DIY Faraday cage for a mobile phone

Hello -

I'm writng some software to run on a PocketPC with a built-in GPRS phone, and I need to test the software when the GPRS signal is weak, intermittent or non-existent. The cheapest way to do this seems to be to build a Faraday cage with adjustable leakiness. It needs to have some holes in it so that I can see the screen and tap it with a stylus. Unfortunately I'm working at a location that has excellent GPRS coverage.

I've already confirmed that a metal tin that previously contained Marks and Spencers chocolate biscuits is 100% effective, but I can't see through it or operate the PocketPC. I've also tried making a cage out of a single sheet of galvanized chicken wire with 12mm square holes, held together with plastic cable ties, with overlapped joints. This only caused a small amount of attenuation, the PocketPC hardly noticed. I'm guessing that it failed because the diagonals of the holes are larger than 10% of the wavelength that GPRS uses (1800MHz ?) or because the plastic cable ties don't provide good conductivity at the joints.

Before I go out and buy some expensive perforated copper sheet, can anyone offer advice on improvements?

Thanks

Reply to
Simon
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Have you tried covering the open side of the tin with two layers of chicken mesh slightly offset.

Dave

Reply to
Dave Stanton

Thanks for the reply. I did briefly try that, but it wasn't very successful. I think part of the problem is that the biscuit tin appears to have a layer of laquer on it, and the chicken wire has a slightly oxidized Zinc coating, so it may be difficult to get a good electrical connection between the two, which I understand is important. I think I will try this again.

Reply to
Simon

would it help if you laid on your back and looking up at the P/PC covered it with tin so you can still operate it inside the tin?

Reply to
Gav

Some fine wire mesh, such as:

Reply to
Grunff

How about some 6mm weldmesh?

Used for bird feeders, get it from the garden centre. If not, I'm sure I've got an offcut - how big are we talking?

Reply to
Nigel Molesworth

Simply purchase a new large fridge, and open the door a crack.

Reply to
Ian Stirling

As a general rule of thumb, grid spacing should be less than (or equal to)

1/10 the wavelength of the highest frequency you are interested in blocking.

Marcus

Reply to
Marcus Fox

What you need to do is use plenty of nuts and bolts with large washers to hold the mesh in place and yes, scrape the laquer off. Blimey this is taking me back a few years to Amateur Radio days. RF will leak around holes so good firm electrical contact is essential.

Dave

Reply to
Dave Stanton

======================== Have you considered using a box made from the reinforcing mesh used with car body filler? It's very easily shaped and reaily available.

Cic.

Reply to
Cicero

Or find a friend with a steel Narrowboat or just take the whole lot to Braunston, Northamptonshire. The signal is next to nothing by the canal there. Brian

Reply to
brian

How about putting it in a microwave oven (with the microwave off, obviously!)

You could control it with PocketVNC with a cable connected to it if you need to operate it with the door closed. You could just open the door a bit the get the required signal strength.

Sparks...

Reply to
Sparks

In article , Simon writes

You'll need quite a fine mesh to do that!. And no gaps either. Solder all of them and operate the keys with non conductive rods of a very small diameter.

Try the lower sub basement floors of a car park or concrete building.

Or failing that take the whole shooting match out into Norfolk where you'll fine a lack of GSM GPRS UMTS and much else!....

Reply to
tony sayer

The message from "Simon" contains these words:

Door from an old microwave should make a nice seethrough radio-opaque wossname.

Reply to
Guy King

I've got an excellent Faraday Cage. It's better known as a caravan!

Reply to
Roger Mills

Try a old biscuit tin (a metal one)- you may find just putting the top in with a "crack" open is enough.

--

73 Brian
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Reply to
Brian Reay

Ooooh - lateral thinking! Nice one Skip!

Reply to
Steve Walker

A microwave oven is a good choice, because it will NOT completely block the signal. This is because the door seals use a quarter wave transmission line structure which means that an open circuit is converted into a short circuit at the operating frequency. Hence they avoid the need for metallic contacts which would become ineffective when covered in food residues. They just need close proximity between the door and the oven.

Mobile phones use different frequencies to microwave ovens, so the oven door seals will attenuate but not totally block the signal. (I just tried it with my GPRS phone on Orange - the signal dropped from 4 bars to 1.

John

Reply to
jrwalliker

In message , Simon writes

Rather than getting the Faraday cage to come to you how about going to it? Any underground car parks nearby?

Reply to
Bill

Rather than trying to see in, why not put a camera (Web Cam ?) and light bulb in the box, and bring the cables out ?

you could then have a sliding aperture to leak in as little or as much as you want..

Nick

Reply to
Nick

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