On 8/12/2013 1:42 PM, Bruce wrote:
I am using a twisted pair, with shield, that is near 55 ohm
impedance for
my receiving delta loop. The cable is designed for audio,
Depending on the nature of that cable, you're probably better off
without the shield if both ends are transformer-isolated as I described.
There are two potential problems. First, the shield provides a lovely
path for common mode current, which can couple noise via Pin One
Problems to a rig. Second, if the cable shield is foil plus drain wire
("rack wire" like Belden 8451), shield current will be STRONGLY coupled
to the twisted pair by a mechanism that Neil Muncy named
"shield-current-induced noise" (SCIN). The mechanism is that the drain
wire has the same lay as the signal pair, and is manufactured so that it
is much closer to one conductor the pair than the other. This results in
more inductive coupling to the closer conductor, converting the common
mode current to a differential voltage.
SCIN is a strong component of RFI to audio systems when the equipment it
feeds lacks adequate low-pass filtering. Until about ten years ago, Greg
Mackie built all of his mixers with DC-to-daylight frequency response in
a misguided attempt to avoid phase shift at higher audio frequencies. He
also built them with massive Pin One Problems. They were otherwise very
nice mixers, great bang for the buck, but if you used one anywhere close
to a TX on the high end of the AM broadcast band you were almost certain
to hear it. And the problem was made worse because the mic cables
running through the walls of a wood frame church had foil/drain shields.
And, of course, the Behringer mixers of that period, which were perfect,
but poorly built copies of the Mackies, complete with mistakes on the PC
boards, had the same problems.
73, Jim K9YC
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Topband Reflector
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