# Wide Scanning Angle Millimetre Wave 1 \(\times\) 4 Planar Antenna Array on InP at 300 GHz
## 2 Design of the Unit-Cell
#### Array Design
Specifically, the near-field interaction between radiating elements can be reduced by creating a conducting wall or providing a path to the ground for near fields. In CMOS or PCB technologies, such a wall is constructed using multiple vias, like in substrate integrated waveguides. In the case of InP, vias cannot be implemented due to the complex fabrication process and low process yield. Thus, for the proposed array design, it is not possible to diminish the near-field interaction or the dynamic part of \(Z_{ij}\). The static part of \(Z_{ij}\) can be increased by providing isolation between the ground plane of the different radiating elements. Furthermore, to attenuate the substrate wave, slots can be introduced in the ground plane. The effect can be increased by providing parasitic metal strips isolated from the ground, as proposed in [21]. The slots in the ground plane disrupt the necessary condition for the propagation of surface waves in a slab waveguide. This effect is further increased by introducing isolated metal strips. Ideally the length of the metal strips and slots should be larger than the length of the radiating element. This is the case when slots are realised in a single dielectric material. In [21], it was proposed that the length of the metal strip can be decreased by using two different dielectric materials for realising the slot. This configuration is challenging for PCB technologies, but in the case of the presented array structure, such a combination of dielectric materials already exists. From the layer stack-up of Figure 1, it can be observed that by removing Gold Layer 1, the two dielectrics BCB and InP can provide the necessary sandwiched dielectric material. InP has a higher relative dielectric constant (\(\epsilon=\)12), thus providing the possibility to realise a slot length (\(L_{s}\)) less than or equal to the length of the radiating patch. The effective relative dielectric constant of both materials is approximately 9. Using a combination of parasitic metal strips with an isolated ground plane (MSWIGP), the ground plane is modified to mitigate the mutual coupling, thereby creating a defected ground plane. The modified layout of the array structure is shown in Figure 8a. The isolation is provided by the slots placed between the ground planes and the parasitic metal strips. The width of the metal strip is 9 \(\upmu\)m (which is obtained parametrically), and of equal length to that of the BCB layer on top, i.e., \(L_{s}=500\)\(\upmu\)m. The slot is created by parametrically varying the distance between metal  
Figure 7: Impedance model of \(1\times 4\) array. strips and the ground plane. The slot is 26 \(\upmu\)m wide with a length of 600 \(\upmu\)m. The distance between two ground planes of adjacent radiating elements (including strips and slots) is 70 \(\upmu\)m. This modified ground plane or MSWIGP is used to create the 1 \(\times\) 4 array. The layer structure and dimensions of elements are kept constant. The modified array was simulated, and the active reflection coefficient was obtained for each radiating element. Figure 9 presents a comparison of the bandwidth improvement in terms of the reflection coefficient. This comparison is obtained by plotting the frequency crossing point for a reflection coefficient of \(-\)10 dB as a function of the steering angle. Figure 9 illustrates the frequencies for which the reflection coefficient of the radiating elements is below \(-\)10 dB while the steering angle is varied from \(-\)50\({}^{\circ}\) to 50\({}^{\circ}\). It can be observed that MSWIGP improves the bandwidth of all antenna elements, especially the central elements (elements 2 and 3). For extreme steering angles, the reflection coefficient is now less than \(-\)10 dB for frequencies below 300 GHz, a clear improvement. There exists some degradation for higher frequencies (>305 GHz), but the unit-radiating element is not designed for these frequencies. The centre frequency can be tuned by optimising the structure of the unit radiating element.