## 2 Design of the Antenna for RF Sensing and That for Communication  
Prior to the stage of the aforementioned assembly, the elemental antennas for the proposed module are designed. The antennas for the two blocks are required to work at mmW bands, which are appropriate for fast data transmission in communication and a very pointy beam in sensing. Both of the antennas take the forms of the array antenna to create narrow beams, and the physical sizes can be small due to the shorter wavelengths of the K and Ka bands. First, the antenna for communication is presented. Figure 1: The sensing and communication functions are integrated into the surface of the moving vehicle, like a car or satellite. Figure 2: The proposed module of sensing and communication both in mmW-bands. Presented in Figure 3a, it is a one-by-four array antenna consisting of the rectangular microstrip patches and the power divider. The patch is 2.8 mm long, the 1 \(\times\) 4 array stretches 21.5 mm, and the substrate is 46.2 mm \(\times\) 23.1 mm wide. The thickness of the metal pattern is 1 oz. The feed is horizontal to avoid EM noise from the patches and other block. Note that it is not difficult to have a larger array, but the one-by-four array is commonly seen in the layout of the commercial 5G mobile handset and adopted in the access point equipment. This structure is made to have good impedance matching at 28 GHz, as this is the target frequency's so-called FR2 band. S\({}_{11}\) is less than \(-\)10 dB, as in Figure 3b. This results in the far-field pattern as shown in Figure 3c,d. The beam pattern is also known as the fan beam. This is quite typical of the one-dimensional array with the resonance at this high frequency. Second, the antenna for the RF sensing block is constructed to be suitable for high directivity, as can be easily found from practices in the development of radar modules. Figure 3: _Cont_. Figure 4 elaborates on the antenna for the RF sensing block. This two-dimensional structure is an 8-by-16 array. The area of the metal pattern on the front side of the antenna is 4.7 cm \(\times\) 9.6 cm, and this is based on the substrate, the area of which is equal to 14 cm \(\times\) 14 cm. RT4350B is adopted as the dielectric substrate to hold 128 0.3 cm \(\times\) 0.3 cm-sized radiating patches. In order to excite the antenna, a complicated-looking power divider that has branches with as many as 128 patches is designed and placed as the metal pattern on the other side of the substrate. A feed-line, laid vertically to avoid the interfering fields from the other block, is centered at the branches. The radiating elements are connected to the ends of the branches through metal vias penetrating the metallic ground between the front side, as in Figure 4a, and the back side, as in Figure 4b, of the structure. The design is validated by investigating S\({}_{11}\) as the reflection coefficient at the input port of the power divider, as well as the radiation performance. If it were a chip-based active beamforming antenna, like AiP, because each of the patches is directly fed by its designated pin of the chip, the designer does not need a power divider. Nonetheless, as the passive array antenna, there is one port that introduces the RF signal as the input power through large branches to the small branches where equally divided power is desired. When the frequency of interest is low, even if there are many branches needed for a power divider, the design is not complicated. However, when the operating frequency must be as high as the mmW bands, it becomes much harder to design a power divider with a great number of branches, since the electromagnetic wave in the component becomes attenuated faster than at low frequencies, and the power division ratios at the end branches tend to be unbalanced  
Figure 3: The antenna for mmW communication.