[TowerTalk] Feedline (choke) question

[Jack Brindle]

Both. Verticals will use whatever wire they can for return current. I have more experience with raised verticals, and know for sure it is a problem there. One symptom is high RFI in the area below the antenna - this was usually inside the house, and was noticed immediately. Having lots of radials helps since the return current will be divided between them. The coax needs to go straight down from the vertical to minimize its share of the return current, but it will still share some. 

Burying the cable helps for ground-mounted verticals, but if the RF can see the shield, it will still use it. As in elevated verticals, return current will be divided up between the radials and any other wire, including the coax.  

Best to apply chokes for both types.


One thing I saw when developing the KPA500 firmware was the effect of RF current on the outside of coax when it gets to an SO-239 that has a directional coupler attached. For these tests, of course, that was a KPA500. The outer and inner currents actually combine, increasing the measured reflected power. As far as the amplifier was concerned this was actually anomalous - only the current inside the coax mattered for this measurement. Choking the coax eliminated the issue. I have since seen this with other SWR bridges/directional couplers. Basically, where ever the first SO239 is hit, the currents combine. If that is a component that measures antenna properties, the reading will probably be anomalous, especially for reflected power.  I have never seen this documented anywhere to this point. It would seem to be a good QEX article for someone who has the time to do the measurements and testing and write it up.

73,
Jack, W6FB

> On Sep 26, 2025, at 7:35 PM, john at kk9a.com wrote:
> 
> Are you talking about a ground mounted vertical or an elevated vertical?
> 
> John KK9A
> 
> 
> Jack Brindle W6FB wrote:
> <snip>
> I have found that the one antenna type that absolutely must have a choke is
> a vertical. Without it the coax will provide a return for the current (no
> matter how many radials) and give all sorts of problems.
> 
> 73,
> Jack, W6FB
> 
> 
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