Exactly!
In the case of Tennadyne, the booms are spaced to give a 50 Ohm feed point.
To do the drooping coax get yourself the Home Depot/Lowe's and buy a 12"
x 24" sheet of UV protected Lexan.
Cut it into eight 3" strips a foot long, drill to clear #8 hardware. Get
some 1.5 inch stainless fender washers with a number 8 hole, and some SS
hardware. Drill the bottom boom through to clear the #8 hardware equally
along the boom. Drill the bottom of the Lexan to clear the cable P-Clips
hardware. I'd alternate sides with the support strips. I'd also use a
stout Common Mode Choke where the coax comes back to the mast and heads
down the tower.
Go ahead and use the Collins choke at the feed if $$$ is an issue.
Otherwise the Tennadyne Yahoo group files sections has a file by a Ham
who rebuilt his T10. Instead of the coax coil at the feed he had some
fancy machining done to support a Balun Designs 1:1 choke at the feed.
Much better isolation of the coax than the Collins choke. Alas, he taped
the coax to the boom. Ignore that part.:-)
On 7/8/2016 9:04 AM, James Wolf wrote:
Think of the dual boom as an open wire transmission line having a
characteristic impedance of something like 300 ohms, and a spacing of
several inches.
The feedline should drop down vertically from its attachment point to the
two booms and then turn and run parallel to the two booms, but below them by
6 to 12 inches, so there is minimal disruption to the open wire line. IMHO,
taping it to either boom makes no engineering sense at all. It does make
marketing sense, in that the antenna might sell better if they show a nice
"clean" installation vs the feedline hanging down.
Some TV antennas are designed this way, and you never see the coax taped to
the booms. It just hangs down.
Rick N6RK
_______________________
This is my current approach. Unfortunately, there is not enough clearance
to run the coax through the boom to the back of the antenna due to the way
the elements connect to the boom. However, what if I were to electrically
connect a piece of aluminum tubing to the bottom boom and run the coax
through it to the read of the antenna where the shorting stub is and then
drop down and bring it back to the boom. That would likely require no choke
at all since it is the "cold" point of the antenna?
Jim - KR9U
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