# Drone SAR Imaging for Monitoring an Active Landslide Adjacent to the M25 at Flint Hall Farm
## 3 Materials and Methods
### Hardware Development
#### 3.2.3 Antennas  
Two antennas are mounted to the drone payload, for the simultaneous transmission and reception of RF signals. This section provides a summary of the antenna development, with a full account provided in [24]; this publication includes deriving the dimensions, simulating antenna performance metrics such as S\({}_{11}\) and gain, and testing the fabricated antenna performances in a RF anechoic chamber.  
The antennas are custom-made for control over the component dimensions and fabrication materials, whilst providing an improved understanding of the signal propagation mechanisms. The ideal antenna performance has good directionality and energy transfer to the target. The antenna type was selected from several options, based on their typical performance, size and weight for drone deployment. Aperture antennas such as horns and waveguides have good bandwidth and gain potential; horn antennas typically provide a highly directive main beam of radiation with smaller minor lobes, since the flared geometry acts to maintain the beam focus.  
The 5.4 GHz centre frequency (f\({}_{0}\)) utilises the upper portions of the E312 frequency range, thus enabling a smaller horn antenna to be produced due to the shorter wavelength. This frequency is within the 5.35 to 5.46 GHz range allocated for civilian use in the UK and avoids the 5.73 to 5.83 GHz range used by the drone remote controller. This frequency facilitates the comparison and fusion of drone and satellite radar products, since the Sentinel-1 satellite constellation utilise a centre frequency of 5.405 GHz.  
Copper Clad Laminate (CCL) is utilised as a conductive, lightweight, and inexpensive manufacturing material. Copper was chosen as a suitable antenna fabrication material due to its high conductivity. The primary material utilised here is a 1.6 mm thick, single-sided, FR-4 material grade CCL epoxy board. Previous demonstrations have shown antennas either integrated into the substrate or lined with this material [34; 35; 36; 37; 38; 39; 40], with this being the first demonstration of horn antennas fabricated from the CCL itself. The board base consists of eight layers of woven glass-reinforced fabric laminate epoxy resin. This provides a lightweight and rigid structure to the antenna at a suitable thickness for dimensional stability during drone deployment. The 35 \(\upmu\)m thick copper at the internal surfaces provides a boundary for the electromagnetic field. CCL is deemed preferable to copper sheets for achieving a lightweight component for drone deployment, given their respective material densities of 1.8 and 8.96 g/cm\({}^{3}\). The CCL pieces are machined using an XYZ Computer Numerical Control (CNC) machine and soldered together with multicore wire lead solder. In addition, 1 mm thick silver anodised aluminium is attached to the external edges of the flared sections using a water-resistant epoxy adhesive to support the soldered joints. The waveguide input feed was made by soldering a brass rod into the solder cup of a 50 \(\Omega\) straight flange mount SMA connector (Figure 9).  
Regarding SwaP-C, the two antennas have a combined weight of 0.99 kg; despite being the heaviest payload components, this weight falls comfortably within the potential payload capacity of 5.5 kg. Antenna testing was performed to compare the simulated and fabricated antenna performances. The S\({}_{11}\), gain, radiation patterns, and beamwidths were measured in a RF electromagnetic anechoic chamber using a calibrated vector network analyser (VNA). Despite a slight performance disparity, which is attributed to artefacts of the manufacturing and testing processes, both antennas perform well, have a wide bandwidth of potential operation, and are suitable for deployment in the drone radar payload [24].