Best is to insert two CMCs. One at the feedpoint of the dipole or beam
feedpoint and the second at the shack end of the coaxial transmission line.
They are also good for keeping local RFI from the receiver. I recently ran
another test without / with the CMC only in the shack at the end of the
feedline. This time I chose 20-meters as my 450-foot long doublet just
happenes to show less than 2:1 SWR on that band so I don't have to invoke
the L-Network for a match. I measured 9.6 dB less white noise (no
suggestion of 60 Hz or harmonics) with far more signals on the waterfall
with the CMC than without on the Icom 7300.
Dave - WØLEV
On Mon, Jun 28, 2021 at 10:31 AM Kim Elmore <cw_de_n5op@sbcglobal.net>
wrote:
> Having just come off of filed day, the value of a CMC was, for the
> umpteenth time, demonstrated in the most practical sense. We were
> operating an FTDX5000MP for the CW station. We probably had the best
> antenna of any station at the site, up about 70 ft or so. But we had RFI
> in the shack: we could hear big thumps through the headphones as we
> keyed the rig and the WinKeyer would occasionally lose it's mind as
> would the laptop we were using for logging.
>
> The owner of the rig had a CMC choke he'd left at home, went home and
> got it, we inserted it and, like magic, all RFI problems were gone!
>
> His was straight out of the latest K9YC cookbook CMC (14T of RG-400 on a
> type 31 2.4" OD ferrite core) in a nice, weatherproof box and the only
> place it would fit was outside where two cables had to be connected
> together with a PL-258 (barrel connector). We had a brief discussion
> about the optimal placement of a CMC: where in the line should it go? I
> take the view that it should be at the antenna feed point, but he
> thought somewhere in the middle of the line is optimal. Now, though, I
> wonder of it really matters where it is in the system. Does it? If so, why?
>
> Kim N5OP
>
> --
> Kim Elmore, Ph
>
> Kim Elmore, Ph.D. (Adj. Assoc. Prof., OU School of Meteorology, CCM, PP
> SEL/MEL/Glider, UAS, N5OP, 2nd Class Radiotelegraph, GROL)
>
> /“I occasionally play works by contemporary composers and for two
> reasons. First to discourage the composer from writing any more and
> secondly to remind myself how much I appreciate Beethoven." – Jascha
> Heifetz/
>
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>
--
*Dave - WØLEV*
*Just Let Darwin Work*
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