# Portable L-Band Radiometer (PoLRa): Design and Characterization
## 2 Hardware
### Antenna Design and Characterization  
The unique dual patch array antenna is both compact and lightweight, and provides sufficient directivity to obtain reasonable ground resolution, low back-lobe contribution, and small polarization crosstalk. The printed circuit board (PCB) patch arrays use two PCB layers separated by an air gap to obtain high gain and high radiation efficiency. The patches are fed with uniform magnitude and phase from a microstrip feed network printed on the same PCB as the patches. The microstrip feed network is fed with a coaxial probe and connected to the front-end switch with a 1 m SMA cable. The antenna consists of two FR4 PCBs of 1.5 mm thickness, spaced with 6 mm PTFE spacers. The PCBs are attached using nylon screws running through the spacers and PCB layers. The total dimensions of the antenna are 0.6 m \(\times\) 0.3 m \(\times\) 9 mm.  
The physical temperature of the antenna and the feed cables are monitored, as indicated in Figure 1. The antenna Ohmic losses and the loss of the coaxial feed cables are determined empirically as part of the ACS characterization described in Section 3. Figure 3 shows photographs of the antenna during ground-based sky measurements and during drone-based measurement.  
The return loss of the antenna was simulated using commercial finite-element electromagnetics software (ANSYS Electromagnetics Suite) during the design process. The feed network and patch dimensions were optimized to reduce the simulated return loss. The return loss was measured with a  
Figure 3: Patch array antenna (**a**) mounted on tower during sky measurement; (**b**) mounted on multi-copter drone during flight measurements.  
vector network analyzer (VNA) while the antenna was pointed towards the sky. The resonance of the antenna, or the minimum in return loss, is highly sensitive to the electromagnetic permittivity of the FR4 substrate. The final presented design required a number of iterations to accurately determine the FR4 permittivity supplied by the particular PCB vendor.  
The angular-dependent power sensitivity pattern of the antenna was simulated with ANSYS Electromagnetics Suite finite element software. Additionally, the antenna power sensitivity pattern was measured using the solar overpass method described in [23]. The antenna was positioned such that the boresight pointed towards the azimuth and elevation angle of the highest solar zenith angle on that day. The relative antenna pattern measured with the solar overpass method characterizes the gain as a function of the total angle \(\alpha\) between the sun and the antenna boresight direction. The spherical polar angle \(\theta\) will only exactly equal \(\alpha\) when the sun passes directly overhead, but the response with respect to \(\alpha\) should be between slices of constant \(\phi~{}=~{}\left\{0^{\circ},~{}90^{\circ}\right\}\). The solar overpass data are only shown through the \(-6\) dB power level, because at high angles the horizon became cluttered by trees, and the measurements became unreliable. Figure 4 shows (a) the simulated and measured antenna return loss, and (b) the simulated and measured antenna power sensitivity pattern (normalized antenna gain).