On Mon,1/12/2015 8:49 PM, KD7JYK DM09 wrote:
Wrap a half-dozen turns around
your hand in a loop four to five inches across and secure with plastic ties.
See what happens, try a few more or few less turns if the problem persists.
That is NOT a choke, it is an inductor, which can resonate with a cable
whose length makes it capacitive. An effective choke must cause the
common mode circuit to have a high common mode Z at the frequency(ies)
of the interference. The only practical way to do that is with
multi-turn choke wound around a lossy ferrite core, placing the broad,
low Q (typically 0.4) in the center of range where suppression is
needed. That is EXACTLY how #43 clamp-ons work with a single turn at VHF
-- their resonance is centered around 150 MHz.
Further, the choke must raise the common mode Z enough that the common
mode current falls by enough to eliminate the interference. If the
common mode Z is 500 ohms without the choke, we must raise it to at
least 1,000 ohms to get 6 dB of suppression. What if we add a 500 ohm
inductor and the cable is 500 ohms capacitive? Now the common mode
circuit is series resonant, and interference gets much worse!
Study k9yc.com/RFI-Ham.pdf
73, Jim K9YC
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