> -----Original Message-----
> From: email@example.com [mailto:firstname.lastname@example.org]
> Behalf Of Jim Brown
> Sent: Tuesday, December 23, 2003 00:04
> To: RFI List
> Subject: Re: [RFI] shielding question
> 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
> lots of data on real cables, and show the effect of various cable
> on RF detection in equipment. They also document my test setup, and
> report of field testing of cables and equipment when exposing it to
> transmitters on Field Day, and to broadcast transmitters.
What did you lab setup consist of?
> The basic mechanism is inductive -- when current flows on the shield,
> it induces voltage on each of the signal conductors that run inside
> 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
braid shield with a drain wire? Don't think I've ever seen that.
> current is quite evenly distributed over the shield. But if there is a
> drain wire, the current will generally divide between the drain and
> 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,
> effect causes the current to be more evenly distributed, and my
> research suggests that braid-shielded cable loses its very advantage
> roughly 20 MHz (this will, I think, vary with cable construction).
I would expect so. What type of foil was on the cables in your tests?
I have seen a wide range of 'foil' material over the years. Some that
looked like it had less metal than those shiny plastic static protection
bags, up to this stuff I got now that is a relatively thick apparently
solid metal foil. The gauge of the drain wire compared to the
conductivity of the foil material would probably be important also.
> Now, consider that in most foil/drain cables, the drain wire is
> at the same rate as the signal pair, and is usually constructed so
> the drain wire is much closer to one of the signal conductors than the
> other. This is the principal cause of SCIN.
In a 2 wire cable how can it be much closer to one than the other? If
the drain is twisted along with the two conductors it doesn't have much
choice but to lay directly between them. If it is laid straight with
the foil it would cross each conductor once in each twist.
> 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.
how about putting this in perspective. I can make statements all day
that sound horrible taken alone, but when compared to other factors are
way down in the noise (literally). How does 'scin' compare to other
currents on the wires. i.e. unshielded current, braid shielded current,
and foil/drain shielded currents all in the same environment. if 'scin'
is already 100db below unshielded current pickup its not going to bother
me even if it is 50db worse in foil/drain cables than braid shielded
David Robbins K1TTT
AR-Cluster node: 145.69MHz or telnet://dxc.k1ttt.net
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