1. The problem statement, all variables and given/known data For a given rectangular waveguide the cut off frequency of the fundamental mode is 6.5GHz. What is the fundamental resonant frequency of a 30mm long cavity made from the same waveguide? 2. Relevant equations Unsure 3. The attempt at a solution I would have thought that if the rectangular waveguide dimensions didn't change then the fundamental mode would still be 6.5GHz. I am unsure of how the length contributes to the resonant frequency... Any help would be great.