Combined approach of geometry-optimisation and surface shaping of sub-reflector of Gregorian antenna to recover gain degradation due to fabrication error in main-reflector surface

This paper reports an innovative technique to recover gain degradation due to fabrication error in main-reflector for contoured-beam Gregorian antenna. A 2.5 m Ku-band shaped-beam Gregorian antenna has been designed and manufactured. Due to large size, main reflector has undesirable surface roughness, which cause gain degradation in the coverage. This technique uses combined approach of parameter-optimisation and surface re-shaping of sub-reflector of the Gregorian reflector antenna. The main intricacy is to compensate surface deviation in large-sized main reflector using very small sized sub-reflector. The sub-reflector is approximately 0.28 times of main-reflector in size. Multi-level coordinate search (MCS) and minimax optimisation techniques of Ticra Tool have been used for geometry optimisation and delta-surface shaping, respectively. Physical optics (PO)/physical theory of diffraction (PTD) has been used for analysis of the Gregorian antenna. The surface deviation normal direction (in z-direction) is required to be in the limit of ±5 mm for feasible re-machining for existing mould of the sub-reflector. During the delta-shaping of sub-reflector, its minimum radius of curvature (RoC) has also been controlled, which is essential for its fabrication. Measured results are matching well with predicted results of re-shaped Gregorian antenna, which demonstrates the effectiveness of the proposed method.