UK Mobile Phones (uk.telecom.mobile) Mobile telephone equipment and networks.

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Old April 16th 19, 09:05 AM posted to uk.telecom.mobile
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Default Faraday cage question

I was in a lift yesterday and continued to get 4G reception. I
thought a lift was an earthed steel structure (Faraday cage) and there
would be no radio reception. Am I missing something or should I make
a claim against my former physics teacher and the education authority?

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Old April 16th 19, 09:19 AM posted to uk.telecom.mobile
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Default Faraday cage question

Scott wrote:
I was in a lift yesterday and continued to get 4G reception. I
thought a lift was an earthed steel structure (Faraday cage) and there
would be no radio reception. Am I missing something or should I make
a claim against my former physics teacher and the education authority?


You need proper rf tight seals on all the apertures. Constructing a proper
Faraday cage is much harder than you might at first think. Being earthed is
not relevant, however. I design electronics to go in spacecraft, where the
enclosures are constructed to prevent rf getting in or out, and we’ve
certainly not got an earth....


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Old April 16th 19, 09:49 AM posted to uk.telecom.mobile
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Default Faraday cage question

In article ,
Scott wrote:
I was in a lift yesterday and continued to get 4G reception. I
thought a lift was an earthed steel structure (Faraday cage) and there
would be no radio reception. Am I missing something or should I make
a claim against my former physics teacher and the education authority?


To be effective, any gaps in the metal structure need to be small
compared to the wavelength. 4g signals are around 10-30cm.

-- Richard

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Old April 16th 19, 09:53 AM posted to uk.telecom.mobile
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Default Faraday cage question

On 16/04/2019 10:05, Scott wrote:
I was in a lift yesterday and continued to get 4G reception. I
thought a lift was an earthed steel structure (Faraday cage) and there
would be no radio reception. Am I missing something or should I make
a claim against my former physics teacher and the education authority?


Lifts don't make particularly good Faraday cages - too many long thin
gaps. Even when you have a real Faraday cage you have to look after the
beryllium copper spring door seals carefully or it will tend to leak RF.

Your mobile phone will probably still be able to get enough signal
inside a microwave oven with the door closed to ring when you call it.
And that is intended to be a nominally microwave proof Faraday cage.

(DO NOT MICROWAVE YOUR PHONE)

--
Regards,
Martin Brown
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Old April 16th 19, 10:20 AM posted to uk.telecom.mobile
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Default Faraday cage question

On Tue 16/04/2019 10:05, Scott wrote:
I was in a lift yesterday and continued to get 4G reception. I
thought a lift was an earthed steel structure (Faraday cage) and there
would be no radio reception. Am I missing something or should I make
a claim against my former physics teacher and the education authority?

Lifts often look as though they are metal lined but many are actually
thick plastic made to look like metal. The roof lighting also often does
not have full metal behind it. The only part of the box that is metal is
the floor but even then not always......!

As a point of interest you got 4G but did you try to get 3G or 2G?
Because of the way that 4G works it will often get a usable signal where
3G and 2G won't. If you travel in the lift again and your phone does not
have VoLTE, try making a call or sending a text. My money says it
probably won't work.

Funnily enough DAB+ will work where DAB doesn't - and that is also down
to the way DAB+ works.

--
Woody

harrogate three at ntlworld dot com


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Old April 16th 19, 10:21 AM posted to uk.telecom.mobile
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Default Faraday cage question

On 16/04/2019 10:49, Richard Tobin wrote:
To be effective, any gaps in the metal structure need to be small
compared to the wavelength. 4g signals are around 10-30cm.


And they have to have a good electrical connection. I was told of a
demonstration of a top of the range screened room where it was still
possible to talk on a hand held radio from inside the screened room.

It was eventually realised that people tend to run their fingers over
the edge of the door as they go in and out. That was enough to leave a
think film of grease on the door. It was recommended that the spring
fingers on the door are cleaned regularly.

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Old April 19th 19, 12:19 PM posted to uk.telecom.mobile
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Default Faraday cage question

On Tuesday, 16 April 2019 10:05:07 UTC+1, Scott wrote:
I was in a lift yesterday and continued to get 4G reception. I
thought a lift was an earthed steel structure (Faraday cage) and there
would be no radio reception. Am I missing something or should I make
a claim against my former physics teacher and the education authority?


I noticed this in the Barbican 25 years ago (so only 900MHz).

It must leak through the ceiling or gaps on the lift cabinet.
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Old April 19th 19, 06:48 PM posted to uk.telecom.mobile
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Default Faraday cage question

In message
"R. Mark Clayton" wrote:

On Tuesday, 16 April 2019 10:05:07 UTC+1, Scott wrote:
I was in a lift yesterday and continued to get 4G reception. I
thought a lift was an earthed steel structure (Faraday cage) and there
would be no radio reception. Am I missing something or should I make
a claim against my former physics teacher and the education authority?


I noticed this in the Barbican 25 years ago (so only 900MHz).

It must leak through the ceiling or gaps on the lift cabinet.


You only need a gap a few centimetres long to have a leak. It forms
a slot antenna. The gap between doors is relatively enormous.

Just as a piece of metal in air works as an antenna, so does a bit
of air in a piece of metal. Regard it as a discontinuity in the
medium.

A Faraday cage has to be sealed everywhere, with all non-conducting
lengths small compared with the shortest wavelength of interest.

David


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