 An iris-coupled filter consists of a cascade of impedance transformers in the form of waveguide resonant cavities coupled together by irises. In high power applications capacitive irises are avoided. The reduction in height of the waveguide (the direction of the E field) causes the electric field strength across the gap to increase and arcing (or dielectric breakdown if the waveguide is filled with an insulator) will occur at a lower power than it would otherwise.
