Towertalk
[Top] [All Lists]

Re: [TowerTalk] Feedline (choke) question

To: towertalk@contesting.com
Subject: Re: [TowerTalk] Feedline (choke) question
From: Jack Brindle via TowerTalk <towertalk@contesting.com>
Reply-to: Jack Brindle <jackbrindle@me.com>
Date: Fri, 26 Sep 2025 22:24:37 -0500
List-post: <mailto:towertalk@contesting.com>
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@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
> 
> 
> _______________________________________________
> 
> 
> 
> _______________________________________________
> TowerTalk mailing list
> TowerTalk@contesting.com
> http://lists.contesting.com/mailman/listinfo/towertalk


_______________________________________________



_______________________________________________
TowerTalk mailing list
TowerTalk@contesting.com
http://lists.contesting.com/mailman/listinfo/towertalk
<Prev in Thread] Current Thread [Next in Thread>