Essentially a piece of metal poised over a ground plane with some dielectric between the to pieces to enhance the capacitive field.
To a first approximation the field lines assummed to be parallel and orthoganel to the live (upper plate). However this neglects the fringe capacitance that case of lesser sized plate can cause an under estimate of the capacitance.
Like wise we need to take into account the plate thickness (slightly affects fringe capacitance) if the metal loss is to be calculated along with the dielectric loss.
Since, at least broadcast frequencies are in use, then frequency dispersion needs to be accounted for, especially since high power means a thick dielectric and a thick dielectric will let dk slip more than a thin one. Just try out a few microstrip calculations to see the effect.
The method of calculation is the same in each case
