## 2 Slot-Type Doppler Radar Antenna Design  
As alluded previously, the slot-type Doppler radar antenna [19] is designed in this work, since slot antennas are less affected by human body effects compared to other antennas. Figure 1a shows the overall view of the designed slot-type antenna. Both Tx port (Port 1) and Rx port (Port 2) are fed by the end-launch K-connectors and they are connected to the microstrip line through via-holes. The slotted aperture is embedded between the superstrate and the substrate, which are Rogers RO4350B (thickness = 0.254 mm, \(\varepsilon_{\text{r}}\) = 3.48, tan \(\delta\) = 0.0037) and FR-4 (thickness = 1.6 mm, \(\varepsilon_{\text{r}}\) = 4.4, tan \(\delta\) = 0.02), as depicted Figure 1b. The FR-4 substrate is employed for rigidity because the antenna on Rogers RO4350B may be easily bent. The antenna has a slot of \(0.8\times 2.8\) mm\({}^{2}\), as shown in Figure 1c. The radiation pattern of slot antenna is basically omnidirectional pattern. In order to obtain a higher radiation pattern towards the human body than the back side, the slot antenna has a reflector underneath the FR-4 substrate as shown in Figure 1d. Figure 2 shows the microstrip line-based feed circuit. The microstrip lines are printed on the top layer of Rogers RO4350B superstrate and ground planes with slotted aperture are mounted on bottom layer of Rogers RO4350B superstrate. The width of microstrip line is 0.5 mm and its length is 7.3 mm. The surface current is induced around the slotted aperture by electromagnetic coupling with the microstrip line. Therefore, the slotted apertures work as a slot antenna [20]. The impedance characteristic of the slot antenna can be controlled by modifying the length and width of the microstrip line. The surface current distributions at 24.125 GHz is shown in Figure 3. The designed slot-type  
Figure 1: Designed slot-type Doppler radar antenna: (**a**) overall view; (**b**) slotted aperture; (**c**) side view; (**d**) bottom view. Figure 2: Feed circuit of slot-type Doppler radar antenna: (**a**) designed microstrip-line on Rogers RO4350B superstrate; (**b**) perspective view of microstrip-line and slotted aperture. Doppler radar antenna is fabricated and measured in Figure 4. As shown in Figure 4, simulation and measurement results are obtained by considering the slot-type Doppler radar antenna system composed of Tx and Rx antennas, not just only one antenna. Figure 5 shows the simulated and measured S-parameters of the Tx antenna. The simulated reflection coefficient (S\({}_{11}\)) of the slot-type antennas is below \(-10\) dB from 23.2 to 24.78 GHz (bandwidth BW = 1.58 GHz). The simulated mutual coupling (S\({}_{21}\)) from the Tx antenna to the Rx antenna is lower than \(-27.4\) dB over the target frequency band. The measured \(-10\) dB S\({}_{11}\) bandwidth is 23.74 to 25.56 GHz (BW = 1.82 GHz) and the measured S\({}_{21}\) is lower than \(-30\) dB over the target frequency band. The discrepancies between simulation and measurement are regarded as error by the manufacturing process. Although the resonant frequency is shifted to a higher band, the fabricated antenna still covers the target frequency band. The simulated and measured radiation patterns of the Tx antenna are shown in Figure 6, when port 1 is excited. The simulated realized gain and front-to-back ratio (FBR) along z-axis are 4.24 dBi and 9.23 dB, respectively. The simulated cross polarization (X-pol) level is significantly lower than the simulated co polarization in the front side. The measured realized gain and FBR along z-axis are 2.1 and 33.3 dB, respectively. The discrepancies between simulation and measurement in terms of radiation patterns are caused by the same reasons for S-parameters. It should be also noted that the radiation pattern of the designed slot-type antenna is different from that of the basic slot antenna since the designed antenna works as a radiator based on a combination of the slot, the microstrip, and the grounds.