Dominat and degenerate modes in a waveguide
Degenerate modes: -
Whenever two or more modes have the same cutoff frequency, they are said to be degenerate modes. In a rectangular guide the corresponding TEmnand TMmn modes are always degenerate. In a square guide the TEmn,TEnm,TMmn, and TMnm modes form a foursome of degeneracy.
Dominant Modes : -
The mode with the lowest cutoff frequency in a particular guide is called the dominant mode. The dominant mode in a rectangular guide with a > b is the TE10 mode. Each mode has a specific mode pattern (or field pattern).
It is normal for all modes to existr simultaneously in a given waveguide. situation is not very serious, however. Actually, only the dominant mode propagates, and the higher modes near the sources or discontinuities decay very fast.
Example : -
TE10 mode in a rectangular waveguide :