Good faraday cage to block bluetooth beacons

Question

For a project I need to test often with bluetooth beacons linked to an iOS app.

When I'm at home (office at home) I like to reduce exposure to radiation as much as I can. Not in a obsessive way, but just preventing letting any radiosignalling devices constantly bounce through the air.

I currently have only 3 beacons here, but soon might hold many more. Removing all their batteries is honestly too much of a hassle because of the way the bodies of the beacons are produced.

So I tried to make a small faraday cage, but actually am not really sure what parameters to take into consideration. Online you read a lot of conflicting things (ground or not to ground, multi-layering principals, size and conductor thickness).

I have this test-setup:

I'm running my app to scan for one of the beacons and clearly says around -70 decibels. When I put the lid on the box I'm already seeing drops to -90 and worse, sometimes even disappearing fully from the ranging in the iPhone that I'm testing with. Finally when I wrap the outer (big piece of) foil I'm getting a certain stop of signal. The iPhone fully fails to see the beacon, even with the phone literally laying on the box.

My question; what do I need to take into consideration on having this being most effective? For these three beacons, but also having a lot of them in a later stage. Also is there any form of electrical/magnetic buildup inside the box? Are there dangers?

Answer 1

Any continuous conductor should do a good job of blocking the signals. The weakest area in your setup is where the lid connects to the container. Make sure you have good electrical contact in this area. If you can find a large metal cooking pot somewhere, that would make a good faraday cage (if you have more beacons). Make sure the lid makes good, bare-metal contact with the pot itself. Make sure the top is metal.

As far as dangers go, I don't know of any actual dangers. But since the beacons both transmit and receive, perhaps the receivers could be damaged by placing a large number of units very close together? I just don't know.

Answer 2

As @mkeith says, any electrical conductor will block radio signals. Fine wire mesh does just as good of a job for high frequencies as tin foil.

However I would like to target something else in my answer. You say that you are receiving a signal strength of about -70dB - presumably that is -70dBm (the m is important). In that case the receive power is only 100pW (picowatt). At -90dBm it is only 1pW. You are probably being exposed to hundreds of times that power continuously from any Wi-Fi network or mobile phone in your house and likely your neighbours house too.

Furthermore, the beacons will not be transmitting continuously. They transmit for only a few milliseconds at some update interval (typically 1-10Hz), so the average power will be even less.

Basically what I'm trying to say is there is little point going to the effort of shielding them, there are so many other radio signals around that it will make no noticeable difference at all to the amount of energy in radio waves flying around in your house. Unless of course you are also going to wrap your entire house in tinfoil.

Answer 3

You can purchase ready-made pre-tinned shields, known as "cake pans".

Make sure to get the tinned ones, not the teflon-coated or aluminium ones. Useful for testing low noise stuff. Easy to drill, stick BNCs and feedthrough caps in the edges, etc. Flip the top one, put it on the other, clamp, you get a shielded box.

Cookie tins work too, but are usually painted, so you got to sand off the varnish on the edge where it has to make contact with the lid. That's more work, but you get chocolate.