WT4 Millimeter Waveguide: TEo1 Transmission in Waveguide with Axial Curvature

The transmission characteristics of the T E 0 1 mode in circular waveguide are quite different from the characteristics of cable or wire transmission media currently used in the Bell System. The waveguide medium has the unique property that its heat loss decreases with increasing frequency. Heat losses1 as low as 0.3 dB/km are obtained at 110 GHz in 60 mm diameter waveguide. However, since the diameter of the 1849 waveguide is much larger than the operating wavelength of 2.75 mm at 110 GHz, several hundred spurious modes can also propagate, in addition to the TEoi mode. The spurious modes have slightly different phase and group velocities than the TEoi mode. Spurious modes are excited in practice because the waveguide is not a geometrically perfect right circular cylinder, and there is a continuous exchange of energy between the TEoi and spurious modes. This mode conversion-reconversion results in added loss and delay distortion. A detailed understanding of the relationship between spurious mode generation and waveguide geometry is required since even small distortions in the guide cross section (of the order of 1 ^m) can cause substantial loss.2 The mode conversion problem can be modeled by an infinite system of coupled transmission line equations3 for which approximate solutions have been obtained by first-order perturbation theory.4 At the beginning of the WT4 field evaluation test the first-order theory was considered to be a valid and accurate method for predicting the TEoi mode conversion loss.