[TowerTalk] Test Fixture for Common Mode Chokes

[Jim Brown]

Of course. But what matters most is the common mode current at the 
feedpoint, which is where common mode current couples to the receiver. 
which For my dipoles for 80, 40, and 30M, this is 120 ft in the air, 
suspended between redwoods. How do you propose I make that measurement?

I also use a choke farther down the feedline to prevent it acting as a 
parasitic element to my 160M vertical. 18 years ago, I observed its 
effectiveness on the air when I no longer observed that happening.

After I added my first tower, 120 ft tall, 150-200 ft from my 160M Tee 
vertical, N6BV and NI6T (separately) suggested that I look at 
interaction. I spent the summer doing that, first measuring and plotting 
their physical alignments in AutoCad, then spending more than a month 
studying them in NEC. I had also rigged two wire verticals sloping away 
from the tower, one at about 70 degrees Az, the other at 270 degrees, 
insulated from the tower and fed from the base against elevated radials, 
and with on-ground radials for the tower.

NEC showed that the tower gave me a few dB to VK/ZL, shorting the Tee at 
the feedpoint kicked the few dB the tower gave the sloping verticals 
30-40 north to EU and JA. I provided the shorts in the shack with 
suitably terminating the unused feedlines (tweaking lengths of those to 
the tower, switching in a stub for the Tee, which is just outside the 
shack). I observed the directivity on the air.

What the model didn't show was the absorption of the dense redwoods 
surrounding the tower, so the directivity of the two sloping wires was 
only useful on receive. :)

73, Jim K9YC

On 1/25/2026 11:09 AM, David Gilbert wrote:
> why don't you folks just use a clamp on RF ammeter, and actually
> measure the CM current,  at several different places along the coax cable
> ??   The CM current will vary along the length of the coax due to swr on
> the outside braid of the coax, even though the ant swr may well be  1:1.