Screening Properties of a Squirrel Cage of Wires

In this paper is calculated the energy radiated by a long current filament which is screened by a squirrel cage of equally spaced thin wires. It appears that the screening ratio depends on two distinct factors; one is the peripheral spacing between the wires of the cage and the other is the radius of the wires themselves. If the peripheral spacing is not greater than about about ¿/10 then the radius of the wires is likely to be much more important than the space between them. If the wires are not too thin the screening effect is out of all proportion to the space blocked up by them: this is reminiscent of the properties of the grid of a triode valve, a problem which is a particular case of the present one. To obtain good screening effect the radius of the wires should not be less than about ¿/100. Provided this condition is fulfilled then the energy radiated from the screened filament will be only of the order of 2% of that from the unscreened filament even when the peripheral distance between wires is as much as ¿/10, whatever the radius of the cage. Thus a comparatively open cage is found to be a very effective screen, provided the radius of the screen wires is not excessively small. The analysis will not be exact when the radius of the wires is comparable with the distance between them, for then the current distribution round each wire will not be uniform: the wires must be thin in this sense. This analysis should form a useful qualitative guide to the screening effect of the screens of finite dimensions used in practice. The equations required to solve this problem lead readily to a resonance condition for a coaxial chamber: it is shown that this condition tends very rapidly to the radial distance between the inner and outer conductor of a coaxial box being one half of a wavelength. As a final example the Q is calculated of a cylindrical chamber having no inner conductor.