## 2 Design Procedure  
Many investigations concerning microstrip antenna and the increase of its bandwidth have been developed in the last few decades. Several solutions have been proposed, many of which are reported in [33]. Various authors have proposed the ACMA in wideband applications using different design strategies, but the basic condition for obtaining satisfactory results in increasing the bandwidth is the optimization of the set of geometrical parameters of a multilayer antenna structure. To ensure less coupling and therefore better polarization purity, we use a so-called T-shaped configuration for the slots. The basic antenna structure that we propose consists of four dielectric layers with two stacked radiating patches. The driven patch is coupled to the feed-line through a slot in a common ground plane, while the other patch is parasitic. The feeding network allows dual polarization, with two shaped slots deployed mutually orthogonal. Figure 1 shows an exploded schema of the proposed antenna together with the design parameters. For the sake of clarity, only the H-polarization parameters are shown. Those of the V-polarization will be the same, but will assume obviously different values. To distinguish them, we will use the superscript H or V above the variable name (for example, \(W_{s}^{H}\) and \(W_{s}^{V}\)). In order to achieve excellent polarization purity, the feed network is implemented in a separate layer underneath the ground plane, which has the main advantage of blocking the possible interference between the patch radiation and the parasitic radiation of the feed-lines. The increase of bandwidth in the proposed antenna is obtained using a design technique that excites three distinct resonances in the desired frequency band. The tuning of four specific geometric parameters permits the adequate collocation of the resonance frequencies to achieve the wide matching bandwidth; they are: the length of the slot, the radiating edge of the driven patch, and the size and distance from radiating edge of the parasitic patch. The thickness and the permittivity of substrate materials also play an important role in determining the bandwidth, but they may not be free to be tuned due to technological constraints. The main advantage of this architecture is the simple industrial process for realizing a layered antenna structure. On the other hand, the design process requires a non-negligible effort based on an accurate cross-optimization of the slots shape, the feeding lines, and the size of patches in order to fulfill the given specifications in dual polarization condition. In this paper, the antenna reflection coefficient for both polarizations has been optimized using the Ansys HFSS optimizer [37]. The algorithm chosen is sequential non-linear programming (SNLP) [38]. The main advantage of SNLP (gradient) is that it handles the optimization problem in more depth. This optimizer assumes that the optimization variables span a continuous space. As a result, there is no minimum step size specified in this optimizer, and the variables may take any value within the allowable constraints and within the numerical precision limits of the simulator. The optimizer modifies the variable values until the minimum is reached with acceptable accuracy. Before starting the optimization process, the substrate (dielectric 1) must be chosen. In particular, a trade-off must be made between materials with low and high values of permittivity in order to have an antenna with high efficiency, but small size. The choice of substrate height (\(h_{1}\)) will also be a trade-off between line width and loss, and in some specific cases, weight. The substrates below the patches have the sole role of mechanically supporting the patches and are, therefore, chosen sufficiently thin (\(h_{4}\) and \(h_{5}\)). Certainly, the permittivity of dielectric 2 and 3 must be chosen with sufficient stiffness to prevent patch flexing.