An efficient and accurate technique based on the impedance theory of cavity-backed slot antenna is introduced to calculate the resonant frequencies of a rectangular enclosure with a slot on its wall.Expressing electromagnetic fields in terms of cavity Green's function inside the enclosure and the free space Green's function outside the enclosure

an integral equation for equivalent magnetic current is obtained by enforcing the boundary condition across the slot.The Method of Moments is used to solve the integral equation for the input impedance.We can get resonant frequencies from the zero reactance or the minimum resistance on the frequency-impedance curves.Our method is verified successfully by experimental results and CST.The effects of the cavity size

of the slot length

and of the slot offset on the resonant frequency are discussed.