[TowerTalk] Test Fixture for Common Mode Chokes

[David Gilbert]

I didn't write the text that you responded to.  VE7RF did.

Dave   AB7E


On 1/25/2026 3:23 PM, Jim Brown wrote:
> 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.
>
>
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