The Development of Circular Waveguide TEOON Mode Filters

Unwanted circularly symmetric modes of the TE^O_ON family are generated at discontinuities in a circular waveguide and can cause serious impairment within the guided millimetre-wave transmission system. This paper describes the development of a TE^O_ON mode filter which operates using the phase difference between two or more sectorial waveguides, and provides cancellation to the undesired TE^O₀₂ and TE^O₀₃ modes whilst passing the wanted TE^O₀₁ mode. To establish the principle, an experimental model of a two-section filter has been constructed and its TE^O₀₁ and TE^O₀₂ mode transmission performance has been measured. TE^O₀₁ and TE^O₀₂ mode transmission losses of ⩽0.3 dB and ⩾10 dB for a 12 GHz frequency band centred at 36 GHz respectively have been measured. Ultimately, a broadband, 30 to 110 GHz, four-section filter is proposed which exhibits a theoretical TE^O₀₂ mode transmission loss of greater than 20 dB.