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Electromagnetic field screen

:: EPE Chat Zone ­:: ­Radio Bygones Message Board :: » Everything else! » Archive through 22 October, 2012 » Electromagnetic field screen « Previous Next »

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rob_guyer
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Username: rob_guyer

Post Number: 75
Registered: 08-2011

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Posted on Thursday, 06 September, 2012 - 05:54 am:   Edit Post Delete Post Print Post    Move Post (Moderator/Admin Only)

I am legally obliged to accept the smart meter now measuring my house mains power consumption. I trawled the internet looking for pros and cons of these devices and found mostly opinion posing as fact.
I intend to fit a electromagnetic screen on an inside wall opposite the meter box. The easiest way is to make up a sandwich of magnetic whiteboard and aluminium kitchen foil but first I need to estimate the degree of screening. An early textbook* informs that a continuous conductive wall of thickness “x” reduces the field magnitude to a fraction e raised to the power minus mx of its original value. The attached gives “m” but uses CGS units which are not one-for-one transferable to SI units.
The internet could not help so I hope that a CZ member can advise if this could be worthwhile .
* “Elements of Electricity and Magnetism-J.J.Thomson, Cambridge University Press, 1909”field attenuation
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vlf
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Username: vlf

Post Number: 361
Registered: 10-2005


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Posted on Thursday, 06 September, 2012 - 06:14 pm:   Edit Post Delete Post Print Post    Move Post (Moderator/Admin Only)

A very long time ago in my R&D days, we used mu-metal that has a special combined properties; was commonly used around electrostatic deflection scope CRT's for screening.

My vague experience now, is that you would need just two sheets of mu-metal spaced about 2mm for, as a rule-of-thumb, 1000 gauss; based on close proximity to the 50 Htz electromagnetic source.

However mu-metal is hard to find now days, and most likely expensive too.

Regards, Brian L.
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ant
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Username: ant

Post Number: 912
Registered: 05-2006

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Posted on Thursday, 06 September, 2012 - 08:52 pm:   Edit Post Delete Post Print Post    Move Post (Moderator/Admin Only)

Hello Rob,

I've no idea about the maths, sorry. But rather than duplicating your research, why exactly are you anticipating needing the screening?

Could you screen the box rather than the inside wall? Or even mount a screen behind the box, with a slot cut out to go round the cables? I suppose you can't screen it completely without masking their cellphone signal - on the other hand whose problem might it be...?

If you're looking to screen the wall, the most economical way might be a sheet of steel from your local stock-holder? Or even more economical, if it can be made good cosmetically, something made of washing machine casing flattened out?

Regards Ant
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rob_guyer
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Post Number: 76
Registered: 08-2011

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Posted on Saturday, 08 September, 2012 - 02:06 am:   Edit Post Delete Post Print Post    Move Post (Moderator/Admin Only)

The internet brings much subjective opinion on electromagnetic field safety but very little practical advice on minimising risk. I looked again at the 12mm plasterboard separating my bed-head from the mains wiring in the open-backed steel meter box on the outside wall and thought that a small screen on the interior wall might reduce radiation at 50Hz noise/ harmonics. A second look at my maths and comments from Brian and Ant triggered new lines of thought;
(1) Size to just overlap the meter box footprint.
(2) At 50Hz, mild steel permeability of 40-100 makes up for poor conductivity so aluminium foil is out. At cellphone frequencies the skin effect dominates screening again favouring mild steel as the economical choice.
(3) 2mm steel double screening will increase effective screen depth, keeps weight down and stops capacitive coupling. But which plate should be connected to ground?
(4)Screening the ferrite cellphone antenna mounted on the meter box is illegal and probably counterproductive since the cell computer will increase transmitter power to compensate. I hope that the ARRL antenna handbook on order will advise on a suitable corner reflector. Directing the radiation might persuade the cell computer to order a decrease in radiated power?
(5) All this does nothing to reduce conducted noise and perhaps the 12mm gap will bypass radiated fields, but if it helps mt sleep.........
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ant
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Username: ant

Post Number: 914
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Posted on Saturday, 08 September, 2012 - 05:15 pm:   Edit Post Delete Post Print Post    Move Post (Moderator/Admin Only)

Hello Rob,

It occurs to me that there might be refraction round the edge of the screen, maybe perhaps needs to be a bit larger than the meter box? I presume that it'd be worse at low frequencies than high...

Regards Ant
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rob_guyer
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Username: rob_guyer

Post Number: 77
Registered: 08-2011

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Posted on Monday, 10 September, 2012 - 09:56 am:   Edit Post Delete Post Print Post    Move Post (Moderator/Admin Only)

I agree. Increasing the screen overlap to, say 1/2 wavelength should direct a good part of the cellphone stray field towards the outside world with perhaps the rest taken up by diffraction creating a chaotic field pattern near the edges. As you say, screening at 50Hz is a different beast altogether. I had overlooked my basics and the fact that flux density is more concentrated as the air gap reduces. This is not what I wanted anywhere near my pillow, so I will increase screen dimensions from 450x600mm to 600x900

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