> Much has been said about ferrite solutions, but they have
been a big disapointment for me. I have NEVER had a snap on
type of core make any difference on any frequency. One piece
cores sometimes help, but it takes a dozen or more to raise
the impedance enough to be effective on 160, so that's
usually not practical.
They are a BIG disappointment for most people Larry.
All a hunk of ferrite does is increase the common mode
impedance of the cable. If you have a cable ten feet long
between two solid grounds, the bead can make a many-fold
increase in impedance and common mode rejection.
If you have a poor ground at either end of the cable or a
long cable, a ferrite bead as a general rule does little or
nothing to the system.
Think of the bead as the series "L" in a pi network. If
shunt Z and series Z is very low, adding a bead choke of
modest impedance will have a large effect. If the shunt Z is
high *or* the series Z is high, adding the bead will do
nothing.
If the bead is primarily inductive, adding the bead can
actually increase problems if the cable length is such that
the common mode impedance is capacitive.
The bottom line is just slapping a bead on a cable without
planning, even though that is the first thing we tend to do
nowadays, is most often a wasted effort. Odds are about 1
out of 4 it will help.
It's an impedance ratio problem. One 6 ft ground rod in the
right spot would do more than ten beads.
73 Tom
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