DIY Faraday cage for a mobile phone

Guy King wrote:
> The message <[email protected]>
> from "Simon" <[email protected]> contains these words:
>
>> Before I go out and buy some expensive perforated copper sheet,
>> can anyone offer advice on improvements?
>
> Door from an old microwave should make a nice seethrough
> radio-opaque wossname.

Ooooh - lateral thinking! Nice one Skip!

---

› See More: DIY Faraday cage for a mobile phone

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

In message <[email protected]>, Simon
<[email protected]> writes
>Hello -
>
>Before I go out and buy some expensive perforated copper sheet, can anyone
>offer advice on improvements?
>

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

--
Bill

In message <[email protected]>,
[email protected] writes
>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.
>
Didya switch the microwave on ?
`
--
geoff

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

In article <[email protected]>
Simon <[email protected]> wrote:
> 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?
>
Get a large cardboard box (or old wardrobe), wrap it in kitchen foil and
sit inside it. :-)

You'll need to make very good joins betweeen the sheets of foil to make
that work.

Copper tape with conductive adhesive is ideal for this, but you had
better be sitting down when you look up the price.

Bringing a cable into an enclosure is the best way to bring in mobile
phone signals as well. When I needed to do almost exactly what you are
doing in a commercial test chamber I had to filter every signal
separately with feedthrough capacitors.

The chamber was quite impressive. They tested military vehicles in
there - even tanks - and could rotate them on a huge turntable.

John

Bill wrote:
> In message <[email protected]>, Simon
> <[email protected]> writes
>> Hello -
>>
>> Before I go out and buy some expensive perforated copper sheet, can
>> anyone
>> offer advice on improvements?
>>
>
> Rather than getting the Faraday cage to come to you how about going to it?
> Any underground car parks nearby?
>
Try placing the phone inside some chicken wire or rabbit wire...not sure
what the attenuation would be, but pretty high.

Rob Morley wrote:
> In article <[email protected]>
> Simon <[email protected]> wrote:
>> 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?
>>
> Get a large cardboard box (or old wardrobe), wrap it in kitchen foil and
> sit inside it. :-)
>

Right. Another thing to try is metal building lath.

This is used as a base for rendering. Its a deal smaller bore than
chicken or rabbit wire.

And its dirt cheap.

The Natural Philosopher wrote:
> Right. Another thing to try is metal building lath.
>
> This is used as a base for rendering. Its a deal smaller bore than
> chicken or rabbit wire.
>
> And its dirt cheap.

I did some work with GSM signals, our faraday cage used mesh with ~6mm
dimaond perforations.

--
Spamtrap in use
To email replace 127.0.0.1 with blueyonder dot co dot uk

Simon wrote:
> 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

Why not control the PPC remotely using software such as SOTi Remote
Controller? Then you could szeal the PPC in the biscuit tin and have only a
single cable to the PC?

--
Darren Griffin
PocketGPSWorld - www.PocketGPSWorld.com
The Premier GPS Resource for News, Reviews and Forums

Why not open the phone up and remove the antenna. Loads of attenuation
can be achieved by that method

In article <[email protected]>, Rob
<[email protected]> writes
>Why not open the phone up and remove the antenna. Loads of attenuation
>can be achieved by that method
>

And possible PA damage!...
--
Tony Sayer

"Simon" <[email protected]> wrote in message
news:[email protected]...
> 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
>

I would use the tin idea with a small hole cut and connect the USB cable and
then use "active sync remote display" to control and view the screen. Easy!

Then you can chop the tin up and creat some leaks to simulate different GPRS
strength.

Paul

tony sayer wrote:

> In article <[email protected]>, Rob
> <[email protected]> writes
>
>>Why not open the phone up and remove the antenna. Loads of attenuation
>>can be achieved by that method
>
> And possible PA damage!...

Same could apply operating it in a biscuit tin, unless you introduce
some RF absorbent lining.

So how about a dummy load connected via a length of coax into the outer
of which you cut some slots, leaky-feeder style?

--
Andy