RF shields do not need to be "air-tight"

Posted Nov 22, 2005 2:57 UTC (Tue) by stevenj (guest, #421)
In reply to: RF shields do not need to be "air-tight" by stevenj
Parent article: Richard Stallman's Tin-Foil Hat (Bruce Perens' Journal)

...the amount of radiation that gets through is proportional to the fraction of the surface area occupied by the holes

Whoops, sorry, it's worse than that: the transmitted radiation is proportional to the hole's fraction of the surface area multiplied by (d/λ)⁴, where d is the hole diameter and λ is the wavelength. (H. Bethe, Phys Rev. 66, p. 163, 1944.)

However, once the hole occupies a significant fraction of the solid angle around the antenna (as when you bring your radio close to a window), then the problem completely changes because you are in the near field and you can't treat the incident field as ~constant amplitude in the vicinity of the hole. This shouldn't be the case for an object wrapped in foil, however.