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There seems to be conflicting information regarding which signals can be effectively blocked by wrapping the device in aluminum foil.

With effectively blocked I mean block the signal as such that the service becomes unavailable.

Faraday bags exist, but they have been shown to be rather expensive and will become unusable after wear and tear. Also, most bags do not specify which signals are reliably blocked by the bag.

Aluminum foil seems a viable alternative, but I could not find any reputable source investigating the issue.

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    \$\begingroup\$ Wouldn't this be easy to check? Get yourself a big roll and cell phone and have at it! \$\endgroup\$ Commented May 25 at 22:49
  • \$\begingroup\$ @StainlessSteelRat My experiments yielded the following results: Mobile phone reception is blocked, Wifi isn't. Could not test GPS however. I have read that GPS is blocked too because it is a weak signal, but it was not from a reputable source. \$\endgroup\$ Commented May 25 at 22:51
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    \$\begingroup\$ Aluminium foil supplanted tin foil in the mid 20th century. In the United Kingdom and United States it is often informally called "tin foil". It might make a difference if you are looking up the electrical properties of the foil. \$\endgroup\$
    – Transistor
    Commented May 25 at 22:53
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    \$\begingroup\$ Add this to your question to show what effort you have exerted to the problem. Download a wifi strength app and see if there is a change in signal strength. \$\endgroup\$ Commented May 25 at 22:54
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    \$\begingroup\$ @StainlessSteelRat Changed to aluminum foil, which is what I tested it for. \$\endgroup\$ Commented May 25 at 22:55

2 Answers 2

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Tim is absolutely correct. I am an EMC engineer and use aluminum foil on occasion to wrap equipment to prove whether emissions of equipment are from the unit or its power cable (or other cables).

In theory if the aluminum shield were perfect this would give infinite attenuation at all frequencies, but in practice this isn’t the case. If the foil is well wrapped and taped at all the seams I can usually get in the order of 40dB of attenuation up to ~1Ghz. Past that the attenuation drops because the wave lengths get shorter and any small seam acts like a slot antenna (albeit a poor one).

So to answer your question, all of those signals can be blocked reliably by aluminum foil if you are diligent about how well you wrap them and close the seams. Oxidation is also an issue because you need good contact at the seams, but in practice this is not normally a much of issue unless you want very high attenuation or you are working at very high frequencies.

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    \$\begingroup\$ Just to clarify. You use seam in the sense of gap? \$\endgroup\$ Commented May 26 at 9:23
  • \$\begingroup\$ Yes the seam is must be perfectly closed for perfect results. This means the oxidation would need to be removed and a good bond made between the pieces by soldering them together for a near perfect result. I also should mention that receive signals are obviously much weaker than transmitted signals, so this technique will work much better on received signals than transmitted ones. \$\endgroup\$ Commented May 27 at 2:20
  • \$\begingroup\$ @EricEverton Whether RX or TX is affected more by shielding depends on the environment. If the signal is limited by noise level rather than receiver sensitivity (like e.g. wifi often is), shielding can affect TX more than RX. On RX side also the noise level drops and SNR remains the same, but on TX side the other end still hears the same amount of noise as before. \$\endgroup\$
    – jpa
    Commented May 27 at 6:42
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    \$\begingroup\$ @CuriousIndeed: Adding extra wraps of foil will only give you marginally better results. The issue is not that you need more material, but rather that you need a better sealed enclosure. Imagine the unit as if it were spewing water in all directions. Adding more foil wouldn’t help plug the gaps and seams very much. Instead you would need to ensure that there was no gaps anywhere for the water to leak out. \$\endgroup\$ Commented May 28 at 0:18
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    \$\begingroup\$ @EricEverton Actually you do not need contact at the seams. Overlap with a thin insulating gap (like oxide) is enough! Obviously for the $E$-field this would be enough since the capacitance would effectively close the gap, but also for the \$B\$ field this works. It's not as good as a real contact but whenever I tested it (having worked on EMC myself) it worked well enough for the 20 ... 40 dB attenuation range. Of course not for much higher attenuations. \$\endgroup\$ Commented May 28 at 6:21
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Aluminum foil is more than thick enough to block plane waves of these frequencies (ballpark 1GHz) to a high ratio.

It is not easy to employ in practice, however. Seams are transparent to RF fields, unless they are a particular width of overlap, or the seams have been shorted over with conductive EMI tape.

Note that it's not enough to simply lay foil together, or use generic aluminum (actual-duct) tape: the surfaces may be coated or oxidized, or a layer of adhesive insulates the foil backing. Special care must be taken to create a contiguous conducting shield.

Finally, for full shielding, the device to be shielded must be sealed inside the shield. A room lined with foil must be completely lined, floor to ceiling, with no gaps or seams, and any openings, doors, windows, etc. must be covered over with mesh or foil, and spring strip around the seams so they seal when closed every time. Outlets, lights and other connections that penetrate the shield, must be filtered for RF, lest they carry signals in as well.

To enclose just a single device, the best bet is to make a foil pouch by folding a sheet in half lengthwise, then crimping the sides down, and finally crimping down the end. Hopefully, the crimping action creates an electrical seal, but you will need some way to verify that this is the case.

Even a five-side-sealed pouch geometry isn't enough, because as long as the opening of the pouch is wide enough to transmit wavelengths of interest, energy can be exchanged with the device within. The frequency of 5G for example might be 5GHz or more, and the critical opening width (wavelength) is just a few inches.

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    \$\begingroup\$ +1 Aluminum exposed to air quickly develops an oxide layer, meaning it's difficult to get good electrical contact. A seam is often a good slot antenna, so crimping and multiple folds are probably necessary to ensure good contact and lower chances of leakage. \$\endgroup\$
    – John D
    Commented May 26 at 1:13
  • \$\begingroup\$ And do you think OP was talking about an EMC testing enclosure or room? I mean, WiFi can lose connectivity with just a couple of sheet rock or plaster walls between the two nodes. \$\endgroup\$
    – SteveSh
    Commented May 26 at 1:25
  • \$\begingroup\$ @SteveSh Got it. A couple inches of sheet rock can absorb or reflect well, but it entirely depends on the environment: there might be multipath through open doors or even windows, for example. And reception is simply worse in poor environments, say if all the neighbors have their WiFi on full; a household situation isn't very well controlled. The description of a test enclosure (of sorts) helps to illustrate the importance of assembly (at least, that's the hope). \$\endgroup\$ Commented May 26 at 3:26

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