# The Leaky-Wave Perspective for Array-Fed Fabry-Perot Cavity and Bull's-Eye Antennas
## 4 Conclusions  
In this paper, several different 2-D planar antenna structures fed by phased arrays have been reviewed, emphasizing the advanced radiative features achieved at limited complexity cost. The common property of the presented devices consists in the excitation of cylindrical leaky waves on the planar surface constituting the antenna aperture. The simplicity in  
Figure 19: Electric field \(E_{\theta}\) along the cone of maximum amplitude \(\theta=\theta_{\text{LW}}\) radiated by a BEA fed by a circular phased array. (**a**) Magnitude in \(\text{dBV/m}\) and (**b**) phase in degrees. Reproduced from [60]. Figure 20: BEA fed by a circular array for the generation of twisted Bessel beams: \(E_{z}\) component magnitude and phase distributions over a transverse \(z=z_{\text{NDR}}/2\) plane for different azimuthal orders \(\ell=0\), 1, 2, 3 at a frequency of 18 GHz, with \(\beta/k_{0}=-0.4\). (**a–d**) Magnitude (dB). (**e–h**) Phase (rad). Reproduced from [60]. providing an elevation scan, resulting from the frequency dispersion of relevant supported leaky-mode wavenumbers, greatly simplifies several critical design details, which would otherwise require considerable additional efforts. The review focused on two categories, Fabry-Perot cavity antennas and _bull's-eye_ antennas, both capable of producing narrow pencil beams at broadside or conical scanned beams. Array-fed Fabry-Perot cavity antennas have proven capable of achieving reconfigurable far-field polarization, or reconfigurable orbital-angular-momentum order, or 2-D beam scanning in elevation and azimuth. _Bull's-eye_ antennas exhibit a richer modal spectrum, allowing for negative radial wavenumbers, corresponding to cylindrical backward leaky waves. This valuable characteristic has been shown to be useful for producing diffraction-resistant Bessel-like beams that, when excited by circular phased arrays, may exhibit a higher azimuthal order and thus carry non-zero orbital angular momentum. Finally, when compared to traditional phased-array configurations, both considered categories of leaky-wave antennas require a very small number of simple, inexpensive, and broadly available primary radiators, whose reduced complexity substantially simplifies the design and realization of feeding networks. Conceptualization, A.G. and P.B. ; Methodology, W.F., D.C., A.G. and P.B. ; Software, W.F. and D.C.; Formal analysis, P.B. ; Investigation, M.M. and E.N. ; Data curation, M.M., E.N., W.F. and D.C.; Writing--original draft, M.M. ; Writing--review & editing, M.M., E.N., A.G. and P.B. ; Visualization, W.F. and D.C.; Supervision, W.F., D.C., A.G. and P.B. All authors have read and agreed to the published version of the manuscript. This research was supported by the European Union under the Italian National Recovery and Resilience Plan (NRRP) of NextGenerationEU, partnership on \"Telecommunications of the Future\" (PE00000001-program \"RESTART\"). Data is contained within the article. The authors would like to thank all the co-authors for their valuable contributions and collaboration to the referenced research. The authors declare no conflict of interest. References  
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