On Thu, 11 Mar 2010 23:46:37 -0500, Jon D. Garner wrote:
>I only use Belden Cat5e sheilded. and who I recommend.
While Belden makes the best Ethernet cable by far, I have not found it to
make any improvement in RFI. As others have noted, it's all common mode on
the cables, and BOTH ENDS OF EVERY CABLE IS A NOISE SOURCE! Radiation is
NOT the result of any defect in the cable, but rather inferior output
transformers that put common mode onto that cable.
In general -- consider the cable a long wire antenna, and put a toroidal
choke (or a multiturn clamp-on) within 1/10 wavelength of both ends. If the
cable is shorter than 1/4 wavelength at the highest frequency were you have
a problem, a single choke in the middle is probably enough. Chokes wound
with #31 are good for a 6:1 frequency range if you set the number of turns
to center them in that range. If you need to cover more frequency, use a
second smaller choke at each end tuned to the higher frequency.
EVERY cable is a potential antenna, including a cable to a cable or DSL
modem, and the power supply for each of those boxes, so any or all of these
cables may need a choke. What generally happens is that they all
contribute, so if you have multiple cables, you may not hear much
improvement until you've choked most of them.
See http://audiosystemsgroup.com/RFI-Ham.pdf for detailed winding advice
and part numbers for Fair-Rite parts that work best.
[There is, however, a good reason for using Belden's cables -- faster
Ethernet due to lower error rates. I use Belden Mediatwist, which is their
best, but even their lower grade cables are better because the pairs are
molded to hold their spacing more uniform.]
Jim Brown K9YC
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