Twisted Pair Wiring and RFI Rejection

Posted by Jim Brown-10 on Aug 30, 2010; 3:11pm
URL: http://elecraft.85.s1.nabble.com/BL2-choking-Impedence-measurements-tp4686857p5478634.html

On Mon, 30 Aug 2010 10:14:14 +0100, David Woolley (E.L) wrote:

>That's another of Jim's suggestions that the UK amateur radio trade
>and RSGB shop have failed to take note of.

I don't want to take credit for repeating fundamental principles that
have been well understood for at least a century. The great telephone
companies began using twisted pairs for both short and long lines
early in the 20th century. In the early days, the only source of
interference was 60 Hz power, so wires were run as parallel
conductors, with the twist in the form of a crossover every other
pole. This worked fine, because it was much much shorter than the
wavelength of the 60 Hz power, and telephone lines were able to run
directly under power lines without interference!

Today, CAT5/6/7 cable rejects noise SOLELY by virtue of its high
quality twisted pair construction. Crosstalk is further reduced by
the fact that the pairs are twisted at different rates.

These fundamental principles are WELL understood in the EMC world,
and those of us working in EMC in pro audio have had to learn them to
keep hum, buzz, and RFI out of our systems.

I find it ironic that RF folks look down on audio engineering,
because "it's only 20-20,000 Hz." In fact, audio systems are far more
complex than RF systems -- they span 3 decades of frequency, require
transducers that can produce high power over this wide frequency
range with controlled dispersion far more complex than most antenna
systems, must work in an acoustic environment that is at least as
complex as the ionosphere, have dynamic range requirements of 100 dB
or more, and must regularly achieve that with signal levels in the
millivolt range in an environment that is full of all sorts of
electrical noise.

73, Jim Brown K9YC


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