Planar Bragg reflector operating in the sub-terahertz wavelength installed at the upstream end of a sheet beam backward wave oscillator(BWO) is very promising to minimize the whole circuit structure and make it more compact. In this paper, a sub-terahertz wavelength(0.18–0.22 THz) tunable planar Bragg reflector is numerically analyzed by using multi-mode coupling theory(MCT). The operating mode TE10 and dominant coupling mode TE01 are mainly considered in this theory. Reflection and transmission performance of the reflector are demonstrated in detail and the results, in excellent agreement with the theoretical analysis and simulation, are also presented in this paper. Self- and cross-coupling coefficients between these two modes are presented as well. The reflector behaviors with different Bragg dimensions are discussed and analyzed in the 0.16–0.22 THz range. The analysis in this paper can be of benefit to the design and fabrication of the whole BWO circuit.
