Hi Bob,
Shield coverage does not matter! Study the SCIN paper -- what matters
is the UNIFORMITY of the shield. In the world of pro audio, we learned
long ago that twisting is at least as important as shielding, both at
audio and RF frequencies. From the earliest days of telephony, telco
cables ran on the same poles as power. In those days, power was
relatively free of AF noise components. The only noise rejection
employed was that the telco pairs were crossed over every few poles.
This was sufficient because of the long wavelength of 60 Hz and the low
harmonics associated with running motors.
At an RFI workshop I taught in 2005 with other members of the AES
Standards Committee WG on RFI, I demonstrated the use of one pair of a
CAT5 cable carrying mic audio from a pro condenser mic to the audio
mixer with both conductors of another pair connecting the Pin 1s. That
connection provided a bond for the mic shielding and a path for phantom
power needed to power the mic. I injected RF along the cable using a
Nextel phone and a TH-F6A Kenwood talkie, moving both along the cable,
and keying the talkie on-off continuously to generate AM. Another of the
same mic type was connected to the same mixer via Belden 8412. The mics
had some RFI susceptibility at VHF and UHF. The two cables were equally
good at rejecting the RFI. In his workshops, Neil Muncy demonstrated
the value of twisting by running a very long chain of mic cables around
the facility where he was presenting it, then moving a tape demagnetizer
along the line. The only hum heard was where the twist was interrupted
at mated XL connectors.
Someone asked about cross-references to the cable types tested. The
foil/drain cables included everything I could get my hands on at the
time. The braid-shielded cables included a variety of what was in my
working stash, a few samples that Belden provided, and a nice selection
from Gepco. Based on my work, Gepco later produced a miniature
braid-shielded twisted pair with no drain wire (comparable in size to
standard rack cable and mic snake pairs). The braid/foil pair came from
a member of the SynAudCon pro audio community working in Brazil. Based
on Neil's work, he got a local cable company to build it for him.
In general, all you need to know is to avoid foil/drain cables, if the
cable is shielded, it should have a braid shield with no drain wire, and
that a twisted pair cable with a drain wire is worse than no shield at all.
73, Jim K9YC
On 1/16/2018 10:40 AM, Bob McGraw K4TAX wrote:
The braid shield type is necessary where flexibility is required however, just
as in coax, not all braid is the same. Some provides 85% shield (poor) while
others approach near 100% shielding (good). There is a difference in price,
usually for this reason.
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