Computation of the RATAN-600 radio telescope power beam pattern with allowance for diffraction effects in the southern-sector-with-flat-reflector mode

We report the results of computation of the power beam pattern (PB) of the RATAN-600 radio telescope operating in the Southern-sector-with-flat-reflector (South+flat) mode throughout the entire operating frequency range of the radio telescope with the allowance for diffraction effects in the space between the secondary, primary, and flat mirrors. We also computed the gain-phase field distributions at the mirror surfaces when operating in the South+flat mode and in single-sector observations at the close to horizon elevations. The width of the power beam pattern in the vertical direction is shown to decrease and the maximum of the diagram to shift towards lower elevations with increasing wavelength. When the radio telescope is operated in the South+flat mode the above two changes in the power beam pattern begin to show up at shorter wavelengths compared to the case of single-sector operation. The shift of the power beam pattern maximum is shown to depend on the height at which the antenna is mounted. The power beam pattern was computed both for the case of a focused antenna and for the case of the off-focus primary emitter. We report the results of the computation of the PB performed in terms of the Fresnel diffraction and geometrical optics approximations, and also based on the available experimental data. The diagrams computed using this technique agree well with the measured power beams.