Mode coupling in coplanar waveguide bends: a simple four-port model

This paper presents a novel four-port model for a 90° coplanar waveguide bend based on the decomposition of the waveguide into two coupled slot lines. The effect of the bend discontinuity is that of a non-uniform coupled line system due to the different slot lengths in the structure. The new model consists of uniformly coupled lines and cascaded uncoupled transmission line sections. The parameters for the coupled line system are obtained from the geometry of the bend. The modal transmission coefficients obtained with the model match well with full-wave simulations. The salient feature of the model is the simplified illustration of the frequency-dependent effects of the bend discontinuity. The occurrence of mode-conversion in the two-mode system and its effects on two-port measurements are also described.