Evanescent Mode Attenuation

does anyone know how to find the attenuation for an evanescent mode rectangular waveguide? i've looked everywhere, but have no idea. my guess is:

j*k * sqrt(1-(fc/f)^2).

that would mean that the solution in dB/m would be extremely large, right?

When you are below cutoff the attenuation/length is severe. Also the propagation constant makes the wave travel unusually slow.

If you are considering using this method for a known attenuation I would advise against it. It is more reliable to use a portion of line with a carbon plane that is inserted into a slit in one wall of the guide.