# Eco-Friendly Metadome-Antenna Innovations for Wearable Millimeter Wave Radar Sensing
## 2 Materials and Methods
### Design of the Metasurface-Based Radome  
Starting, first of all, with what refers to an ordinary radome, there are five basic styles defined according to dielectric wall construction: (1) Monolithic half-wave wall solid, (2) thin-wall monolithic (with thickness \(\leq\) 0.1 \(\mathrm{\lambda g}\) at the highest operating frequency); (3) A-Sandwich multi-layered wall (a core layer between two skin layers with relative dielectric permittivity higher than the one they overlay, at 0.25 \(\mathrm{\lambda g}\) thickness); (4) Multi-layered wall comprising five or more layers (odd number of high-permittivity layers and an even number of low-permittivity core layers. The broadband performance improves as the number of layers is increased); and (5) other wall constructions not fitting into the aforementioned ones, including the B-Sandwich that comprises a high-permittivity core between two low-permittivity skins. The monolithic half-wave wall radome at 24 GHz requires a thickness of 4.1 mm for PP and being arranged at least 6.21 mm from the antenna, which is not convenient at all for a wearable application because it increases the thickness of the device too much, making it bulkier, heavier, less discreet and less comfortable to wear. It goes without saying that all other styles of multilayer radomes (styles 3 to 5) are not suitable for the intended purpose either, both because of the thickness required and consequent weight and lack of comfort, and because of the complexity and higher cost of their implementation. On the other hand, a thin-wall monolithic radome, requiring a thickness \(\leq\) 0.83 mm if PP-based, will be considered for comparison with the metasurface based radome approach, which is proposed as a much more convenient solution to protect the antenna without performance degradation for the target ETA sensing application. To achieve an operational metasurface design suitable for use as a radome in the 24 GHz frequency band, a systematic procedure has to be followed. First, the specific performance requirements for the intended application must be defined: transmission should be maximized (S\({}_{21}\) (dB) as close to 0 dB as possible) and reflection minimized (S\({}_{11}\) (dB) bellow \(-\)10 dB) across the target bandwidth (24.05-24.25 GHz), while aiming to maximize angular stability (S\({}_{21}\) and S\({}_{11}\) meeting the aforementioned requirements for the highest possible incident angles, for both TE and TM polarized incident plane-waves. Next, the geometry of the unit cell has to be chosen (square unit-cell in this case) and the specific geometry of the unit-cell metallization defined. To achieve a metasurface that resonates at the high frequency of 24.15 GHz and is suitable for fabrication with conventional microwave circuit techniques, thereby keeping costs affordable, the metallization geometry cannot be very complex. Moreover, the designer must conceptualize an equivalent LC resonant circuit for the dielectric-backed metallization geometry (which will be further explained, along with the metasurface operation). In this case, an aluminum metallic cross on a PP dielectric slab has been chosen. Once the unit cell geometry and the materials are chosen (implying a specific thickness, relative dielectric permittivity and loss tangent for the dielectric, and conductivity for the metallic parts), the dimensions are optimized using electromagnetic software to meet the desired specifications. Iterative simulations and adjustments are crucial to refine the design. Throughout the iterative simulation process, potential size constraints of the geometric elements must be considered to ensure compatibility with the chosen fabrication technique.