The design of waveguide-to-finline taper transitions at millimetre wave frequencies

Traditional techniques for designing the waveguide-to-finline taper transitions are based on the design a suitable impedance transformation profile first, and then the conversion of this profile into a finline contour. These techniques disregard the electrical discontinuities involved in the transition, such as the step model of two finlines with different widths. This introduces a phase error in the design, because the propagating wave in the taper is not a plane wave if the gradient of the finline contour is not zero. The change of the slotline gap causes changes in the impedance and propagation constant of the slotline, and thus alters the phase in the travelling wave. In this paper, it is demonstrated that there are significant effects related to the electrical discontinuities in the taper shape and that a better understanding of the discontinuities is needed. By compensating the taper phase errors, the return loss could be improved.