The effects of mode conversion in long circular waveguide

Coupling to spurious modes at imperfections in a long circular waveguide may cause the TE₀₁ loss to increase and to fluctuate randomly with frequency. The statistical properties of the TE₀₁ loss-frequency curve can be expressed in terms of the statistical properties of the random geometric imperfections of the guide and therefore in terms of the mechanical tolerances of the waveguide. Such imperfections as poor joints and elliptical waveguide have negligible effect under reasonable tolerances. However, random deviations of the guide axis from a straight line may have very serious effects under certain conditions. Experimental measurements verify these conclusions.