HE₁₁mitre bends and gaps in a circular corrugated waveguide

The HE11 transmission loss in a mitre bend or gap in a corrugated waveguide is derived using mode matching techniques. For a gap length L in a waveguide of diameter D = 2a at a wavelength λ, the predicted loss for D ⋙ λ is approximately l·7[Lλ(2a2)]3/2dB. Measured data versus gap length oscillate about this predicted value due to the diffraction of very high order modes. For a mitre bend, L = D. The loss in mitre bends where the corrugations are continued up to the mirror should be reduced by half from the above formula due to the reduced diffraction. Measured data on mitre bends that are in agreement with these expectations are presented. In addition, the design of a mitre bend assembly for a 1·25 in waveguide at 110 GHz using an HE11/HE12 mode mixture to reduce the diffraction is described. The measured HE11 transmission losses in six of these assemblies in series were 0·46% and 0·54% per bend for H-plane and E-plane orientations, respectively. The inferred mode conversion loss of 0·38% is less than one-third that of a simple mitre bend. The inferred ohmic loss of 0·08% for H-plane bends is in reasonable agreement with theory.