# Enhanced Efficiency and Reduced Side Lobe Level Convex Conformal Reflectarray
## 1 Introduction  
Printed reflectarray (RA) antennas are nowadays a well-assessed technology for a wide range of applications. In comparison with other high-gain antenna configurations, they present several advantages, such as reduced weight, the possibility of being easily stored when folded and easy deployment. All these features make RAs suitable for several functions [1, 2, 3, 4, 5, 6, 7, 8], including mobile communication systems. In such applications, in addition to good performance, it is often also required that the adopted antennas present a low profile. Traditional reflectarrays, unfortunately, do not satisfy this requirement since they adopt a spatial feed system. A solution to this problem was firstly introduced in [9] and then improved by several authors (see [10, 11, 12, 13, 14]): it consists in using a folded reflectarray, in which the presence of a polarization layer allows to approximately halve the thickness of the entire RA with respect to a traditional configuration. An alternative solution to reduce the antenna volume can be that of fitting the reflector to the surface, where it has to be mounted, and fixing the feed not to the reflector itself, but to another part of the structure where the antenna is located. In the case in which the supporting structure is the fuselage of an airplane or the curved sidewall of a ship, the RA can be no longer flat. The available literature on conformal reflectarrays is still quite limited. In [15], the differences among the performance of a planar, a concave and a convex reflectarray are discussed, while in [16, 17, 18], the effect of the curvature on the radiation features of several convex configurations is numerically investigated. As expected, what emerges from theseanalyses is that concave RAs are characterized by better performance, but they are less interesting from the application point of view since they are very close to conventional reflectors, while convex reflectarrays are more attractive, but their performance decreases as the surface curvature increases. In [19; 20], some preliminary results on the experimental characterization of cylindrically conformal reflectarrays are shown: in [19], a center-fed RA is considered, realized with variable-depth grooves in a completely metallic structure, while the antenna presented in [20] is an off-set configuration, using printed re-radiating square patches. The results presented in both these papers confirm those obtained through the analysis in [16; 17; 18]: a convex RA is more critical than a planar one in terms of maximum gain, and therefore, efficiency, bandwidth, and side lobe level (SLL), especially in the plane affected by the curvature; for this reason, particular care must be taken in its design, adopting, when possible, the techniques developed for planar RAs and, if necessary, introducing new ones. In the literature, many alternatives are presented to increase reflectarrays' bandwidth and efficiency, most of them based on the introduction and use of re-radiating elements with enhanced properties [21; 22]; some of the solutions presented in these papers can be extended also to the case of convex conformal reflectarrays (indicated in the following of this paper as CRAs), and they could enhance its bandwidth, but they would not affect its efficiency and would not help to reduce the SLL. In order to improve the performance of a CRA, an innovative approach is here introduced, according to which the feed is considered a further degree of freedom of the whole antenna system, and it is designed to optimize the entire antenna performance. This is quite different with respect to what it is generally done for planar RAs since the most-used RA feed is a suitable commercial horn, selected to guarantee good antenna performance, such as high efficiency and low cross-polarization.