# The Naked Scientists Forum

### Author Topic: How do you Calculate the Effectiveness of a Faraday Cage?  (Read 617 times)

#### Simple Simon

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##### How do you Calculate the Effectiveness of a Faraday Cage?
« on: 24/07/2016 13:06:32 »
In short: I'm wondering how many watts (approximately) of energy it takes, for charge to break through a Faraday cage. Specifically with aircraft. Whether it would be tens, or hundreds, or even thousands.

This is more direct than my other questions. I want to work out how effective a Faraday cage UAVs and other aircraft have. More specifically, how much charge you need to hit them with till you can get through the Faraday cage.

Microwave jammers capable of messing with aircraft have been gaining interest lately.

#### evan_au

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##### Re: How do you Calculate the Effectiveness of a Faraday Cage?
« Reply #1 on: 24/07/2016 22:58:25 »
The effectiveness of a Faraday Cage doesn't just depend on power, but also on frequency and time.
At low frequencies, the mesh can be fairly coarse, while maintaining protection.
However, higher frequencies will find their way inside through the mesh, or through seams between sections of mesh.

Joints between sections of mesh will have a maximum current carrying capacity, and will burn out if the current is maintained for too long; fortunately, lightning only continues for tens of microseconds. So duration is also important.

Note: Do not mess with aircraft - authorities jump on people who shine laser pointers at aircraft; microwave weapons will be viewed even more dimly - especially by their operators or nearby bystanders, who are likely to fry their eyeballs. High intensity microwaves are best kept inside a sealed oven.

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#### alancalverd

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##### Re: How do you Calculate the Effectiveness of a Faraday Cage?
« Reply #2 on: 24/07/2016 23:20:38 »
And what, exactly , would you  hope to jam? The only microwave gear in a plane is the radar system, which is of tertiary importance and frequently temperamental anyway. Or the oven, which will just annoy the first-class passengers - the rest of us eat steam-reheated meals or sandwiches.

A lightning strike delivers around 10^10 to 10^15 watts and is pretty harmless. I was in a 737 struck twice whilst descending to Dublin a few years ago: the only real problem was that the thunderstorm deposited 5 cm of hailstones on the runway between final turn and touchdown, adn that can indeed be lethal.
« Last Edit: 24/07/2016 23:24:10 by alancalverd »

#### wolfekeeper

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##### Re: How do you Calculate the Effectiveness of a Faraday Cage?
« Reply #3 on: 28/07/2016 00:54:57 »
The main practical problem of lightning strikes for Faraday shields is that magnetic fields aren't shielded, only the electric field; you've got a massive current flowing over the outside of the shield, and although it cancels to some degree, it WILL penetrate the shield and ground loops and similar can be problematic, but shielded and twisted cables are proof against it.

#### evan_au

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##### Re: How do you Calculate the Effectiveness of a Faraday Cage?
« Reply #4 on: 28/07/2016 09:26:50 »
Quote from: wolfekeeper
shielded and twisted cables are proof against it
Optical fibers and error-correcting protocols also help...

#### The Naked Scientists Forum

##### Re: How do you Calculate the Effectiveness of a Faraday Cage?
« Reply #4 on: 28/07/2016 09:26:50 »