A new configuration for a flushmounted resonator antenna is presented in which the antenna is formed by a conducting cavity with a dielectric post, and radiates through an annular slot in the upper ground plane. An aperture magnetic current integral equation is used to model the slot, and the effect of the post resonator and cavity are included via Green's function techniques. The formulation is presented for the azimuthally-independent TE and TM cases. The moment method is applied to the integral equation, and the complex resonant frequencies for the antenna structure are found by searching for the zeroes of the determinant of the impedance matrix in the complex-frequency plane. The radiationQfactor, slot-field distribution, and radiation field are subsequently computed at the resonant frequency. Various TE and TM modes have been investigated for different geometrical configurations.