A transmission line-to-waveguide transition that includes a microstrip impedance transformer for matching the impedance of an input transmission line to that of a flared slotline is disclosed. The slotline's width is sufficiently small such that when the transition is inserted into a waveguide the slotline is spaced inward from the waveguide's inner walls. A balun bi-directionally couples the unbalanced signal on the microstrip to a balanced signal on the slotline. The signal propagates along the slotline and is capacitively coupled to the waveguide. A trimmable tuning stub is used to adjust the resonant frequency of a parasitic cavity formed between the transition and the waveguide to increase the transition's effective bandwidth. A tapered dielectric insert is positioned inside the waveguide to decrease its size and to improve the coupling efficiency of the transition.