Hacker News Reader

Comment by WatchDog | Hacker News Reader

The section on trying to attenuate outside wifi signals interested me.

There is a bunch of hand wavy information on building faraday cages online, some people suggesting to utilize a microwave oven, since they operate at the same frequency.

There are even wifi faraday cages for sale on amazon.

However I can't really find much actual benchmark data online about how well these various approaches actually attenuate signals.

View on HN

Because the stuff on amazon is mostly targeted at consumers, not professionals.

Less about protocol development and more fear of 5G and wifi.

The prices of this things can be quite high. Which is why I bought my small one as used surplus.

One can find a great amount of models from various manufacturers here: https://www.everythingrf.com/search/shielded-test-enclosures

And none of them look cheap. But more "please call for quote"-type of things.

If the product doesn't list some form of dB loss (at at least 1 frequency) I assume it's more or less just a standard box. Sometimes that'll be right, sometimes it won't. Some of it comes down to how you use the box as well. If you're feeding un-isolated power in via a big hole in the back then it doesn't really matter how perfect the front is at blocking signals.

I'm surprised their paint can only gave them a 10 dB difference. I've found simply wrapping things in aluminum foil is good for about 40 dB when it comes to Wi-Fi.

At least for microwave ovens, with domestic models reaching 1 kW and commercial (e.g. in food) 2 kW, the shielding on these needs to be pretty darn good to make sure that someone standing right next to it doesn't get their body fried. The maximum allowed leakage power density is 5-10 mW/cm² [1], according to a 2004 study even then most ovens barely get up to 1% of the legally allowed emissions limits [2, page 7].

Out of random interest I tested the "put a phone in it and call it" and it didn't work (as did wifi) that's mentioned in [2].

[1] https://www.hea.de/fachwissen/mikrowellen/sicherheit

[2] https://www.sfu.ca/phys/346/121/resources/physics_of_microwa...

Don't underestimate the dynamic range of a modern wifi transceiver. It is transmitting at ~20dBm and will receive down to at least -80dBm. You need to achieve >100dB attenuation, well beyond what a microwave oven provides.

Even a solid metal box with a few small holes will struggle.

A microwave oven is an excellent (30-40dB) attenuator specifically around 2.4GHz. The legal requirements essentially mandate 30dB + manufacturing safety margin. Your mileage will vary for other frequencies, as they're not actually faraday cages, so it's usually easier to simply use aluminum foil or a copper mesh/PVC box depending on your needs.

Thing is, 30-40dB is not sufficient to properly block wireless connections, they can still work with 40dB attenuation.

To add on to this - I just tested with my home microwave - I get 35-38dB attenuation, but I can still quite reliably get wifi packets to/from an ESP32 in that microwave.

Maybe you are looking for something like this?

https://www.mattblaze.org/blog/faraday

It's a few years old, but seems pretty thorough.

Not sure on specific numbers but based on the picture he posted of it in a can he would have probably done better with metal netting from a window screen or metal chicken wire it destroys wifi signal pretty damn effectively

Reply on news.ycombinator.com