Aperture admittance of a rectangular aperture and its use

It has been found that a rectangular aperture not more than one wavelength long in a conducting plane can be characterized by two complex numbers (called the aperture admittances) for the regions on each side of the aperture for the purpose of obtaining an accurate estimate of the tangential electric field inside the aperture. To define these admittances, the aperture is closed with an infinitesimally thin perfectly conducting plate. The aperture admittance for the region on one side of the aperture is that of the magnetic current sheet M₀ placed on this side of the plate. The aperture admittance for the region on the other side of the aperture is that of M₀ placed on the other side of the plate. M₀ is parallel to one of the long edges of the aperture, and along this edge is a simple half-cycle of a sinusoid. M₀ is independent of the coordinate that is measured along the width of the aperture