In this paper, an open cavity is proposed to measure the permittivity of dielectrics. The cavity consists of an ellipsoidal mirror and two planar mirrors. The relationship between the parameters of the beam in the open cavity and the cavity geometrical parameters is presented. The transcendental equation of dielectric loaded cavity is presented, from which the permittivity of the dielectric can be solved. The resonance frequencies of the vacuum cavity and loaded cavity are computed by the resonance frequency formula and the transcendental equation. they are compared to the results from FDTD simulation. The results from two methods are almost same with each other. The advantage of the proposed open cavity over the conventional open cavity composed of spherical mirror and planar mirror is demonstrated.
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