[Elecraft] K3S Microphone Cable

[Jim Brown]

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.