elliptical coupled waveguides as filter

Hello specialists,
I'm studying a filter realized by three elliptical waveguide coupled by a hole on the broad wall and tunable by screws. Each cavity is very similar to the "E250" elliptical cavity from Flexwell?; it has a post thick on the upper side and a hole centered on the broad wall and 3,3 [mm] from the bottom in order to yield a coupling to the other cavity. Opposite to the post there is a tunable screw so to tune the frequency. The first and the third cavity are coupled by electrical probes placed in that way: the first cavity probe is on the broad wall and is opposite to the coupling hole, the third cavity probe is on the thin wall and both are in the same plane of the coupling holes
The system is tunable on the C band.
I suppose each cavity to work on the evanescent mode
I would like to know what kind of field pattern best describes this structure
thanks