RadioBanter - View Single Post - Faraday shields and radiation and misinterpretations

Lostgallifreyan · December 1st 09, 09:42 AM posted to rec.radio.amateur.antenna

K7ITM wrote in news:c52a1b1d-ef32-4d69-bf61-
:

It's fairly straightforward, actually, if you believe in Faraday's law
of magnetic induction. That law says that for any closed loop
(through air, through a conductor, through anything), there is an
electromotive force (a voltage source, if you will) whose magnitude is
proportional to the rate of change of magnetic flux enclosed by the
loop. As there is no voltage drop along a perfect conductor, if your
closed loop follows the path of a perfect conductor, there is no
voltage drop around that loop, and therefore the rate of change of the
total magnetic flux enclosed by that loop must be zero. If the
perfect conductor is a closed box, then you can draw loops anywhere
through that conductor and you will never see a changing magnetic
field enclosed by that loop. Thus, the inside of the box and the
outside are magnetically independent; things happening on one side
(magnetically) are not sensed on the other side.

You can understand how this works if you realize that a changing
magnetic field outside the box that would penetrate the box if it
weren't there will induce currents in the conducting box (or even just
in a closed loop of wire). Those currents will (in a perfect
conductor) be exactly the right magnitude to cause a magnetic field
that cancels the external one everywhere inside the closed box (or the
net flux enclosed by a loop of wire). An example: if you short the
secondary of a mains transformer, the primary will draw lots of
current at its rated voltage: it's very difficult for the primary to
change the magnetic flux in the core.

Does the electric field shielding from a perfect conductor need any
explanation?

Of course, an imperfect conductor will be an imperfect magnetic
shield. But a perfect conductor won't let any change of field
through, no matter how slow (no matter how low an EMF it generates),
so a perfect conductor works as a shield all the way down to DC. A
box made with an imperfect conductor is essentially a perfect shield
if the box's wall thickness is at least many skin-depths thick at the
frequency of interest.

That's a quick beginning. You can find lots more about this in E&M
texts. There's even useful stuff about it on the web. ;-)

Cheers,
Tom

Thanks, that helps, especially the paragraph about creating a magnetic field
in response that tends to cancel the original one, and the thickness of metal
with regard to frequency. The OP (Art Unwin) mentioned cancellation in more
complex terms, so I'm still not clear if this validates what he said or not.
It appears to but he mentions stuff I'm not likely to grasp in just an hour
or two of effort.. What I'm getting at is that I'm not sure if his calling
orthodoxy into question is all that drew the flak, or if there's something
obviously wrong in his post that I'm missing.

Also (though I'll likely find out about this when I look deeper), why is it
often ok for a Faraday cage to have holes in it?

Braided screens, meshes,
perforated metal sheets, etc, I've seen many shields that are not a complete
'seal'... UHF TV cables especially seem to be very loosely shielded but they
work. Conversely, I found some nice coax in a skip once that had two heavy
braids amounting to almost complete coverage around a single fine stranded
core. (Found outside a telephone exchange, but I don't know what frequency
they were intended for, though I used some for an outdoor VHF receiving
quarter wave dipole with good results, and I suspect it will do for a SW
longwire once I get a matching transformer for it).