When the Method of Moments (MOM) is applied to analyze problems involving metallic cavities, one can choose to use either the free space Green's functions [1] or the Green's functions of cavities [2]. Relatively speaking, the Green's functions of cavities are more computationally efficient, in that the number of discrete unknowns is minimized. However, if the Green's functions of cavities are directly applied, MOM would fail around the resonant frequencies associated with the cavity, since the Green's functions are singular at these resonant frequencies. Indeed, as long as the cavities are not fully enclosed by perfect metal, these singularities do not exist physically hence can be avoided numerically. In this paper, a technique is proposed to numerically remove the singularities through re-arrangement of discrete unknowns, so that MOM yields correct results for all frequencies. This technique is presented using rectangular cavity backed slot (CBS) antennas as examples, and validated by experimental data.
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