what is waveguide condition?

Hi, what is waveguide condition?

Or what happens to the wave that does not satisfy such condition?

The wave will not go through such waveguide? If so, then where does the wave go?

thanks

An EM field that is not supported in a waveguide structure is said to be below the cutoff frequency. When a signal is injected into a waveguide that is less than the cutoff freq, it will propagate, but not well... it's amplitude will decay rapidly. For signals that are at or above the cutoff, they will readily propagate down the waveguide with low loss.