On Sun,8/23/2015 11:49 AM, Roger (K8RI) wrote:
The reason for chokes was "it was handy". Being lazy, I decided to
try the second choke where the coax reached the tower as there was
already a connector there. The only extra work that entailed was
removing the weatherproofing from the connectors. It worked and that
let me get away without adding more weight hanging off the center of
the dipole.
That's a good move. The second choke reduces the common mode current,
reducing the stress on both chokes.
With the 5 core, 6 turn choke
http://www.rogerhalstead.com/ham_files/AntennaFeed2.htm plus nearly
100 feet of LMR-400 already hanging off the center. That dipole
already has well over 100# of tension to minimize the sag
I've got about 90# tension on mine, in the form of a water jug filled
with dry sand. Looking at your photo -- this choke is optimized for
30-15M. The wide spacing of the turns minimizes capacitance, so the
resonance is too high for 75M. Below 20M, it's best to closely space the
fewer number of turns that can be achieved with big coax.
I am building a single, larger choke, but using the 1/4" coax with a
Teflon dielectric and a double shield of silver plated braid. The
cores are spaced 1/8th inch and epoxied into the Lexan spacers. I've
not settled on the # of cores or turns of coax. The spacers are strips
of 1/4" Lexan 3/4" wide by a length to be determined. The slots are
milled 1/8th inch deep with 1/8th in spacing. I did one trial using
Plexiglass spacers with 1/4" deep slots, spaced 1/4", but they were
too fragile. The Plexiglass strips were cut from scrap and were
free. The Lexan was cut from a relatively small piece. Still they
were less than $2.00
Going to the Teflon coax is a good move in your system, because it
allows the greater number of turns needed for an effective choke. I have
not measured chokes wound with this type of coax -- the dielectric
affects the capacitance between turns, which can shift the resonance a
bit. I suggest that you follow my cookbook for RG8X-size coax.
I'll have to shoot some photos of the spacers and the construction.
Nice looking, Roger. You say 75M fan -- I'm assuming multiple wires to
increase the SWR bandwidth on 75M only. My long fans are 2-band, with a
single wire for 80 and a second for 40.
73, Jim K9YC
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