# Feasibility Investigation of SIW Cavity-Backed Patch Antenna Array for Ku Band Applications
## 2 Single Antenna Design  
The design of high-performance antennas requires several trade-offs. High permittivity substrates can allow the construction of very compact microstrip antennas. Thick substrates with lower dielectric constants offer a better radiation efficiency and wide bandwidth, but larger sizes are needed. Further performance improvements can be obtained by employing SIW circular cavities backing the microstrip patch antennas. They also allow the suppression of surface waves, thus obtaining very high isolation from the surrounding elements of antenna arrays. In this paper, a low-loss and low dielectric constant substrate was used. In particular, the Rogers Duroid 5880 with a dielectric constant of \(\epsilon_{\mathrm{TS}}=2.2\) and loss tangent of \(\delta=0.0009\) was chosen for both (top and bottom) substrates. Among the Rogers Duroid 5880 commercially available substrates, the top substrate (on which the patch antennas were obtained via etching) had a thickness of \(\mathrm{h_{s}}=0.381\) mm and the bottom one (including the cavity walls) had a thickness of \(\mathrm{h_{c}}=1.575\) mm; the metal layers of both substrates were \(\mathrm{h_{m}}=0.035\) mm thick. Metalized via holes circularly spaced constitute the SIW cavity side walls, as shown in Figure 1a, which illustrates a global view, and Figure 1b shows a section view of the single antenna. The design was performed by using the 3D commercial software tool CST Microwave Studio(r). Several simulations were performed to reach the best trade-off in terms of insertion loss reduction, bandwidth broadening, and radiation pattern characteristics. The proposed antenna combined the attractive features of both cavity-backed patch antennas and patch antennas with truncated corners. Moreover, as a further improvement with respect to [18; 26; 27], two suitable cuts on the sides of the rectangular patch were designed to provide a wider frequency band and a frequency downshift. The radiating element is then smaller, allowing a more compact size of the whole array. In order to identify the different contributions, a patch antenna without a backing cavity and with the feed located in the center is shown in Figure 2a. Figure 2b shows the modified patch obtained by performing suitable lateral cuts and by moving the feeding microstrip in the corner of the patch. Both patches had a side \(\mathrm{b}=7.7\) mm, and a substrate thickness \(\mathrm{h_{s}}=0.381\) mm with a metal thickness of \(\mathrm{h_{m}}=0.035\) mm. Figure 1: 3D Layout of the SIW cavity backed patch antenna: (**a**) global view and (**b**) section view. Figure 2: Layout of the linear patch antenna: (**a**) without lateral cuts, and (**b**) with lateral cuts and feeding stripline in the corner. The impedance matching network is not reported. The comparison between the modulus of the scattering parameter S\({}_{11}\) of the antenna illustrated in Figure 2a (solid curve) and the modulus of the scattering parameter S\({}_{11}\) of the antenna illustrated in Figure 2b (dashed curve) is reported in Figure 3. For the antenna with a lateral cut and feeding strip line in the corner, the minimum of the modulus of the scattering parameter S\({}_{11}\) shifted toward the lower frequency of about 1 GHz with respect to that of the antenna without a lateral cut and central feeding. The \(-10\) dB bandwidth slightly increased (about 0.1 GHz), changing from 11.6-12.6 GHz to 10.7-11.8 GHz. A suitable SIW cavity backing the patch antenna was designed. The optimized antenna patch parameters are shown in Figure 4, illustrating the top view of (a) the top layer and (b) the bottom layer, and are listed in Table 1. \begin{table}
\begin{tabular}{c c} \hline
**Parameter** & **Size** \\ \hline b & 7.700 mm \\ w1 & 0.245 mm \\ w2 & 0.658 mm \\ w3 & 1.12 mm \\ l1 & 3.15 mm \\ l2 & 4.32 mm \\ l3 & 2.80 mm \\ n & 1.443 mm \\ \hline \end{tabular}
\end{table} 
Table 1: Parameters of the optimized antenna. Figure 4: Layout of the SIW cavity backed antenna. (**a**) Top view of the first layer and (**b**) top view of the second layer.