# Development of a Wideband Slotted Antenna Array with Low Profile and Low Sidelobe
## 2 Design of the Antenna Array
### Basic 2 \(\times\) 2-Slot Subarray
The corresponding dimensions of Figure 2a are also provided by Table 1. Obviously, the cavity has been simplified, and the overall size, especially the height, of the subarray has been obviously reduced. As presented in Figure 2c,d, the coupling slot in \"Layer3\" is designed right under the middle of the two distantly designed radiation slots, with the short edge close to the edge of \"Layer2\". The length \(l_{cs}\) and the width \(w_{cs}\) of the coupling slot are carefully designed, so that the desired \(TE_{220}\) mode can be excited in the resonance cavity. In order to maximally transmit the power from \"Layer4\" to \"Layer2\", the edge of the feeding waveguide of \"Layer4\" is designed beneath the coupling slot of \"Layer3\", as shown in Figure 2d. The final design of the radiation unit is presented in Figure 2b and the corresponding dimension details are presented in Table 1 as well. For comparison, the two radiation units shown in Figure 2a,b with the dimension information listed in Table 1, are analyzed by the ANSYS HFSS simulations. Figure 4 presents the reflection coefficients and the gains of the primary and the improved subarray units. It is noted that the reflection coefficient of the improved unit is always smaller than \(-15\) dB in the frequency range from \(17.6\) GHz to \(22.3\) GHz (\(23.6\%\)), while the corresponding bandwidth of the primary unit is from \(18.4\) GHz to \(21.5\) GHz (\(15.5\%\)). Thus, the bandwidth of the proposed resonance cavity-based unit is much broader than the primary unit. The gain at the desired frequency \(20\) GHz is approximately \(13.6\) dBi for both the primary and the improved subarray units, which means that the introduced resonance cavity has almost no effect on the gain. The simulated radiation patterns of the two subarray units are presented in Figure 5. It is expected that the main lobe of the improved subarray in both \(E\)-plane and \(H\)-plane is located at \(0^{\circ}\), indicating that the phases of the radiated waves through the four slots are the same. This is in a good agreement with the primary subarray unit. The \(3\) dB beam-width of the improved subarray unit is slightly larger (\(12.0\%\) and \(11.9\%\) in \(E\)-plane and \(H\)-plane) compared to the primary subarray. The SLLs in the \(E\)-plane and the \(H\)-plane for the primary subarray are approximately \(-6.6\) dB and \(-11.2\) dB, respectively, whereas the corresponding \(E\)-plane and \(H\)-plane SLLs for the improved subarray are approximately \(-13\) dB and \(-19\) dB, respectively. This means that the improved subarray unit can provide a good SLL when it is used as a radiation antenna. \begin{table}
\begin{tabular}{l l l l l l l} \hline \hline \(h_{1}\) (\(h_{1}\)\({}^{\prime}\)) & \(h_{2}\) (\(h_{2}\)\({}^{\prime}\)) & \(h_{3}\) (\(h_{3}\)\({}^{\prime}\)) & \(h_{4}\) (\(h_{4}\)\({}^{\prime}\)) & \(h_{5}\) (\(h_{5}\)\({}^{\prime}\)) & \(l\) (\(l^{\prime}\)) & \(w\) (\(w^{\prime}\)) \\ \hline \(0.5\) (\(0.9\)) & \(1.33\) (\(4.5\)) & \(0.9\) & \(4.5\) & \(3.6\) & \(21.2\) (\(26\)) & \(21.2\) (\(26\)) \\ \hline \(l_{1}\) (\(l_{1}\)\({}^{\prime}\)) & \(l_{l}\) & \(l_{w}\) & \(s_{l}\) & \(s_{w}\) & \(w_{1}\) (\(w_{1}\)\({}^{\prime}\)) & \(d\) (\(d\)\({}^{\prime}\)) \\ \hline \(9.32\) (\(8.2\)) & \(6.49\) & \(1.09\) & \(2.44\) & \(1.72\) & \(2.98\) (\(3.28\)) & \(16.54\) \\ \hline \(l_{c}\) (\(l_{c}\)\({}^{\prime}\)) & \(w_{c}\) (\(w_{c}\)\({}^{\prime}\)) & \(l_{cs}\) (\(l_{cs}\)\({}^{\prime}\)) & \(w_{cs}\) (\(w_{cs}\)\({}^{\prime}\)) & \(a\) (\(a\)\({}^{\prime}\)) & \(g\) & \(rim\) \\ \hline \(21\) (\(16.28\)) & \(21\) (\(21.2\)) & \(8.13\) (\(8.96\)) & \(0.5\) (\(3.97\)) & \(8.6\) & \(1.8\) & \(0.1\) \\ \hline \end{tabular}
\end{table} 
Table 1: Dimensions of the \(2\times 2\)-element subarray (unit: mm). Figure 3: Amplitude distribution of the \(E\)-field for \(TE_{220}\) mode in the cavity.