Analysis of a multiple-beam waveguide for free-space transmission of microwaves

The transmission of several quasi-optical beams via one system of oversized focusing mirrors is discussed. The analysis of this multiple-beam waveguide shows that such a system can substantially reduce the total mirror surface compared to the corresponding number of individual lines. With the help of analytic coupling coefficients for Gaussian beam modes deduced from the coupling integrals, configurations of four mirrors are found, which give low losses due to the reconversion of spurious modes generated at the different mirrors. Optimization of the mirror surfaces as a function of system parameters is performed and expressions for the loss as a function of the number of transmission channels are deduced. The results are confirmed by diffraction calculations and laboratory scale experiments. The application to a high-power system promises a very efficient transmission with high mode purity.