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Old 26th Jun 2014, 6:33 pm   #1
don200002
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Default No signal in a metal clad building

Hi all
I work in a metal clad building where we have at last been granted the use of radios. I thought it would be nice to use one of my Hacker Sovereigns but there is no signal at all even on a modern radio. Some people have managed to get a weak signal by connecting the aerial to earth, which seems a little strange but I suppose it is blocking some of the RF generated by computers and SMPS in the Factory. Does anyone have any ideas how to improve reception in this situation?
Thanks,
Don.
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Old 26th Jun 2014, 6:45 pm   #2
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Default Re: No signal in a metal clad building

Can you throw a length of wire out of a window?
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Old 26th Jun 2014, 6:58 pm   #3
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Default Re: No signal in a metal clad building

The radio signals are been "blocked" by the "Faraday Cage principle" see link below. Agree with Paul above
regards S-W

http://en.wikipedia.org/wiki/Faraday_cage
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Old 26th Jun 2014, 7:08 pm   #4
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Default Re: No signal in a metal clad building

Yes, I had thought about the Faraday Cage problem so I ran a piece of wire outside (unfortunately there are no windows) but it just picks up more RF generated inside the Factory. Maybe a piece of Coax with the screen earthed but it would have to be about 30Mtrs long to get outside the building.
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Old 26th Jun 2014, 7:19 pm   #5
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Default Re: No signal in a metal clad building

Many buildings built in the last 20-odd years, apart from being metal-framed, have incorporated aluminium-foil-faced plasterboard/drywall partitions, foil-faced 'Kingspan' or 'Celotex' foam slabs in the wall-cavities and foil-encapsulated wool-insulation [a.k.a. 'space blanket'] in the attic in order to meet mandatory energy-efficiency insulation requirements.

In addition, a lot of window-glass (double- or sometimes triple-glazed] now includes solar-reflective coatings which are not exactly RF-transparent either.

All this makes great RF shielding. You're discovering one effect of it - I'm more familiar with its efficiency at interrupting 3G/4G mobile data coverage and messing up predicted on-site WiFi coverage.

There's no easy answer to this unless you can put up an external antenna.
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Old 26th Jun 2014, 8:22 pm   #6
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Default Re: No signal in a metal clad building

Sorry to be a nit-picker, but a FARADAY cage doesn't stop radio signals.

It's a special sort of screen used to block only electric field components, while letting magnetic field components pass right through.

You should have no problems operating a portable radio inside a real Faraday cage.

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Old 26th Jun 2014, 8:23 pm   #7
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Default Re: No signal in a metal clad building

I would cheat and use a tiny MP3 player hidden inside your Sovereign, loaded up with podcasts of the programmes you want to listen to.

BTW, are you trying to listen on FM or AM? I suspect the latter would be utterly impossible in the circumstances.

N.
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Old 26th Jun 2014, 8:37 pm   #8
don200002
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Default Re: No signal in a metal clad building

Yes AM would be impossible with the RF "noise" kicked out by the computers etc but I will give the earth screened Co-Ax idea a try.
Thanks for everyones input,
Don.
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Old 27th Jun 2014, 1:41 am   #9
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Default Re: No signal in a metal clad building

If there is an internet connection availabe (wired or wifi) you could stream the radio programming using a computer, tablet, smartphone or internet radio. Sadly you won't be able to use vintage tech though, at least for the reception bit.
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Old 1st Jul 2014, 4:21 pm   #10
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Default Re: No signal in a metal clad building

I had this problem some years ago in a large coal mine workshop. Metal clad building with steel girders for a framework. We were allowed to have a radio for entertainment but could not get decent reception.
I solved it by deciding that I could use the steel upright supports as an auto transformer. The earth from the radio was firmly connected to the base of one of the upright supports of the shed. The antenna lead was firmly connected to the same upright but about 15 feet above ground level. I found I had a good noise free signal but was quite weak. Thought it might be an impedance issue so I made a simple impedance matching transformer to go between the radio and the shed framework. The radio had a high input impedance and the steel framework was a low impedance. I would not have a clue what impedances were involved with the transformer. I just guessed everything wound it on a length of plastic electrical conduit and covered with tape. I found the setup was quite immune to electrical interference. Quite surprising really as it was a big workshop for the maintenance of open cut coal mining machinery and contained a lot of bits of electrical equipment such as welders. The last I saw of the setup was when I left the mine some 18 years later and it was still going strong then. The mine was in a country area so radio station signal strength was not perfect.
Good luck with it.
Cheers, Robert.
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Old 2nd Jul 2014, 5:18 am   #11
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Default Re: No signal in a metal clad building

Quote:
Originally Posted by Radio Wrangler View Post
Sorry to be a nit-picker, but a FARADAY cage doesn't stop radio signals.
Sorry to pick your nits RW, but you are wrong there. The only EM effects not blocked by a Faraday cage are static or slow moving EM fields.

Back in the 1970's I turned my (small) workshop in my employers premises into a Faraday cage because I was running their radio-telephone service department and my transmissions were playing havoc with the TV department next door. It was not a perfect solution as I didn't have the budget, but we got it very close to that and I couldn't use an am radio inside it.

So, if you have experienced a Faraday cage that didn't block radio signals, then it was leaking like a sieve and thus was not a Faraday cage at all.

Best you go refresh your recollections on the subject.

Google is your friend.

Cheers

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Old 2nd Jul 2014, 9:14 am   #12
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Default Re: No signal in a metal clad building

Here at the University of Bath, the engineering buildings are of the CLASP design dating to 1969. They consist of a glass and aluminium clad steel frame mounted on earthquake proof springs. It is impossible to receive AM signals but FM TV and DAB are fine. The radio boffins upstairs said that AM waves were too big to pass through the frame unobstructed whereas shorter wavelengths pass through easily.
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Old 2nd Jul 2014, 9:31 am   #13
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Default Re: No signal in a metal clad building

Quote:
Originally Posted by Billy T
Sorry to pick your nits RW, but you are wrong there. The only EM effects not blocked by a Faraday cage are static or slow moving EM fields.
You're right, but there's a little bit more to it than that. To include the obligatory Wikipedia quote:

"Faraday cages cannot block static or slowly varying magnetic fields, such as the Earth's magnetic field (a compass will still work inside). To a large degree, though, they shield the interior from external electromagnetic radiation if the conductor is thick enough and any holes are significantly smaller than the wavelength of the radiation." (My emphasis).

This video gives an excellent illustration of the point (watch from the 5 minute point if you don't want the background science).

Hugh
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Old 2nd Jul 2014, 9:33 am   #14
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Default Re: No signal in a metal clad building

Quote:
Originally Posted by don200002 View Post
I work in a metal clad building where we have at last been granted the use of radios. I thought it would be nice to use one of my Hacker Sovereigns but there is no signal at all even on a modern radio. Some people have managed to get a weak signal by connecting the aerial to earth, which seems a little strange but I suppose it is blocking some of the RF generated by computers and SMPS in the Factory. Does anyone have any ideas how to improve reception in this situation?
No signals at all? Don't tell GCHQ or they will want one too! People go to huge trouble to stop E-M signals leaking out of their buildings, so unless you are working in one of those specially engineered buildings, I suspect you are suffering from a combination of high noise level and some modest attenuation of external signals.

If you want VHF signals, then running coax to a standard Band II aerial on the roof would be a standard solution. Not cheap unless you are allowed to do it yourself (unlikely in most companies).

For MW/LW, the easiest solution is a length of wire thrown out of a window. Again, management in most companies I've been in would throw a wobbly at anything "unusual" - and would definitely ban a window being left slightly ajar to allow a wire out.

A radical alternative would be to use a phone with unlimited internet access (e.g.3) and amplify the signal out of the headphone socket with a small speker/amp. That assumes you can get a good enough mobile signal of course (typically you need HSDPA). The much shorter wavelength of mobile signals usually allows quite a lot of penetration into most metal buildings.

If you have got company internet access via landline, you might just plug in a suitable PC/tablet/etc into the company network and pick up radio 1/2/etc. That will avoid the need to find a phone.

Richard
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Old 2nd Jul 2014, 9:52 am   #15
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Default Re: No signal in a metal clad building

I think we're just going to have to disagree on that one.

The 'Faraday' prefix seems to be being used in front of almost all mentions of screening.

This is a shame because we seem to be losing the term for a specific form of shield. It's as if the word 'gearbox' became always prefixed by 'automatic'.

As far as my recollections go, they start with lectures, lab sessions and exam in electromagnetics (passed, by the way). Then on to getting a few dozen HP products to pass the newly introduced EMC regulations in the 1980s. Nowadays getting avionics products to meet the rather stiff EMC requirements they are subject to. Oh, and a few transformers with true Faraday screens to minimise electric coupling that go to hundreds of MHz.

The frequency range any form of screening is good over is always limited by details of implementation, like skin depth in solid metal walls, for example. So what you made to stop signals from radio telephones was good in that band because it did what was needed. However screens can be made to pass magnetic field components up into the GHz region. You can also find electric field screens in transformers operating well above 100MHz.

If you look inside the jaws of one of those lovely Tektronix wide-bandwidth current probes, you'll see a tiny Faraday screen.

Let's not argue. This area is getting muddied enough as it is.

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Old 2nd Jul 2014, 10:38 am   #16
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Default Re: No signal in a metal clad building

Whoever wrote the Wiki article appears to have (largely) missed the point of what Faraday was originally trying to achieve- screening electric fields without unduly affecting magnetic fields (which would have been slowly-changing in the context of his experiments, anyway) and strayed into what we would term "screened rooms". It's a dividing line whose definition depended on the definition of "slowly changing" and is becoming more and more ambiguous as more and more people use it ever more glibly.

In the end, definitions change over time with popular culture and there's not much that can be done to stem the tide. I've given up telling people that "volatile" doesn't mean "imflammible", it means "evaporates readily", giving the example of water as a highly volatile substance. As "volatile" gets substituted for "imflammible" or even "explosive" ever more and more, even in news reports and written literature, I realised that acting King Canute was futile and that "volatile" had shifted its meaning.

So, unfortunately, with "Faraday cage".
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Old 2nd Jul 2014, 1:56 pm   #17
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Default Re: No signal in a metal clad building

It is interesting to compare a metal clad building with a screened room. We had some experiments in the lab at work that were potentially going to produce very severe interference. To avoid disruption these were put in a commercial screened room. One of the experimenters brought in a portable radio/tape recorder as background when he was writing up notes. With the room door open he could receive radio signals adequately.

On one occasion he left the radio playing when he was preparing for a run. As he was securing the door he noticed that the radio received signals even when the door was almost closed. Only when the door was properly secure did the reception stop.

It hints that in reasonably good signal areas even in a metal building there may be places where reception is possible. Windows are a good bet, though all the replies suggest it is a bit hit and miss and depends on wavelength. This is not much use if the place of working is fixed though a Sovereign with MW LW and FM stands a better chance than many.
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Old 2nd Jul 2014, 3:16 pm   #18
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Default Re: No signal in a metal clad building

When I was installing some VHF/UHF kit for the Saudi National Guard, Belling-Lee were also on site installing screened rooms. They went to great lengths to bond all metal interfaces, but there was an air vent. It was made up of metal slats, about 2cm deep in a grid pattern with similar spacing (from memory), low down near the ground.
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Old 2nd Jul 2014, 6:56 pm   #19
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Default Re: No signal in a metal clad building

They were Belling-Lee screened rooms that I used. Those doors pulling a flanged edge into a groove filled with springy fingers are rather good.

Just for fun I took a 2-metre transceiver inside, closed the door and had a contact through the Burntisland repeater. The trick was in using the room as an antenna by coupling the RF to two separated places.

The ventilation grid works as a waveguide-below-cutoff attenuator. The length of the channels puts the attenuation up, and the cross section size sets the cutoff frequency.

Those places don't half get oppressive when well stuffed with absorbent pyramids.

We had chambers for emissions finding (definitive measurements were then open site) and also calibrated chambers for susceptibility testing.

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Old 2nd Jul 2014, 8:18 pm   #20
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Default Re: No signal in a metal clad building

I recall working on an RFA vessel, deep in their radio room on which I commented to the radio officer regarding the all-steel-perforation-clad walls.....

"faraday cage mate, keeps our signals from 'leaking' to the enemy"....

....when my mobile phone started to ring....
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