High Frequencies and Microwaves
Among the modes that can propagate in a given waveguide the most important is that which has the lowest cutoff frequency. This mode is named the dominant mode in the waveguide; all other modes are named higher order modes.If the actual frequency of the source is higher than the cutoff frequency of the dominant mode but lower than the cutoff frequency of the first higher mode (fig. 1), only the dominant mode can propagate through the waveguide. In practice, this is the usual situation. A simultaneous presence of more than one propagation mode that propagate with different velocities, leads to the apparition of the modal dispersion, degrading the performances of the guide as a transmission channel.
In a waveguide, a forward-traveling wave that propagates from generator toward load is superimposed with a backward-traveling wave, which propagates from load toward generator. In lossless waveguides, the two waves propagate without attenuation.
The superposition of the forward-traveling wave with the backward-traveling wave generates a stationary wave depending on the phase-shifts between the superposing signals. The amplitude of the resulting oscillation varies from one point to another along the line.The maxima and minima of the amplitude distribution issued from this superposition can be found by formulating the phase condition for getting a maximum, or a minimum.
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