Lots of meanings to the word induce. The one I was using was: to
bring on, or about; effect; cause... I was not intending to imply
transformer action. As an engineer I should know better than to
try to use English to describe electrical phenomenon.
My intention was to find out what: " ...the skin depth of the coax
shield gives up well before 60Hz..." meant.
Real life examples show that coax does just fine at shielding all
the way down to DC... As long as you keep the currents flowing
through the outside of the shield to a minimum. Which is done
routinely by not connecting the ends of the shield so that current
loops occur. Hardly a PA system exists that doesn't have a fairly
long bit of unbalanced shielded cable at some high impedance high
gain input.
-Chuck Harris
Alexander Pummer wrote:
No, the current passing the outside f the shield will not induce any voltage
inside
of the coax, but the voltage drop caused by the current on the ohmic resistance
[!!!]
of the shield will show upbetween the two ends of the cable -- and that will
show up
as it was added to to the voltage which is carried on the center conductor of
the coax.
73
Alex
On 7/20/2014 6:10 PM, Chuck Harris wrote:
I'm not sure what you are saying.
skin depth = (2.6/sqrt(fhz))inches for copper.
So, at 60Hz, skin depth = 0.336 inches.
and at 100KHz, skin depth = 0.008 inches.
and at 1MHz, skin depth = 0.0026 inches.
Are you saying that at 60Hz, because the
skin depth is deeper than the coax shield is
thick, that current passing through the outside
of the shield will induce voltage inside of
the shield, and that at say, 100KHz where the
skin depth is a little less than the shield
thickness, or at 1 MHz, where the skin depth
is only a small fraction of the thickness of
the shield that it won't?
Or something else?
-Chuck Harris
Bob Camp wrote:
Hi
The “coax is an antenna” problem comes in well before you get to DC. Even with
no
transformer involved, the skin depth of the coax shield gives up well above 60
Hz
(and likely well above 100 KHz). If you want to do full isolation over a very
wide
range you need some combination of shielding and balanced lines.
Bob
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