Microwave holography of large reflector antennas

This thesis describes experiments in Microwave Holography using the 26 m radio telescope at the Mount Pleasant Observatory, and the development of data processing methods to overcome a major limitation of traditional holographic measurements. Microwave Holography is a fast and effective technique for measuring the surface profile of reflector antennas. It requires measurement of the amplitude and phase of the antenna's far field radiation pattern. The Fourier transform relationship of the far field pattern to the aperture current function is used to estimate the aperture phase profile, which can be related directly to the surface profile. Using an unmodulated 12.7 GHz carrier signal transmitted by the AUSSAT-1 geostationary satellite, the 26 m diameter surface was measured to an accuracy of ¬¨¬±53 ˜í¬¿m with a resolution of 0.6 m. Using a beacon signal, weaker by 20 dB, gave a comparable accuracy of ¬¨¬±64 ˜í¬¿m. Each map took about 4 hours to record. In practice only a small part of the complete radiation pattern can be measured, so information about high spatial frequencies in the aperture is lost. This causes detailed structure in the surface error map to be smoothed out. Most large reflectors are constructed with panels, and misaligned panels give discontinuities in surface error which cannot be resolved. This can lead to incorrect assessment of panel positions. Significant improvement can be obtained by recognizing at the outset that the reflector surface is not smooth and continuous, but a collection of individual rigid panels. The Method of Successive Projections is an algorithm, with a simple geometrical interpretation, which allows information about the panel boundaries to be readily incorporated into the data reduction process. The algorithm is straight-forward to apply and is very flexible: aperture blockage effects do not disturb its operation, and panel distortions can be included in the analysis with minimal extra effort. Good results may be achieved while minimizing measurement time. This is an important consideration for high-performance antennas with a busy operating programme.