[TowerTalk] OCFD

[Doug Ronald]

The tapered line concept is actually simple and logical. My LPDA antenna
uses one which I pulled apart to inspect when I was constructing it. The
outside diameter of the coax stays constant, in my case it is 1 5/8" EIA air
dielectric aluminum tube, but the center conductor consists of brass tube
whose diameter decreases in linear sections. One constant diameter brass
tube will go for a few feet, and then a smaller tube is fitted into the
larger one, and brazed. That section continues for another few feet, and
then another transition happens. I measured the ID of the outside tube, and
the OD of the inner conductor at the final end point of the tapered section
to calculate the final Z0 which is around 80 Ohms. Of course it starts out
at the input to the antenna as 50 Ohms. This transmission line (and the
antenna) can handle 50,000 Watts PEP. When I used a VNA in the time domain
mode, I could see each of the transitions in the distance vs impedance plot.

-Doug
W6DSR

>##   I looked up what the typ 50-500 kw stations use, and most will
>use 300 ohm balanced feeders, and either 20 ft or 26 ft tall  aluminum 
>supports for the 300 ohm line, spaced XXX feet apart.   Took a while
>to see what they use for a balun, but it appears they start  with 50 ohm
coax 
>from the TX, then into whats called a tapered line section balun, 
>where it changes from 50 ohms unbalanced.... up to 300 ohms  balanced.  
>I still can't fathom how that concept works exactly. 
>
>Jim  VE7RF

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