# Development of an L-Band SAR Microsatellite Antenna for Earth Observation
## 5 Discussion  
Since the mesh material is fixed on the reflector structure by being stretched and stitched around ribs and rims, there is an added volume of mesh material over these structures. Hence, in both measurement methods for geometric surface verification, some deviation from the theoretical model is expected. Additionally, the reflector position in each measurement setup was different; in the laser measurement, the reflector was facing upwards, whereas in the 3D scanning measurement setup, it was facing almost horizontally. Therefore, reflector surface distortions due to gravity are also expected to be different in each measurement setup. Results from the laser measurements presented largest discrepancy from rib number 9, with 3.6 mm RMS, and overall ribs measurements presented 1.92 mm RMS, lower than the required surface accuracy of 4.22 mm RMS. The 3D scanning results, on the other hand, presented higher RMS error of 3.86 mm. This difference in surface accuracy error can be attributed to the low image SNR of mesh material when 3D scanned, possible image misregistration of scanned patches, and more significant reflector distortion due to gravity compared to the laser setup. In the future, errors in the image processing steps can be reduced by using geo-referenced reflective tags on the parabolic surface under the same 3D scanning approach. Results from the near-field antenna pattern measurement showed good agreement between simulation and measured data in both azimuth and elevation planes. The overall antenna gain can be improved by increasing the helical feed gain to comply with the required 30 dBic. That can be simply done by increasing the number of helical loops. Measured axial ratio also reasonably agreed with simulations, but showed considerable axial ratio degeneration of 0.7 dB when compared to simulated results. This together with other small deviations could be attributed to unavoidable misalignment in the setup between the helical antenna and center of parabolic reflector, as well as effects of the mesh weaver pattern in the circularly polarized wave. Some degeneration of simulated versus measured values of axial ratio, co- and cross-polarization can be seen at boresight, as referred in Table 7 and Figure 10. These differences can be attributed to unavoidable misalignments in the setup, as well as effects of the knitted mesh pattern for circularly polarized waves, since, in the simulations, the reflector was assumed rigid. Adopting a mesh type with a pair of wires instead of single wire can reduce the variance of reflectivity dependant on the direction of electric field of the incident wave, as previously reported for 20 GHz frequencies [19]. Hence, further investigations on the mesh reflectivity for CP waves in the L-band must be carried out. Moreover, measured sidelobe levels presented good agreement with simulation results, but still present higher values than the specification. Further adaptation of feed tower must be carried out to minimize cross-polarization and sidelobe values. Figure 11a is a holographic amplitude map at the antenna aperture, which can be interpreted as the illumination efficiency of feed on the main reflector. A near 10-dB taper feed can be seen, as expected from measured and simulated helical antenna patterns. Additionally, the amplitude map  
Figure 11: Hologram at aperture level in center frequency of: (a) field amplitude; and (b) phase. shows few areas with uneven illumination. That can be explained by a series of factors: diffraction due to struts and other elements, an improperly focused antenna, reflector surface distortions and potential misalignment in the setup. Precise alignment between probe and reflector platform was not possible due to unavoidable vibrations from the lifting table, which could have resulted in slight movements of the antenna configuration. The holographic phase map gives a good approximation of surface distortions, and jumpy phases of \(\pm 180^{\circ}\) can be seen as the phase is still wrapped.