diff. between cutoff and resonant frequency

I want to design a TEM cell and I have doubts about some definitions. It is not very clear to me what is the difference between cutoff frequency and the resonant frequency of a TEM cell. I found two different formulas, so I suppose there are two different phenomena. Are they related?
Thanks in advance!
D.

I have found myself the answer. For the ones who have the same question as I had, here's the link to an article, explaining it:
http://www.dtic.mil/cgi-bin/GetTRDoc...f&AD=ADA246466