AB7E wrote:
...I assume that we want to place the CMC as close to a current maximum
as we can..."
In the models I've looked at, that's invariably at the feedpoint. If you
still have CM issues after installing a choke there, you might want to
try a second choke at a different spot some distance away. You can use a
clamp-on current probe to find a maximum (if one exists). Or you can
just pick a convenient spot. A good place might be a free-space quarter
wavelength down the line. The idea is that if an unanticipated shield
current null occurs at the feedpoint, a maximum will occur a quarterwave
away. (This does not apply to a traveling wave, which slowly decays.) If
you measure the shield impedance at the chosen spot using the procedure
I mentioned earlier, you can calculate the shield attenuation in dB a
proposed choke will provide without having to sever the coax and install it.
https://k6sti.neocities.org/cmcb
The last plot looked fine at the new image hosting service. No
extraneous stuff.
The decaying traveling wave prompts a second test, this time with an
animated GIF. This magnified view sights down the horizontal section of
the shield conductor, which is coincident with the green dot. The
current traces are phasors. The distance from the wire to the trace is
magnitude, while the angle with respect to the wire is phase. A slowly
decaying spiral is characteristic of a dissipating traveling wave. This
is the feedline for the Yagi pictured earlier.
https://iili.io/fPoIrOl.gif
Brian
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