A study of coupled dielectric waveguides and their application to millimetre-wave components

This thesis studies the propagation characteristics of coupled dielectric waveguides and their application to millimetre-wave components. Firstly, by applying the transverse boundary conditions, exact characteristic equations for single and coupled dielectric slab waveguides are obtained. By assuming that the loss tangent of the guiding and surrounding region is small, approximate expressions for the attenuation constant are then derived. These expressions relate the attenuation constant directly to the lossless propagation constant and the waveguide parameters; their evaluation is then straightforward. Secondly, by defining an effective loss tangent, the slab guide solutions are used in the effective dielectric constant (EDC) method and the dual effective-index method (DEIM) to yield a simple, yet accurate method for the evaluation of the complex propagation constant of single and coupled dielectric waveguides with rectangular cross-sections. Furthermore, it is shown that the presented results are in good agreement with both finite element values and experimental data obtained using the open resonant cavity technique. By considering transitions between single and coupled dielectric slab guides, and the resulting coupling effects, several dielectric waveguide components are described. Each device relies on a short length of coupled waveguide formed by placing a movable block of dielectric parallel to a dielectric guide. It is shown that if the block is of the same material, and has identical cross-sectional dimensions to the waveguide, the effects of coupling can be exploited to achieve a variable attenuation. It is further shown that the separation at which maximum attenuation arises is dependent on the operating frequency; the structure can therefore be used as a wavemeter. By introducing large asymmetries into the device, the effects of coupling are prevented. Tinder these conditions, the device operates as a low-loss phase- shifter.