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What makes a good Faraday cage?

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What makes a good Faraday cage?

Old 27th Jan 2018, 16:21
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What makes a good Faraday cage?

A few months ago I made a lead box key holder a little bigger than a pack of 20 cigarettes to act as a Faraday cage in order to prevent my car keys in my tent from allowing access to my adjacent car. The box was made as two 5 faced rectangular trays which are a snug fit one inside the other. This proved to be quite effective at shielding the keys to my older "keyless go" car but I have found that it is not fully effective when used with my new car keys. The lead sheet used is a little over 1.5mm thick.

Would I have been better to make it of another metal sheet? Maybe copper?

I have tried putting the keys inside a posh Barley Sugar tin which is made of thinner steel sheet and this is less effective than the lead box.

What are the rules of thumb for this situation? Thickness? Conductivity of metal? Dimensions relative to wavelength? Moon phase?

Andrew.
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Old 27th Jan 2018, 17:06
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Definitely better conductors will work better. Therefore copper is best, or aluminium if weight is an issue.

The important thing is to have no gaps through which the radiation can travel, and your one tray inside another approach should do this, but I'd still feel happier with finger-strips where the two parts meet. Maybe you could experiment a bit with kitchen foil - try more than one layer - apparently shoplifters line shopping bags with it to defeat the security devices on high-value goods.
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Old 27th Jan 2018, 17:17
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Originally Posted by UniFoxOs View Post
Definitely better conductors will work better.
... ... ...
Daniel Barenboim, Charles Mackerras, Thomas Beecham, Sir John Barbirolli, Claudio Abbado ... ?

Sorry! (Coat, hat ...)
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Old 27th Jan 2018, 17:19
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Any good conductor will work well for the E part of the EM, but if you are going to be pretty close to the car, close enough that the M part may just be enough, then I'd suggest making the box from Mu Metal, as that will provide good magnetic as well as electrical screening. The snag is that Mu Metal is expensive.

If it were me then I'd just hunt around for a small tinplate box, like the ones you used to get small sweets in, and use that. A Minty's or Altoids tin would be ideal (both US products, so not easy to find here), if your fob will fit in it. As these tins are popular for electronic projects, you can buy blank ones, and there's at least one UK seller: https://thepihut.com/products/adafru...-gum-sized-tin
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Old 27th Jan 2018, 17:33
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The snag is that Mu Metal is expensive.
It's not THAT expensive in small quantities. The real problem, as I discovered after I'd bought some to screen some delicate electronics, is that to make it effective after construction you have to anneal it in hydrogen at something like 800C. Good luck doing THAT at home.
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Old 27th Jan 2018, 18:54
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VP959's suggestion of a steel box is excellent. On a pedantic note, since the keyless fobs operate at 300-450MHz depending on locale, almost any metal with sufficient thickness to be mechanically strong will work, provided there are no gaps. The skin depth (or equivalently penetration depth) for e.g. Cu or Al is a few micrometers, so a few tens of micrometers thickness will attenuate the RF by more than 10000X. Ordinary steel is about 10X better than Cu, because it is ferromagnetic.

For electromagnetic shielding, there is little value in using mumetal at frequencies above a few hundred Hz, because practical thicknesses of steel are already effective. Lead is not a good choice, because it has little mechanical strength, and would be prone to gaps. Note that 4 layers of ordinary household Al foil would be sufficient.
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Old 27th Jan 2018, 19:41
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Reckon gaps no bigger than 0.1lambda and good bonding, remembering skin effect and poor joints give junction effects and non-linearities which generate harmonics.
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Old 27th Jan 2018, 19:58
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A mate that does quite a bit of camping and traveling about in his modern car uses a Band-Aid tin. These things are made out of steel.

Uncoated lead may not be the best of metals as you may want to wash your hands after handling it.
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Old 27th Jan 2018, 20:01
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Given the crapness of the radio comms of all the car keys I've ever had, I can't help wondering whether people may be over-thinking this? The original question was about being in a tent: if this is a tent in the UK it will be at least slightly damp, if it's not actually raining right now, and surely that's enough of a Faraday cage?
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Old 27th Jan 2018, 20:48
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Originally Posted by Trossie View Post
Daniel Barenboim, Charles Mackerras, Thomas Beecham, Sir John Barbirolli, Claudio Abbado ... ?

Sorry! (Coat, hat ...)
Which reminds me of this one I haven't heard for a few decades, could just be new to younger readers here (are there any??) ...

Passenger puts one foot onto the platform of a bus. Conductor rings bell, bus pulls away, passenger falls underneath the following lorry, gets killed.

Company commiserates with the conductor for having witnessed such an unfortunate accident.

Next day, a second passenger puts one foot onto the platform of the bus. Conductor rings bell, bus pulls away, passenger falls underneath the following lorry, gets killed.

Company warns conductor that he really does have to be more careful.

Next day, a third passenger puts one foot onto the platform of the bus. Conductor rings bell, bus pulls away, passenger falls underneath the following lorry, gets killed.

This time the police take an interest, conductor is convicted of murder, and sentenced to the electric chair.

Executioner presses the button. Conductor just grins. WTF??

Executioner presses the button again. Conductor laughs out loud.

Executioner presses the button a third time. Conductor widdles himself laughing.

"WTF?" askes the executioner, well aware of the rule that after three failed attempts the prisoner goes free, "how did you survive that?"

"Easy peasy" comes the response: "I'm a bad conductor".
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Old 27th Jan 2018, 21:20
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VP, I tried selling some tobacco tins on eBay as either vintage artefacts or keysafe. Oddly no interest at all. They have gone to charity's.
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Old 27th Jan 2018, 21:41
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Actually the problem is that at UHF frequencies even a tiny 'slot' (couple of cm) where the lid and tin don't completely touch is enough to let the radio signal through and even make the outside of the tin a pretty efficient aerial. Professional 'box' designs use many flexible metal fingers to ensure perfect contact every few mm.

In the normal everyday world the pouch /pod solutions are still not 100% efficient but scrunching tin-foil tight round the keys works every time.
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Old 27th Jan 2018, 22:05
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To be strictly accurate, the slot doesn't let the signal 'through' in the sense of it squeezing though the gap which will usually be only a tiny fraction of a wavelength.
What happens is that the circulating currents inside the cavity and on its surface meet a discontinuity at the slot, and the different potentials each side of the slot cause it to radiate. Although the contact fingers do block most of the gap, their significant function is to equate the potentials on each side to stop it radiating.
Slot antennas can be very useful when a low physical profile is required.
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Old 27th Jan 2018, 22:13
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Would a tin within a tin be more effective?
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Old 27th Jan 2018, 22:22
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Why not? You could market it as Tin Tin
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Old 27th Jan 2018, 22:32
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I'm not an avionics person, but those I know use special bags for ESDS
(Electrostatic Discharge Sensitive) components, to protect the contents from unwanted radiation. I wonder if one of those would isolate the key fob ?

If so, you could line the box with the bag material, or just use the bag.
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Old 27th Jan 2018, 23:10
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ESDS Bags dont protect against radiation, they are conductive so bleed any electrostatic charges away from the contents.

To shield your keys, wrap them in alfoil, and then put in a tin box. A Mylar bag may help as well.
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Old 27th Jan 2018, 23:46
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Just drop your keys into the same glass of water as your dentures . This will also guarantee that your wonder of modern tech doesn't try to reverse park itself unaided at 3am in the morning.

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Old 28th Jan 2018, 00:34
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Is it just an American thing, preferring Can-can to Tin-tin?
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Old 28th Jan 2018, 03:30
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Can't you just disablethe the electronics in the car? Pull a fuse, find a secret switch or something, and then use the normal key.

My vehicle has a switch under the steering column to turn off the keyless entry system to prevent it draining the battery if the car is not to be used for an extended period. It is not obvious, not labeled, and you would not know it existed without a careful search of the owner's manual.
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