**4.2 Shared-aperture design of the mm-wave array with the lower frequency band antenna**

To integrate the mm-wave array with the lower frequency band antenna, a low pass (or high pass) filter can be applied [77, 78]. As shown in **Figure 12(a)**, a 3.5 GHz lower band antenna is directly connected to a 28 GHz mm-wave antenna with a low-pass and high-stop (3.5 GHz pass and 28 GHz stop) filter [77]. Thus, the mm-wave antenna and the lower frequency band can be designed in a near space with a single feeding port. To reduce the occupied space of the mm-wave array and lower frequency band antenna, the mm-wave array can be integrated into the lower frequency band antenna [11, 79–83]. As shown in **Figure 12(b)**, the mm-wave slot array antenna is integrated into the clearance of the lower frequency band inverted-F antenna [79]. In addition, a notch on the lower frequency band can be applied to integrate the mm-wave array antenna [81, 82]. Also, the mm-wave array antenna can be deployed on the lower frequency band antenna [83]. To further reduce the occupied space of the mm-wave array and the lower frequency band antenna, the metal pattern of the lower frequency band antenna can be applied to design SIW structure for the mm-wave array antenna [84–87]. As shown in **Figure 12(c)** [84], the lower frequency band antenna is a simple patch antenna. To integrate the mm-wave array on the patch antenna, the patch of the lower frequency band antenna is designed as a SIW slot array. Thus, the mm-wave array antenna and the lower frequency band antenna is designed in the same aperture. In addition, the mm-wave SIW slot array can be integrated into the monopole antenna [85] or inverted-F antenna [86, 87] of the lower frequency band. Besides using the SIW structure, the higher order mode of the lower frequency band antenna can also be directly applied to design the mm-wave array [88–91]. Also in [88], the half-wavelength slot mode of the lower frequency band antenna has the higher order mode of the slot. And multiple feedings are applied to excite the higher-order mode of the slot, which is the connected slot array. Thus, a single slot is designed to work at the mm-wave frequency band and lower frequency band simultaneously. In [89], a single microstrip grid array is designed to cover the mm-wave frequency band and lower frequency band simultaneously. In [90], a surface is the integration of a metasurface at the lower frequency band and a partially reflective surface (PRS) at the higher frequency band. In [91], a single slot is designed to function as a decoupling slot at the lower frequency band and the taper slot antenna at the mm-wave frequency band.

#### **Figure 12.**

*Typical design schematics of co-design of mm-wave array with lower frequency band antenna. (a) Using the filter to integrate the mm-wave and lower frequency band antennas. (b) Using the clearance of the lower frequency band antenna to deploy the mm-wave array. (c) Using the SIW structure to integrate the mm-wave and lower frequency band antennas.*
