Theoretical method for fast optimization of rectangular transition structure in folded waveguide devices

In this paper, the analytical equations based on mode-coupling theory are derived to facilitate the design process of rectangular transition structure in folded waveguide devices. As an important component of the power coupler, two common categories of structures are considered to match the TE10 modes in rectangular waveguide in folded waveguide slow wave structure into the TE10 mode in the standard rectangular waveguide port. By comparison, the tapered transition structure is much longer than the laddered transition structure with the same reflection requirement, whereas the former has a naturally much broader frequency bandwidth and less sensitivity to configuration errors. As an example, both the laddered and tapered rectangular transitions are designed and error-analyzed for 220 GHz FW Backward Wave Oscillator (BWO). The theoretical method adopted in design process is finally verified by numeric simulation, which shows excellent agreement. The design period based on the accurate theoretical method is only several minutes.