[TowerTalk] Feed Point Choke Question

[Kirk Kleinschmidt]

Okay. Let's try to simplify my situation.
Storm cloud are visible, so I don't have time to buy teflon coax and wind an awesome choke, mount it in a box, install SO-239s, etc. I DO have 12-gauge THHN, but I don't have the time to source a workbox and install connectors right now. In the spring this will all be redone properly, as the antenna will be moved to a taller tower, etc.
Think of this as a post-apocalyptic scenario. No shopping. No non-zombies with whom to trade. Etc. Just what I have on hand.

I have a hexagonal beam on top of a 22-foot tilt-over tower. At the base of the tower I have a weather-resistant box that houses a terminal strip (for patching my tower rotator cable to my shack run of rotator cable) and a UHF bulkhead / union, for patching my tower coax, which includes a rotator loop AND a ferrite core, to the length of larger coax that runs back to the shack.

The tower coax is RG-8X or LMR-240. Including the choke and the rotator loop it will be 30-35 feet long.
So, with storm clouds visible and no time to shop or build the best possible choke, what do I do?
I have a length of coax, and I have an FT-240-43 or an FT-240-31. How many turns?  Solenoid or bunched up?  Tightly wound or turns of a couple inches?

Experts get set. Experts get ready. Experts GO!
Thanks.  :)
--Kirk, NT0Z

P.S. LUV the cold-WX signal boost that I have historically enjoyed...but as I approach my early-onset curmudgeon phase, I'll take the T-shirt WX we're now having in MN (2 days only). 

P.P.S. The hexagonal beam manufacturer sells / recommends a sleeve balun made from a bunch of ferrite cores slid onto the coax near the antenna's feed point. That may indeed work from 20-10 meters, but I have never been all that enthusiastic about that approach. At any rate, I only have a few of the 2.4-inchers on hand.
P.P.P.S. I don't know if I have correctly sized/wound them, but I wish I would have learned about K9YC-style chokes much earlier in my ham career. Since I have used them (past 6-8 years) the level of locally received noise on all external and attic-mounted antennas has dropped dramatically. I have new appreciation for the intricasies of winding, designing, and testing them, but right now I just want someone "in the know" to help me wind something that works reasonably well from the materials I have on hand.  :)


My book, "Stealth Amateur Radio," is now available from www.stealthamateur.com and on the Amazon Kindle (soon) 

    On Friday, November 6, 2020, 2:59:10 PM CST, Artek Manuals <manuals at artekmanuals.com> wrote:  
 
 Jim

Show me the ACTUAL antenna range pattern measurement data  with and 
without the common mode choke on a a similar antenna, with a reasonable 
SWR ( LESS THAN 1.5:1)  and the feed line has been brought down 
perpendicular to the plane of the antenna and I will concede you the point

Dave
NR1DX


On 11/6/2020 3:37 PM, Jim Brown wrote:
> On 11/6/2020 12:28 PM, Artek Manuals wrote:
>> The short answer is how tight you make the turns will have little 
>> real world effect. 
>
> That short answer is WRONG.
>
> Use the RG-8X I would probably use the #31 core over the
>> 43 . The poor mans 8 turns of coax 4-5" in diameter without ferrites 
>> will probably actually work as well. 
>
> Define "work." The important thing a choke should do is kill common 
> mode current on the feedline that couples noise to the antenna and 
> fills in nulls in the antenna's pattern. That "poor man's" solution 
> won't do much for that. If "work" means it won't overheat, yes, it 
> would "work."
>
>> Depending on SWR and feed line dressing (always perpendicular to the 
>> plane of the beam the need for choke at all� is arguable
>
> It's arguable only if you don't understand the problem, or don't care 
> about noise or the antenna's nulls.
>
> 73, Jim K9YC
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-- 
Dave Manuals at ArtekManuals.com www.ArtekManuals.com


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