On Fri, 18 Jun 2004 08:20:14 +0100, Ian White, G3SEK wrote:
>We've all seen examples where one clamp-on bead can work, but I don't
>think that negates Tom's point.
Yes, it does, because Tom's post is aimed at the mindless application of
clamp-ons, and mine is aimed at a thoughtful engineering approach.
My original post observed that 1) replacing the parallel wire telephone cable
with cable that is effectively twisted would significantly reduce
DIFFERENTIAL MODE rf at the equipment; and 2) common mode RF at the equipment
could be reduced by an effective choke.
>If one bead is enough to solve an RFI problem, it only proves that the
>problem was a "soft target".
There is a tendency for engineers to take an "all or nothing" approach to
problems, and inflate the costs in the process. Many problems ARE soft
targets, and virtually all detection is square law. A 10 dB reduction in
common mode current can reduce detected interference by 20 dB, a 6 dB
reduction in current reduces detection by 12 dB, and so on. Let's not get
caught in the trap of going after gnats with elephant guns.
I could cite other examples of soft targets in my home: a few turns of
loudspeaker cables around a small toroid took the RF out of a stereo
receiver; a few turns of the line level drive cable around the same toroid
took it out of some amplified loudspeakers hooked up to my wife's computer.
I'm running 100 watts into dipoles that are within about 25 ft of the
susceptible equipment. Obviously, 1 kW needs 10 dB more suppression, but the
antennas may be 6-10 dB further away too.
Jim Brown K9YC
http://audiosystemsgroup.com
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