Note on the radiation associated with the excitation of an open resonator

Open resonators have a number of possible applications at microwave frequencies; the very high Q factor obtainable is one attraction. Coupling to the resonator usually takes place through small coupling holes in the mirrors. An important practical question is to what extent radiation of energy out of the resonator by these holes limits the Q factor obtainable. In this letter a very simple formula is derived. The effect is shown to be very small.