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A flat-roofed sine ridge waveguide (flat-roofed SRWG) is investigated for application in a backward wave oscillator (BWO) operating in the range from 300 to 350 GHz. The truncated ridge structure helps to intense the longitudinal electrical field of the slow wave structure (SWS), then the interaction impedance of flat-roofed SRWG is at least 33% higher than that of the SRWG and much higher than that of the sine waveguide (SWG) with the same dimensional parameters except the ridge over the operating frequency range. When the tube length is the same, the output power of the flat-roofed SRWG is correspondingly at least 21.3% higher than that of the conventional SRWG. An ultra-wideband sapphire Brewster window is designed for such BWO, with a simulated insertion loss of less than −2.36 dB in the frequency range of 300–350 GHz. The flat-roofed SRWG and Brewster window were fabricated by using the nano-computer numerical control (nano-CNC) milling techniques, and their performances were cold tested by using a vector network analyzer (VNA). The measurement results were in good agreement with the simulations and demonstrated that the flat-roofed SRWG and Brewster window achieved a wide frequency bandwidth of ~50 GHz (300–350 GHz) with an insertion loss better than −5.38/−4.77 dB and a reflection coefficient ${S}_{{11}}$ lower than −17.5/−14.6 dB, respectively.