On Tue,6/28/2016 5:50 PM, N1BUG wrote:
I have been using chokes of 16 turns RG-303 on 160 meters, 16 turns
#14 THHN on 80 meters, 14 turns on 40, 13 turns on 30, and 12 turns on
20-10m, all on #31 material. I have yet to experience a failure at
1500 watts on my wire antennas. Perhaps I have been lucky. I
considered it marginal when I built them.
Lucky or duty cycle or not enough choking Z. Most of my high dipoles
have a choke along the coax in addition to the one at the feedpoint,
which reduces the total common mode dissipation and splits the common
mode voltage (and dissipation) between them. The second choke is mainly
there to break the feedline up like an egg insulator to prevent
interaction with nearby verticals. I fried a bifar choke on a high 80/40
fan that I had just rebuilt with new feedline but forgot to add the
second choke down the line. I fried it at the end of the second day of
AA CW on 80M when I had worked out the band and was beacon-CQing at
1.5kW. In other words, high duty cycle.
The physical form of the choke can be seen in k9yc.com/7QP.pdf W6GJB is
responsible for the mechanical design.
I have no evidence to back this up, but I am concerned the TH-11DX
antenna below the new 40/30 meter dipole may introduce significant
imbalance. The 40/30m dipole will be parallel to TH-11DX boom. I want
to design for more choking impedance to be on the safe side.
More Z is conservative for power handling.
I plan to use #43 material for these chokes.
My bifilar chokes are a little different than what you describe in
your tutorial. I made sure the wire was very straight, then bundled
into pairs with a continuous wrap of tightly wound teflon plumber's tape.
I do about the same -- I use small tywraps or Scotch 88 to hold the pair
together, then wind them. I don't think it's critical.
Understood. I have yet to see any objectionable affect on SWR using
#14 THHN.
W6GJB has done a nice rebuild of three C3SS tribanders that we use for
CQP. He found that the matching system that N6BT designed into the C3SS
didn't like the mismatch of the THHN chokes, so he used enameled wire
for those chokes.
73, Jim K9YC
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