[TowerTalk] Test Fixture for Common Mode Chokes

[Brian Beezley]

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