[TowerTalk] Feed Point Choke Question

[Jeff Blaine]

Kirk,

Almost all the chokes I've used in the last 10 years have been built 
loosely around the K9YC cookbook.  Those are generally speaking a couple 
of type 31 ferrites with a few turns of coax running through it, with 
the turns widely spaces (not bundled). Works great, is simple to make 
and quick.

As with so many projects like this, you certainly can spend a lot of 
time working on esoteric optimizations which may or may not be 
noticeable.   So go with what you have, and it will probably work just 
fine.

As to the optimal number of cores & turns, that depends on the bands of 
interest and what you have in the way of core inventory there.  But if 
you will hunt down the K9YC transmission choke cookbook, Jim has done 
all the lab work there - just follow his recommendations (if you have 
enough cores) and leave the rest as a good winter research project for 
when it's too cold to go outside.

73/jeff/ac0c
alpha-charlie-zero-charlie
www.ac0c.com


On 11/6/20 7:38 PM, Kirk Kleinschmidt via TowerTalk wrote:
> 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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