## 3 Antenna Array Design  
As mentioned in the introduction, the antenna is designed for IR UWB radar Non-Contact Human Vital Signs Detection application that requires a narrow beamwidth of 40-50\({}^{\circ}\) and the gain enhancement on the limited area of microwave-integrated circuit. \begin{table}
\begin{tabular}{c c c c c c} \hline \hline
**Type** & **Conventional** & **Inset Fed** & \multicolumn{3}{c}{**Inset-Fed and Two-Slot**} \\ \hline @Frequency (GHz) & 7.5 & 7.5 & 7.1 & 7.5 & 7.9 \\ \hline Gain (dB) & 5.1 & 4.8 & 4.6 & 3.5 & 2.4 \\ \hline HPBW (\({}^{\circ}\)) (H/E plane) & 90.5/92.2 & 91.7/93.6 & 93.0/93.7 & 92.7/93.8 & 89.6/92.5 \\ \hline Bandwidth (MHz) & 360 & 650 & & 1080 & \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Comparison of single antennas’ performance. Figure 3: The antenna reflection coefficient: (**a**) impact on reflection coefficient of Slot-L; (**b**) impact on reflection coefficient of Slot-D; (**c**) impact on reflection coefficient of Slot-W; (**d**) impact on reflection coefficient of inset-L.  
On the contrary, the radiation simulation result of the single antenna revealed a wide beamwidth and low gain. Thus, the design of a planar antenna array based on [17] is employed to satisfy this requirement of small beamwidth and gain enhancement. Figure 4a presents the proposed 2 \(\times\) 2 planar antenna array configuration with the detailed dimension. Note that designing four antennas (i.e., 2 \(\times\) 2) in the given area (i.e., 36 \(\times\) 40 mm\({}^{2}\)) is challenging since the wavelength of the lowest frequency (i.e., 7 GHz) is already 42.9 mm. The corporate feed network is a suitable option for supplying power to the antenna in this small region. The entire integrated circuit is designed on an FR-4 four-layer stack-up, as indicated in Figure 4b. The ground surrounds the feed line on the top layer, and vias go from the top to the second ground (the second layer), resulting in a GCPW structure with a thickness of 0.2 mm. This feed line width of 0.4 mm of GCPW is also employed for the width of the co-operate feed microstrip line network. To obtain the best matching for the antenna array, the dimension of the antenna patch is slightly modified. In this antenna array configuration, when the inter-spacing element is lower than 0.5\(\lambda_{g}\), directivity values are observed to be low, and high values are obtained for the inter-spacing element higher than 0.5\(\lambda_{g}\), where \(\lambda_{g}\) is the wavelength at 7.5 GHz. Hence, inter-element spacing equal to or greater than \(\lambda_{g}/2\) is favored to achieve higher directivity and slightly narrower HPBW [18]. Therefore, in the vertical plane and horizontal plane, the inter-spacing elements are 0.52\(\lambda_{g}\) and 0.64\(\lambda_{g}\), respectively, maximally utilizing the limited antenna area of the circuit board. According to our design specifications, the short transition GCPW of 50 Ohm is also added to the FR4 1.6 mm of thickness substrate to guarantee the best reflection coefficient of the array antenna, as displayed in Figure 5, and keep the inter-spacing elements in the horizontal plane to achieve 0.52\(\lambda_{g}\). As can be seen from Figure 5a, the length of the array feed line should be 10 mm (\(\lambda_{g}/4\)) when it is out of the 50 Ohm GCPW feed line. The mutual coupling between each element is reduced as well by this distance. The proposed array antenna configuration is a rectangular arrangement (2 \(\times\) 2). The antenna part is attached to the main circuit portion by 6.5 mm of GCPW transmission line, as shown in Figure 4a, with geometrical dimension parameters for the antenna array. Figure 4: Antenna array geometry: (**a**) geometry of 2 \(\times\) 2 array antenna on L1 (unit: mm); (**b**) geometry of 4-layer stack-up. As depicted in Figure 5b, the simulation result shows a planar antenna array bandwidth of 950 MHz (13.3%). The bandwidth slightly decreases compared to the single antenna.