ANALYSIS AND MEASUREMENTS OF NONRADIATIVE DIELECTRIC WAVE-GUIDE BENDS

Coupling theory is used to analyze loss characteristics of nonradiative dielectric (NRD) waveguide bends. In addition to the operating LSM//0 //1 mode, the parasitic LSE//0 //1 mode is generated at bends as a result of mode conversion. A rigorous expression for the coupling coefficient between these two modes is derived and then used in the two-mode coupling equations to be solved for the bending loss analysis. Design diagrams for optimizing the strip width against a given curvature radius to build a lossless bend are constructed for 90 and 180 degree NRD-guide bends. It is shown that the field profile at a bend always shifts inwards, confirming experimental prediction. It is found that the periodic spikes on the loss-versus-frequency curve of a bend can be interpreted as being caused by resonances of the parasitic mode between the transition horns fixed at both ends of the bend, and that the true bending loss is normally restricted within a reasonable level. As an application of the theory, a polystyrene lossless 180 degree bend with a curvature radius of 5 mm was fabricated and tested at 50 GHz. The measured bending loss was less than 0. 3 dB.