[TowerTalk] Choke on feed point of dipole

[Jeff Blaine]

No problem on the ferrites cracking here in KS.

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

On 1/13/2026 12:13 AM, Jim Brown wrote:
> On 1/12/2026 9:16 PM, Tom Hellem wrote:
>> I’m going to try base feeding it with an LC network and see if I can get
>> better results.
>
> You might also try it as an end-fed half-wave using a 7:1 transformer 
> on one or two Fair-Rite #61 2.4-in o.d. cores. If you wind one winding 
> on top of the other, the capacitance between windings will will couple 
> common mode current. If you put the two windings on opposing sides of 
> the core, the transformer will block common mode current, not 
> perfectly, but pretty well, because there's still a MUCH smaller 
> capacitance between windings through the core.
>
> Looking at what the EFHW guys are doing, the design that looks best to 
> me have a 2-turn primary and a 14 turn secondary, with 100 pf across 
> the primary. They seem to have not yet discovered that Fair-Rite #61 
> is a much lower loss material at HF, especially below 10 MHz, than the 
> much lossier materials like #43 or #52. Higher loss means more heating 
> of the core. I suggest that you try it with one core, transmit for a 
> while, then go out and check for heating. If you can hold your finger 
> on the core with it feeling hot, I'd say it would be enough. If too 
> hot, rewind it on two cores.
>
> I suggest that you hold the cores together with ty-wraps, but not too 
> tightly so that freeze-thaw doesn't crack them. On this issue, I'd 
> like input from AC0C, who has winters; I did too, for 42 years in 
> Chicago, but not here, 5 miles from the Pacific! FWIW, in the 15 years 
> I've been recommending chokes, no one has ever told me about an issue 
> with freeze-thaw.
>
> Like chokes, they must be either in free air or a well ventilated 
> enclosure. NA6O, an EE retired from the EMC world and part of the 
> engineering team at superstation N6RO, did some lab testing to confirm 
> this, and built ventilated enclosures for the chokes on the 160M 
> antennas. Photos and a description of his work are in the text that 
> goes with my 2018 Cookbook.
>
> The only reason for connecting to mother earth at the antenna is 
> lightning protection -- it doesn't make the antenna work better.
>
> 73, Jim K9YC
>
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