LundyCam - DIY Faraday Cage

I'm currently working as software developer, using the Microsoft .NET Compact Framework to build a GPRS-enabled application that runs on ruggedised PocketPCs. This involves testing how the PocketPC software behaves when the GPRS signal is weak or non-existent. I needed to be able to operate the screen and buttons of the PocketPC, while it was in an enclosure that blocked the GPRS signals. Building a Faraday cage turned out to be more difficult than I expected, so this page describes how I eventually built a low-cost Faraday cage than will block GPRS signals (at most locations) while allowing the user to operate the PocketPC.

One obvious approach to this problem is to put the PocketPC in a microwave oven. One drawback is that there is no way to operate the device while the door is closed. Another less obvious (to me) problem is that most modern microwave ovens do not provide good screening except at the exact frequency on which they operate. This is because they do not rely on a good electrical contact between the body of the oven and the door, instead they have a cavity running round the edge of the door that is λ/4 deep (where λ is the wavelength of the radiation used) to prevent leakage. This means that in most cases a microwave oven will not block GPRS signals.

The rule of thumb when building a Faraday cage is that it should contain no gaps larger than λ/10, where λ is the shortest wavelength that is to be blocked. For GPRS with a frequency of 1800MHz, this suggests no gap larger than about 17mm. The smallest mesh I could find in my local DIY shop was galvanised steel chicken wire with 12mm square holes. The diagonal of these holes is about 17mm. I tried to build a cage out of a single sheet of this, with overlapped seams held together by plastic cable ties. When the PocketPC was put inside, the signal strength meter did drop from 3 to 1, but GPRS still worked.

I then bought some 6mm square galvanised steel chicken wire and built another cage. This did not provide sufficient screening either. I suspect that the seams were not sufficiently sealed, and that the plastic cable ties did not provide good enough electrical contact along the seams.

Next I tried a metal tin, which had previously contained Marks & Spencer chocolate biscuits. With the lid closed, this did entirely block GPRS, but of course it was not possible to operate the buttons or screen.

I found a workable solution with the biscuit tin and 6mm chicken wire (see photographs on left). I cut a square hole in the lid of the tin, and covered it with 6mm square galvanised steel chicken wire. The chicken wire is held firmly against the lid with 8 M6 steel bolts, with 25mm "penny" washers to distribute the load and provide good electrical contact. It may be significant that when viewed from above, the biscuit tin is not the usual rectangle-with-rounded-corners, but has a more complex shape. This may help to reduce the effective length of any gaps to well below λ, and so reduce the amount of GPRS signal that can leak through them. This cage works at 2 of the 3 locations I have tried, but did not screen completely at the 3rd location, probably because there was a cellular transmitter about 100 metres away.

I'd like to say thankyou to the helpful people in the uk.d-i-y newsgoup for providing advice.