[RFI] shielding question
Jim Smith
jimsmith at shaw.ca
Mon Dec 22 16:13:59 EST 2003
Thanks Jim,
I'll look these up.
73 es Merry Xmas de Jim Smith VE7FO
Jim Brown wrote:
>On Mon, 22 Dec 2003 14:59:31 -0800, Jim Smith wrote:
>
>>Could you explain the mechanism (or point me to a source) whereby the
>>sheld is rendered useless by the presence of a drain wire at HF?
>>
>
>I can do both. It's called "shield-current-induced noise," (SCIN). It
>was first described by Neil Muncy (ex-W3WJE) in an AES paper in 1994,
>based on research he did only at audio frequencies. I've done more
>research on it in the last year or so, publishing my research in two
>AES papers -- one in March, 2003, presented in Amsterdam, and one in
>October, 2003, presented in NYC. Neil's 1994 paper and my March paper
>are available from the AES (www.aes.org). The October paper will be
>there in a few months (for some reason, it takes a while after the
>convention to get it there). My March paper extended Neil Muncy's work
>to 4 MHz, the NY paper took it up to 300 MHz. Both of my papers include
>lots of data on real cables, and show the effect of various cable types
>on RF detection in equipment. They also document my test setup, and
>report of field testing of cables and equipment when exposing it to ham
>transmitters on Field Day, and to broadcast transmitters.
>
>The basic mechanism is inductive -- when current flows on the shield,
>it induces voltage on each of the signal conductors that run inside the
>shield. If the current is equally distributed over the shield, it will
>couple relatively equally to the two signal conductors, and the
>differential input will see zero volts (or nearly zero) due to that
>current. But if the shield current couples more closely to one signal
>conductor than the other, the two voltages will NOT be equal, so they
>won't cancel.
>
>In braid-shielded cables, especially those without a drain wire, the
>current is quite evenly distributed over the shield. But if there is a
>drain wire, the current will generally divide between the drain and the
>rest of the shield based on the resistance of the drain and the total
>shield. My research shows that below about 10 MHz, most of the current
>in a foil/drain shield flows in the drain. As frequency increases, skin
>effect causes the current to be more evenly distributed, and my
>research suggests that braid-shielded cable loses its very advantage at
>roughly 20 MHz (this will, I think, vary with cable construction).
>
>Now, consider that in most foil/drain cables, the drain wire is twisted
>at the same rate as the signal pair, and is usually constructed so that
>the drain wire is much closer to one of the signal conductors than the
>other. This is the principal cause of SCIN.
>
>Braid-shielded cables do have SOME SCIN, but it is typically 30 dB
>lower in level than foil/drain cables below about 2 MHz, about 20 dB
>lower below 4 MHz. and about 10 dB lower below 8 MHz.
>
>In the last ten years or so, cable manufacturers have been adding drain
>wires to braid-shielded cables. My research shows that below about 2
>MHz, the drain wire significantly degrades the SCIN performance,
>approximately in proportion to the resistance ratios noted above. The
>degradation is most severe below the AM broadcast band, and gradually
>fades away as skin effect cause the current to flow more uniformly over
>the total shield.
>
>Without the effect of the drain wire (i.e., no drain wire or at higher
>frequencies), the magnitude of SCIN is related to manufacturing
>tolerances in the cable. That is, how equal are the lengths of the two
>conductors, how symmetrically are they twisted, etc.
>
>My research suggests that the ideal shield is a foil shield with a
>light braid, much like is commonly used with MATV coax. The braid
>distributes the current uniformly at all frequencies, and the foil
>improves the percentage coverage. I've tested a rather low quality
>foil/braid shielded cable in the 4 MHz project, and the results are
>quite good. I didn't have time to include it in the 300 MHz work.
>Unfortunately, there is virtually no balanced cable manufactured in the
>US with foil/braid, or even braid only construction that is NEC rated
>(safe for flame spread and noxious fumes) for permanent installation.
>But Belden and Gepco are both aware of my work, and it is my
>understanding that both are studying how they ought to respond.
>
>BTW, Henry Ott has read my two Amsterdam papers which present the work
>up to 4 MHz, and says that he agrees with my conclusions. I don't know
>if he has seen the NY papers.
>
>Jim Brown K9YC
>http://audiosystemsgroup.com
>
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