[TowerTalk] Choke-Winding Wire Effects?

Jim Brown jim at audiosystemsgroup.com
Fri Dec 11 04:33:25 EST 2020

On 12/11/2020 1:10 AM, Kirk Kleinschmidt via TowerTalk wrote:
> Hi, Jim and the group,
> I need to wind a low-band feed point choke on one or two FT-240-31 ferrite cores, and I noticed that various wire and coax chokes (measured) are listed in the cookbook.
> 1. The black and white wires from 12-gauge "romex" are listed, but I have on hand lots of black and wires wires from 12-gauge, heavy-duty extension cords. This is stranded, which I understand isn't as well behaved as solid, but I'm wondering whether the choke performance is similar to same-sized romex? I'm guessing that it is.

It isn't only about conductor diameter, the dielectric properties of the 
insulation, their thickness, and turn spacing when practical chokes are 
wound that matters. So no, I don't recommend using them and expecting 
results predicted by my measurements of other wires.

> 2. If I decide to use two cores in series, do I have to connect them "end to end" (like the wheels on a car), or can I stack them with some minimal spacing (like stacked dinner plates)?

They can be stacked; the key is to stack them far enough apart that 
capacitive coupling between them doesn't de-tune them or couple around 

> This is a 10-100 W choke, so dissipation isn't likely to be a problem. I have RG-58 on hand, but I don't see any current winding data for it and I want to make a choke that "has data" instead of just guessing. This choke will be at the base of an inverted-L and will be in a waterproof box, so WX shouldn't be an issue.

I didn't considered RG58 because it's a very high loss coax. The only 
good reasons to use it are low cost and the ability to fish it through 
tight spaces, like vehicles. RG400 is equally lossy, but it's VERY well 
shield, and it's power handling (thanks to its TFE insulation's ability 
to withstand high temperature gives it very high power handling) makes 
it worth using. Chokes that do anything useful can burn enough TX power 
to overheat if not well designed.

FWIW, I do a lot of QRP, and my smallest coax is a very good RG8 (#10 
solid copper center, shield is very robust tinned copper braid plus a 
pretty good foil; the robust copper braid matters at low frequencies, 
the robust foil at VHF/UHF). If I'm running 5W (or if I'm doing weak 
signal work at legal limit), I don't want to burn half of my transmitter 
in lossy coax! I also do a lot at legal limit.

73, Jim K9YC

> As always, thanks,
> --Kirk, NT0Z
> My book, "Stealth Amateur Radio," is now available from www.stealthamateur.com and on the Amazon Kindle (soon)
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