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Suppose I excite waveguide with frequency less than cutoff frequency. According to solutions of Maxwell equation energy of the wave must decay exponentially. Where does the energy disappear or lost in this case?

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  • $\begingroup$ Your title and question are opposite (i.e., above and below the cutoff frequency, respectively). Which one are you curious about? $\endgroup$ Commented Oct 6, 2016 at 14:30
  • $\begingroup$ Thanks edited the question ? $\endgroup$
    – Sri Ram
    Commented Oct 6, 2016 at 14:32
  • $\begingroup$ And the answer is, your wave guide gets hot. $\endgroup$
    – Jon Custer
    Commented Oct 6, 2016 at 14:32

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Waves in a waveguide with frequencies below the cut-off frequency decay exponentially but not due to any energy loss. Waves below cut-off in ideal, perfectly conductive waveguides do not transport any energy. This is similar to evanescent wave penetration in total reflection. In real, imperfect waveguides, however, there is a small energy loss due to dissipative conductive losses of the associated currents in the waveguide walls.

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