On the penetration of an electromagnetic field through circular and annular holes

A theoretical analysis is performed in which the strength of an electromagnetic field penetrating through a circular hole in an electronic device is compared with the strength of a field penetrating through a narrow annular slot, the dimensions of the holes being small compared with the wavelength. The aim of the study is to determine what kind of metallic screens are most suitable for protecting devices with such apertures. It is shown that even a very narrow slot betwen an additional disk screen and the main screen practically reduces to nothing the screening effect of the disk. In order to reduce the level of the field penetrating through the aperture, it is therefore necessary to assure a reliable electric contact with the device along the whole perimeter of the additional screen.