Transmission Loss Due to Resonance of Loosely-Coupled Modes in a Multi-Mode System

In a transmission system, the presence of a region which supports one or more spurious modes can introduce a large change in the transmission loss of the principal mode when the region becomes resonant for one of the spurious modes. This phenomenon can occur even when the mode conversion is low and the waveguide increases in cross section smoothly to a region which supports more than one mode. In general, the conditions required to resonate the various spurious modes are not fulfilled simultaneously and, in consequence, interaction takes place between the principal mode and only one of the spurious modes for each resonating frequency. Under these conditions the resonating environment can be visualized as made of only two coupled transmission lines, one carrying the desirable mode and the other the spurious one. This simplification makes it possible to calculate the transmission loss as a function of (1) the coefficient of conversion between the two modes and (2) the attenuation of the modes in the resonating environment. The theory has shown good agreement with the measurement of transmission loss of the TEoi mode in a pipe wherein a portion was tapered to a larger diameter which can support the TE n « mode. 899