Twisted Pair Wiring and RFI Rejection

[Edward R Cole]

When I was considering wiring for my new station control panel, I
decided to "harvest" twisted pair from Cat-5e to use as hookup
wire.  I was aware that it would reject low-freq noise and hum
without shielding.  Having a large amount of Cat-5e didn't hurt,
either.  I believe Cat-5e is isolated up to 100-KHz.  I have not
taken photos to post to my website, but have a webpage established
for the control panel project.
http://www.kl7uw.com/controlbox.htm

For a teaser:  It uses a four-stage sequencer for VHF operations,
built-in computer soundcard interface. 17 switches and 22 DPDT relays
to control preamp, amps, relays, remote power supplies, keying to
interface SO2R operation.  With this I control 17antennas on three
towers and one dish and equipment on bands from 600m to 3cm.  All in
a 3x8x6 inch enclosure.

High-tech twisted-pair! ;-)

73, Ed - KL7UW

************************
Message: 33
Date: Mon, 30 Aug 2010 08:11:09 -0700
From: "Jim Brown" <[hidden email]>
Subject: [Elecraft] Twisted Pair Wiring and RFI Rejection
To: "[hidden email]" <[hidden email]>
Message-ID: <[hidden email]>
Content-Type: text/plain; charset="us-ascii"

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



73, Ed - KL7UW, WD2XSH/45
======================================
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[Jim Brown-10]

On Mon, 30 Aug 2010 23:08:34 -0800, Edward R. Cole wrote:

>I believe Cat-5e is isolated up to 100-KHz.  

In essence, the ability of twisted pair to reject HF noise is directly
related to the number of twists per inch, and the uniformity of the
twist. The close spacing of the twist also helps. High quality twisted
pair like any of the CAT5/6/7 cables continues to reject noise well into
the VHF range, although the degree of rejection degrades with increasing
frequency. Noise rejection is also a function of the balance of the
receiver, and to some extent, the balance of the output stage that
drives the line.

CAT5/6/7 cables as a group are constant impedance (100 ohms) and have
relatively good bandwidth (well into the tens of MHz). Like any
transmission line, loss increases with frequency.

73, Jim K9YC


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