Is it possible to make a solid Faraday cage with a hole for a wire (to transmit information inside)?
What is needed to prevent electromagnetic waves from going through the hole?
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Keith Armstrong over at https://www.emcstandards.co.uk/ wrote together a blogpost on this topic last year, see here: https://www.emcstandards.co.uk/constructing-io-panels-for-shielded-rooms
Citing from the blog post:
There are four ways to get power, data, signals, etc., into a radio-frequency (RF) shielded chamber through an I/O panel without destroying the shielding effectiveness of its external surfaces (i.e. its walls, floor, or ceiling):
- Conducted via filters, which must be 360° shield-bonded to the surface of the I/O panel (either on its inside or outside). See Figure 4.
- Conducted via shielded cables, which must be 360° shield-bonded to the surface of the I/O panel (either on its inside or outside). See Figure 4.
- Radiated via RF in waveguides, which must be 360° shield-bonded to the surface of the I/O panel (either on its inside or outside)
- RF-modulated light in free space or guided by optical fibres, passing through a waveguide-below-cutoff, which is a metal tube that must be 360° shield-bonded to the surface of the I/O panel but can be protrude from the panel entirely on its outside, entirely on it inside, or anywhere between those two extremes.
What's important to remember is that a Faraday cage is a theoretical concept that you will never fully achieve, meaning that you will always have leakage and losses. I've built a DIY shielded chamber for EMC testing purposes (i.e. a Faraday cage), and it works ok. You wont have any cell reception when you're inside the chamber, but there is definitely signals that can penetrate the chamber.
When you try to get a signal in or out from a Faraday cage using a wire, you WILL have leakage in and out from the cage. The question is how you minimize it or keep it below an acceptable level.
A small hole will not affect the functioning of a faraday cage for wavelengths larger than the largest dimension of the hole.
However, a conductive wire from inside the cage, to the outside of the cage which is not electrically bonded to the cage can defeat isolation. It is a pathway for both static electric fields and electromagnetic waves to pass through the cage.
To pass a signal between the inside and outside of the cage without compromising the shielding requires special care. Optical coupling is one simple method of transmitting signals while maintaining isolation. If you know the frequency range of the signals you are trying to block, and your communication signal is outside of that range, you could use passive filters to pass your signal, but block the unwanted noise. (That is essentially what optical coupling does, but with frequencies other than optical).