Just knocked up a faraday cage from an old metal bread tin. Made the lid a good fit and managed to get an electrical fit, and the tin conducts current all round. Now whilst it seems to block mobile phone signals it does not block 2 way radios. I assume there is a good technical reason for this, but not my strong point. Does anybody have any ideas about this ?
Faraday cage
- Thread starter Alvariam
- Start date
You can probably find all you want to know and more by searching up on EMC enclosures, which is what you have tried to make.
Are your 2-way radios on UHF or VHF. The answer to why the phone was blocked is likely to be in the frequency used, and possibly the power of the transmission, a cell tower x miles away vs a radio just a few feet away.
I am not strong on electrical engineering, but I know that merely getting the lid and body of a box to be electrically connected is not enough to reliably block electromagnetic waves. Get a thin gap and it can act like an antenna for the right wave. The usual way is to have a lid that fits the body to form a convoluted path, and/or to use a conductive gasket between the two.
I am curious why you decided to make one?
ATB
Chris
Are your 2-way radios on UHF or VHF. The answer to why the phone was blocked is likely to be in the frequency used, and possibly the power of the transmission, a cell tower x miles away vs a radio just a few feet away.
I am not strong on electrical engineering, but I know that merely getting the lid and body of a box to be electrically connected is not enough to reliably block electromagnetic waves. Get a thin gap and it can act like an antenna for the right wave. The usual way is to have a lid that fits the body to form a convoluted path, and/or to use a conductive gasket between the two.
I am curious why you decided to make one?
ATB
Chris
To make your bread bin a true faraday cage it needs to have a total seal. Which means your bread tin needs to be fully sealed i.e. if you can open it ...... it isn't sealed. Unless your lid fits via knife edge baffle it is a very nice tin but not a faraday cage.
Years ago I worked inside a faraday cage which had a double door entrance ( like an air lock) to maintain the seal and a crypto lock on the outside. As the door was a great big thing it also had a Huge handle. Many was the time staff would walk up to the door, type in last weeks code and without waiting, try to open the door. As it was a very big handle the door would move, as it was locked it would not move very far..... just enough to f&ck the lock. Meanwhile the off going staff where now stuck inside!! "Ring Ring" hello boss they have done the door in again, going to need an armed guard on the emergency exit as it is now the only way in and out until we fit a new lock......AGAIN!
Years ago I worked inside a faraday cage which had a double door entrance ( like an air lock) to maintain the seal and a crypto lock on the outside. As the door was a great big thing it also had a Huge handle. Many was the time staff would walk up to the door, type in last weeks code and without waiting, try to open the door. As it was a very big handle the door would move, as it was locked it would not move very far..... just enough to f&ck the lock. Meanwhile the off going staff where now stuck inside!! "Ring Ring" hello boss they have done the door in again, going to need an armed guard on the emergency exit as it is now the only way in and out until we fit a new lock......AGAIN!
I thought it was a frequency thing. I see some homemade cages on ytube so thought I would give it a try. Only tin I had was an old bread tin. I might try an old ammo tin which will probably seal better as the lid pulls down tight.
And the bread was gobbled up a long time ago !!!!
And the bread was gobbled up a long time ago !!!!
It's to do with power as you would expect but also the frequency. A very long time ago I used to work in radar and EMC and IIRC it was to do with waveguide beyond cutoff. As mentioned unless there is a conductive gasket or interlocking edges there will be an gap that EM radiation can sneak through or penetrate.
Gaskets needs to block signals by having an effective thickness to attentuate to zero (log scales I think so think half then half then half etc, never zero) The gaskets we used to use in our test equipment was rubber but coated with metal mesh to give a bigger effective thickness. Any gaps need to be long enough/narrow enough to do the same and that was to do with wavelength interactions or some such....
Think it's been 25 years since I thought about this stuff so I could be talking nonsense!
But some googling should clarify
Gaskets needs to block signals by having an effective thickness to attentuate to zero (log scales I think so think half then half then half etc, never zero) The gaskets we used to use in our test equipment was rubber but coated with metal mesh to give a bigger effective thickness. Any gaps need to be long enough/narrow enough to do the same and that was to do with wavelength interactions or some such....
Think it's been 25 years since I thought about this stuff so I could be talking nonsense!
But some googling should clarify
Most gaps in an "ill-formed" cage are not of a size close to the wavelength and so are not a function of frequency. It is more the fact that EHF and above attenuate quickly over distance. Signal will however leak from gaps in the conductive seal. Using a baffle means signals going up hit a metal screen, signals going left to right hit a barrier, 30 degrees 50, 71, 92, whatever all hit a barrier. Of course it still needs to be more than a bit of cooking foil!
The metal mesh helmets that Rader techs wear have a mesh which is a quarter lambda shorting link, different concept to a faraday cage.
The metal mesh helmets that Rader techs wear have a mesh which is a quarter lambda shorting link, different concept to a faraday cage.
As someone once told me.
There are teams creating Faraday cages that cost tens of thousands of pounds to try and stop a EMP and yet there are people who think they can do the job as well with a tin can and an earth strap.
Really people, don't waste too much time trying to make a DIY Faraday cage, it isn't worth it.
There are teams creating Faraday cages that cost tens of thousands of pounds to try and stop a EMP and yet there are people who think they can do the job as well with a tin can and an earth strap.
Really people, don't waste too much time trying to make a DIY Faraday cage, it isn't worth it.
Yep, a lot of money goes into protecting electronics from EMI.
We used to design and supply military electronics and spent tens (if not hundreds) of thousands of pounds designing, testing, and having qualified the enclosures that house the electronics - both to protect it from EMI and to prevent radiation of noise from the equipment itself.
We used to design and supply military electronics and spent tens (if not hundreds) of thousands of pounds designing, testing, and having qualified the enclosures that house the electronics - both to protect it from EMI and to prevent radiation of noise from the equipment itself.
Can I ask what is the reasoning or thought process behind the WHY you want a Faraday cage?
I can think of 2 places where opening emergency doors gets a rapid response of heavily armed chaps and chapesses. Impressive to watch when its a real emergency.
Much more impressive is watching some devil getting the b@llocking of his life from a Warrant Officer who until then I had thought so benign! Topped off with a senior Navel officer heard to say "What the 7uck do you fly boys do in there!!!"
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