[TowerTalk] Log Periodic Feed

[Kevin]

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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