On 10/7/2010 11:14 AM, Don Moman VE6JY wrote:
>The ISP installation has 4 runs of CAT5 shielded (drain and foil grounded at
>bottom only) to their equipment at the 140' level.
That's a problem -- to be effective, the shield MUST be bonded to the
shielding enclosure at both ends. A shield bonded at one end only on a cable
longer than about 1/20 wavelength is no shield at all.
> So I guess my question is if this method will solve the noise issue or if
> additional things should also be done at the same time ?
My GUESS (and this is only a guess) is that shielding is going to be the
solution. But that solution is expensive, so one thing I would try first
(because it's cheap) is serious ferrite chokes tuned to the frequency of
interest at both ends of every cable. This would help the common mode
component of the problem. Because the noise is very strong, you may
need multiple chokes in series at each end of each cable. I suggest that
you carefully note the effectiveness of each set of chokes as you add
them. That is, add one choke to each cable at each end and note the
noise reduction, then add a second set at each end and measure again.
If I've guessed right that this is common mode, even with shielding,
you're going to need the chokes on any exposed cable at both ends.
Another thing to not rule out is lousy design and poor shielding of the
gear in the head end. I've seen a similar installation that radiated a
lot of HF trash from the equipment itself, which was in an unshielded
rack (glass doors, etc.).
73, Jim Brown K9YC
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