Analysis of very large radio telescope antennas using multilevel physical optics algorithm

Analysis of very large multi-reflector antennas like that of RATAN-600 radio telescope presents a formidable computational challenge. At mm-wave frequencies of interest, the RATAN-600 main reflector measures roughly 20,000 by 1000 square wavelengths. Such reflector antennas are often analyzed as quasi-optical systems in two main steps. In the first stage, the field is propagated from the feed to an equivalent aperture using geometrical optics (GO), while in the second, the radiation pattern is computed by aperture integration, which for planar apertures can be accelerated using the Fast Fourier Transform (FFT) (R.E. Collin, Antennas and Radio Wave Propagation, McGraw-Hill, 1985). This line of modeling completely neglects the wave phenomena involved in the inter-reflector propagation and introduces phase errors due to introduction of the artificial planar equivalent aperture required for the radiation pattern evaluation using the FFT.