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Re: [IRCA] Cable connectors
I'm curious to know about the construction of the RG-6 cable that you used
for
your test?
Brand new, off the roll Belden RG-6 cable. Shield as you described as well
as the foam dielectric. Junk RG-6 wouldn't be worth the effort. In this
case it made no difference from the new Radio Shack coax which is not top
quality, but usually OK.
The cable was cut with an RG-6 specific cutter and the connector crimped on
with specific hex crimp pliers. DC ohm readings were quite low.
Another part of the test was to wind a dozen turns through a large ferrite
toroid. No difference at all. Also tried a new "un-un" to do a ground
isolation. Again, no change.
The problem with any balanced feedline is always line-balance. Since the
electric and magnetic field of the signals extends outside the physical
area of
the twisted pair, there will always be interaction between pairs within a
CAT
type cable. You would need far more spacing between pairs than is
available in
the CAT cable design to prevent interaction between pairs.
I might agree with that except CAT5 is designed to limit the interaction.
The individual pairs are tightly twisted, but not identical turns per foot
for each of them. What that does is cancel the interaction. The four pairs
are also twisted inside the outer cover which further limits pickup. In the
test, I only used a single pair and the others were floating. I set it to
use the Blue-Blue/White pair.
laying the cable on the ground or near to conductive surfaces will also
upset
the line-balance (for UTP cable.) And this prevents the common-mode
rejection
of signals common to both wires of the twisted pair from canceling
completely.
If they weren't twisted, that may well be true. I also have to think that
the loss of the cable on the ground would more greatly reduce the ingress
than increase it. While I didn't add this to the article, I did run a
further test. I laid out a 50' length and terminated it with a 100 ohm
resistor. That made the whole thing balanced when the receive end balun was
included. After taking a set of bandwidth readings, I laid a 50' length of
chicken wire on top of the cable. The goal was to provide some shielding
via capacitance to the ground. End result was no change at all.
Shielded CAT cable is a different animal, outside effects being much less,
but
the inter-pair coupling is still an issue.
I haven't found a source for the shielded cable or appropriate RJ-45
connectors and the different crimp tool. I have had access to a 25'
shielded cable but that again made no difference from the common type.
Bottom line to me is that the CAT5 is better than coax. I have not done an
A-B test with CAT5 and speaker wire. I may get to that.
One of the more important things I've learned in my lifetime is to leave my
ego outside. If someone has an improvement, I'll grab that in a heartbeat.
I do thank you for your comments on this.
Craig Healy
Providence, RI
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