The phase of the reflection coefficient of a TE10 rectangular waveguide mode at the cut-off point in a gentle downtaper is investigated through both experiment and computer simulation. The result shows a very good agreement with the theoretical prediction based on the work by Katsenelenbaum et al., that is, a +90° phase shift occurs at the cut-off point for TE modes if the cut-off point is not too close to the end of the downtaper. An application for the determination of the resonant frequencies for the spurious trapped TE30 mode in an uptaper-downtaper oversized resonant structure is presented.
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