On 2/14/2025 5:01 PM, Jim Lux wrote:
I'd definitely terminate the shield at one end. You don't want it floating. And
terminating at both ends might be a way to get an inadvertent loop. Jim will
probably weigh in - shielded twisted pair is his jam.
FWIW, I would NOT use shielded CAT cable for this system. Rather, I
would use either a transformer or a common mode choke to transition from
the coax to the twisted pair at both ends. Twisted pair inherently
resists crosstalk at frequencies where the twist ratio in turns/unit
length (called the "lay") is high as a fraction of the wavelength, which
is certainly the case for the lower ham bands. And the transformer would
need to have VERY low capacitance between turns. I gave a talk on this
about five years ago, the material was added to the ARRL Antenna Book or
Handbook (don't recall which). http://k9yc.com/RXChokesTransformers.pdf
I achieved the low capacitance with low loss by placing the two windings
on opposing sides of a suitably selected small toroid.
Almost 20 years ago, as part of an EMC workshop for audio professionals,
I set up a tutorial demonstration to show the relative importance of
twisting and shielding. The signal source was two identical electret
microphone known to have RF susceptibility to VHF and UHF signals. One
of these mics fed the mix console with a very high quality shielded
twisted pair. The other used one pair of high quality CAT5 cable for the
signal pair, and both conductors of another pair in that cable in place
of the shield. That connection was required, because the FET follower is
powered by "phantom power," 12-48VDC, with V+ applied through high value
low tolerance resistors to both signal pairs, and V- to the shield (or
in this experiment, the pair replacing the shield.
My signal injector was a Kenwood TH-F6A talkie, which I continuously
keyed on and off as I moved it along each cable. The observed result was
that the two cables were equally good, equally susceptible, the the
talkie. That is, both heard it, but it wasn't overwhelming.
And it should be noted that the dynamic range of audio is in the range
of 80-100 dB for normal program material, and that the peak output of
these mics is on the order of 0.5V - 1V. Another observation is that the
turns/inch of that very good mic cable was probably 1/20 that of CAT5.
In the world of pro audio, we learned long ago that twisting is at least
as important as shielding.
Neil Muncy had a great demo on this for his tutorial workshops. He
connected a dozen or so mic cables in series, running them around the
lecture room, and moved a tape demagnetizer along the line as it passed
through mating XL-connectors. Within the connectors, the pair was not
twisted, and at that point, a very loud hum was heard. Anywhere else
along the cable it was not.
73, Jim K9YC
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