An admittance solution for electromagnetic coupling through a small aperture

Abstract

An admittance solution for electromagnetic coupling through a small aperture in a conducting screen is obtained by the method of moments. This solution gives the equivalent magnetic current in the aperture as a linear combination of the three quasistatic magnetic currents responsible for the three dipole fields seen in the far zone. Because only the dipole effects of the quasi-static currents are known, the elements of the excitation vector and the admittance matrix cannot be evaluated directly. Reciprocity is used to express the elements of the excitation vector in terms of the short-circuit incident fields. The imaginary parts of the elements of the admittance matrix are evaluated by forcing the moment solution to be equal to the solution predicted by conventional small aperture theory. The real parts of the elements of the admittance matrix are extracted from the quadratic expression for the power radiated by the magnetic current. The solution is applied to three examples — an aperture in a conducting plane, an aperture in a transverse wall between two rectangular waveguides, and an aperture in a lateral wall of a rectangular waveguide coupling to a half-space.