> You're missing the fact that although these conductors INTENTIONALLY
> carry DC and low rate signals, they also can PICK UP RF and AF noise.
> Twisted pair, when used to carry a signal, is at least as powerful at
> rejecting RF and AF noise pickup as coax, but ONLY if the pair is
> dedicated to a circuit.
And also only if the twisted pair is properly terminated in a balanced
termination - on both ends. I was field engineer for Bell. I was
sometimes totally amazed at just how good twisted pair are at
rejecting noise. As good as coax, if not better.
But . . . the telco pairs are terminated in carefully balanced
terminations. I don't think the typical ham/audio install is so
carefully designed. They may be, I don't know for sure. I suspect the
terminations are, for the most part, unbalanced. There will still be
some rejection of noise, maybe a good deal. But not as good as if
properly terminated.
I have used twisted pair here with good success, but there is some
luck involved if the pairs are not properly terminated.
73 de dave
ab9ca/4
On 5/9/16 12:03 PM, Jim Brown wrote:
On Mon,5/9/2016 9:41 AM, Lynn W. Taylor, WB6UUT wrote:
If we're talking DC levels, sure, no problem. Low rate signalling
just won't matter.
You're missing the fact that although these conductors INTENTIONALLY
carry DC and low rate signals, they also can PICK UP RF and AF noise.
Twisted pair, when used to carry a signal, is at least as powerful at
rejecting RF and AF noise pickup as coax, but ONLY if the pair is
dedicated to a circuit. That only happens when both ends of the cable
are correctly wired. That is, OR and OR/WH will reject noise, but OR
and GN will NOT.
At audio levels, the shielding from twisting the wires might be
helpful.
It is VERY helpful.
If we're actually using them for Ethernet, then the twisted pairs
aren't wires, they're transmission lines. 100 megabit ethernet is
running at 100 megahertz.
Right. AND -- they are transmission lines at RF frequencies even when
the INTENDED signal is AF or even DC. THAT'S HOW they reject RF and
AF noise.
Wire the cable randomly and you don't have twisted pairs
(transmission lines) at VHF frequencies (or above).
And you don't have them at AF or HF either.
Bottom line -- CAT5 and similar cables are excellent for control
wiring and even for RF, but we must ALWAYS wire them so that every
circuit uses a pair. Wiring them any other way is a recipe for noise
and RFI.
73, Jim K9YC
73 -- Lynn
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