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Comment by NetBeck | Hacker News Reader

On a similar note, I remember the FCC ordering a Bitcoin miner to turn off mining hardware that was emitting 700MHz and disrupting T-Mobile’s network.[1]

[1] https://arstechnica.com/information-technology/2018/02/bitco...

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I wonder if it would be feasible to build a faraday cage around the miners. A few sheets of metallic cloth and a wooden frame wouldn't be too expensive.

https://www.ebay.com.au/i/184562911018?chn=ps&norover=1&mkev...

Faraday cages are a bit of a myth. Unless the metal sheet is continuous with no gaps, and there are no wires passing through the sheet, they will be ineffective.

It means that all joints must be screwed or soldered, all doors to have tight joints, and low-pass filters fitted to all power leads, comms leads, etc.

And often you need two completely independent layers (connected at only one point) to achieve sufficient attenuation.

> Faraday cages are a bit of a myth. Unless the metal sheet is continuous with no gaps, and there are no wires passing through the sheet, they will be ineffective.

Your microwave oven's cage would like to have a word with you.

You can have an effective faraday cage using a metal fence/mesh depending on the frequency you're trying to attenuate and the thickness of the metal. [0]

[0]: https://www.quora.com/What-size-of-mesh-should-I-use-to-buil...

The GP poster is actually correct; I work with RF engineers. Yes, Faraday cages work but, having tried it, my colleagues have attested that it's much harder to homebrew a fully effective Faraday cage than the average HNer would think, at least in the frequency range we work in.

The types of mitigations described by the GP poster are also correct: see, for example, the description of issues for EMI shielding at https://celectronics.com/training/learning/basics/Noise-Prop...

> "These imperfections are:

(1) openings

(2) conductors going in or out and

(3) finite thickness of the shield"

Faraday cages are imperfect and designing a high quality cage does take work but GP is absolutely incorrect in that they are a "myth," or that they would be ineffective without precision engineering.

700MHz is a 42cm wave. A hole 1/10th of the wavelength is about an inch and a half. Not exactly tight tolerances.

Perfection generally isn't necessary - mitigation is usually sufficient. Reducing the interference signal by some percentage (likely a very, very high percentage) goes a long way.

Dropping a wire cage around a device broadcasting in the 700MHz band is going to very significantly reduce the signal.

I made a bet once, claiming that a cellphone wouldn't work in a microwave (obviously, with the microwave not operating) because it's a Faraday cage. We'd get a call going, put the phone in the microwave and close the hatch. See if the call drops. The tech back then was 900Mhz GSM.

I lost that bet.

While we were at it, we tried all sorts of metal containers one can find in a household, and couldn't find anything that would make that GSM call drop.

Not taking any position on the actual effectiveness of Faraday cages, but I'm pretty sure cell phones can handle a order of magnitude or two (10-20dB) drop in signal strength on a good day, especially if your Faraday cage is also blocking all the background noise the signal would usually be competing with.

> "Reducing the interference signal by some percentage (likely a very, very high percentage) goes a long way."

Sure, that's true. But the point is that even a high percentage may not be sufficient; a 90% reduction only improves the situation by 10 dB; a 99% reduction is an improvement of only 20 dB. As the sibling post illustrates, today's receivers are very, very sensitive; cell phones, I believe, have sensitivities below -80 dBm.

So from what I understand an important part of microwave operation is food soaking up the waves. Never heard any other faraday cages use absorption. Kinda wonder why not.

Have you never wondered why there is a metal mesh across the glass in your microwave? The microwave's faraday cage is visible.

Not really. You can test this by running the microwave with no food in it and noting that you don't lose wifi connection while that happens.

I heard it will damage the microwave. My current one detects the situation and refuses to start actually. And my old one would glitch the wifi with or without food lol.

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