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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