[TowerTalk] Choke on feed point of dipole

[Jim Brown]

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