Mode-matching CAD method for broad-wall couplers with square apertures of different size

A rigorous field theory CAD method for rectangular waveguide broad wall aperture couplers is introduced which are coupled by square apertures of different size. Based on the corresponding mode-matching key-building blocks, i.e. rectangular waveguide T-junction and double-aperture step discontinuity, and their modal S-matrix combination, the design takes rigorously into account both the finite wall thickness and the higher order mode interaction between all discontinuities. The presented 5dB-coupler design example in the waveguide Ku-band with a double array of ten square apertures demonstrates the efficiency of the design method. The coupling variation is less than ±0.5dB (together with more than 40dB isolation and return loss) within 11.0 and 14.8 GHz. The theory is verified by excellent agreement with measurements.