# Wide Scanning Angle Millimetre Wave 1 \(\times\) 4 Planar Antenna Array on InP at 300 GHz
###### Abstract  
The design of a uniformly spaced 1 \(\times\) 4 linear antenna array using epitaxial layers of benzocyclobutene over an InP substrate is demonstrated. The array elements are conjugately matched with a uni-travelling carrier photodiode at the input. The phased array is optimised to counteract mutual coupling effects by introducing metal strips with isolated ground planes for each radiating element. The proposed antenna array can provide a gain of 10 dBi with a gain variation of \(\pm\)3 dB. The array operates over a bandwidth of 10 GHz (295-305 GHz) with a wide scanning angle of 100\({}^{\circ}\) in the broadside. millientre-wave communications; Sub-THz antennas; phased array antennas +
Footnote †: journal: Applied  
**Citation:** [PERSON]; [PERSON]; [PERSON] Wide Scanning Angle Millimetre Wave 1 \(\times\) 4 Planar Antenna Array on InP at 300 GHz. _Appl. Sci._**2021**, _11_, 7117. [[https://doi.org/10.3309/app11157117](https://doi.org/10.3309/app11157117)]([https://doi.org/10.3309/app11157117](https://doi.org/10.3309/app11157117))  
Academic Editor: [PERSON]  
Received: 2 July 2021  
Accepted: 26 July 2021  
Subscribed: 31 July 2021  
**Publisher's Note:** MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affiliations. the same die for realising a complete on-chip sub-THz transceiver without the need for interconnects, as in the case of photodiodes (photomixing) for the generation of mm-wave frequencies using uni-travelling carrier photodiode (UTC-PD). Typically, the electrical output power of integrated photodiodes is in the order of microwatts [3,14, 15, 16]. Due to the high path loss suffered by millimetre wave signals, an array of photodiodes is needed to transmit effectively even over a short distance. For an effective sub-THz transmitter/receiver, a phased array antenna is often utilised. An array of planar radiating elements can steer the beam along the desired direction and concentrate energy into the receiver. Sub-THz antenna arrays are conventionally implemented using non-planar structures such as horn antennas [17]. Array arrangements, whilst providing the desired performance measures (in terms of peak realised gain, impedance matching and bandwidth), are bulky and unsuitable for an integrated solution. Additionally, planar array structures on III-V substrates, especially InP, present serious design challenges. The InP has an isotropic etching profile, making it challenging to realise waveguiding structures involving via through holes. These design bottlenecks can be overcome by resorting to an alternative technique of depositing a thin film of a material having a lower relative dielectric constant over a ground plane that provides shielding to the InP substrate [18]. This superficial layer can be used to realise planar arrays by capacitively feeding the radiating elements. Previously reported planar arrays [17,18] were designed for an input impedance of 50 \(\Omega\), which is not typically the case for on-chip photodiodes. Here, we present a linear 1 \(\times\) 4 phased array designed on InP through the employment of layers of benzocyclobutene (BCB) deposited over a ground plane. The designed array provides a front-end for a microwave photonics-based sub-THz transceiver. The input impedance of radiating elements is optimised to provide a conjugate match to in-house-built uni-travelling carrier photodiodes operating at around 300 GHz for maximum power transfer. A capacitive feed structure is used to couple the mm-wave signal to the radiating patch and simultaneously provide the targeted impedance matching. The presented phased array is optimised to counteract mutual coupling effects by implementing parasitic metal strips with isolated ground planes (MSWIGP) for each radiating element. The 1 \(\times\) 4 linear array provides a gain of 10 dBi with a wide scanning angle of 100\({}^{\circ}\) and an impedance matching bandwidth of 10 GHz (295 GHz-305 GHz).