce cross-talk phenomena. A grid of simple planar antennas were designed considering multilayer dielectric structures, which in turn recreated a resonant environment and provided leaky-wave launching effect. Even without considering elements with high-gain features, the narrow-beam radiation element pattern of a single source relaxes requirements for the element spacing, mitigating the parasite coupling effects. The latter possibility was further developed in [43], when a dual-polarized antenna, constituted by two interleaved \(N=2\times 2\) arrays, was employed, obtaining a high-gain radiation pattern in the far field. The seminal works in [42; 43] constituted the starting point for the substantial part of the literature interested in compact high-gain antennas with reduced components cost and low fabrication complexity [44; 45]. Further, novel features for FPCA-embedded sources have been studied, proposing different designs characterized by the common leaky nature of the relevant radiated fields. The electronic reconfiguration of the radiation pattern, in particular of the beam angle in elevation, represents a critically interesting technology for a large number of applications, from satellite communications to radar, to mention but a few. This was studied in [46], once again considering an FPCA-embedded source array, composed of simple nondirectional VEDs in the form of coaxial probes, as illustrated in Figure 8.  
Figure 7: RHCP (solid lines) component frequency-scanning process in the elevation plane. As expected, the cross-polarized component (dotted lines) is negligible in a wide range of frequencies. Reproduced from [37]. A PRS is designed by periodically patterning metallic square patches over a low-permittivity dielectric slab and making the quasi-TEM mode a slow wave, hence preventing it from radiating. The relevant inter-patch spacing is observed to weakly influence the phase constant of the excited TM\({}_{1}\) leaky-mode, while the attenuation constant shows a stronger dependence, as seen in Figure 9.  
An optimal value of inter-patch spacing is found, since placing patches too close together results in a lower attenuation constant, which provides more selective beams in elevation plane but requires wider planar antenna extension to radiate up to 90% of available power and avoid edge diffraction. Additionally, for a single coaxial cable generating the relevant element pattern, an angular frequency-driven scan should be evaluated in the frequency range that ensures a satisfactory excitation of the cavity. Indeed, the overall frequency range, with a single leaky mode dominating the aperture, should be assessed to hold the magnitude of the input reflection coefficient, \(\Gamma_{\text{in}}\), under the threshold of \(-10\) dB. Considering limitations due the magnitude of the input reflection coefficient, an overall elevation scan was obtained from 45\({}^{\circ}\) to 60\({}^{\circ}\)[46].