Rigorous design of sidewall aperture couplers

A rigorous field theory CAD (computer-aided design) of rectangular waveguide sidewall aperture couplers coupled by rectangular apertures of different sizes is introduced. Based on the rectangular waveguide T-junction and the double-step discontinuity key-building block modal S-matrices calculated by the mode-matching method, the design rigorously takes into account both the finite wall thickness and the higher-order mode interaction between all discontinuities. Design examples for rectangular aperture 10-dB, 8.34-dB, and 3-dB couplers in the waveguide Ku-band (12-18 GHz) demonstrate the efficiency of the design method. For computer optimization, the evolution strategy method is applied. As the initial values are derived by using the classical network theory synthesis methods, relatively fast and very reliable designs are achieved. The theory is verified by excellent agreement with measurements.<>