On the field distribution of an aperture

A study of the field distribution in a square aperture on a plane conducting screen illuminated by a plane electromagnetic wave at normal incidence is presented. In solving the problem, the square aperture is first formulated into a complementary square plate scattering problem. The distribution of current on the plate is then found by solving the corresponding wire mesh model with the aid of a computer program for linear wire antennas. A simple averaging process is then introduced in a straightforward manner to convert the currents flowing along the wires into continuously distributed equivalent surface currents on the square plate. The surface current density is, in turn, equal to the value of magnetic field intensity by the boundary condition for a perfect conductor. Finally, a direct application of the electromagnetic form of Babinet's principle gives the electric field distribution of the aperture. The far zone field in terms of a radar cross section of a square plate compares well with an experiment. A discussion is also included comparing the electric field distribution of a square aperture and circular apertures.