Allan Greening wrote:
> I am doing the mod for 17m on my spare 204 BA ,but i am missing the hair
> pin match..my 204ba up the tower "75 feet hi" has a square looking loop
> but the drawing on the mod site shows a different shap loop, wide at the
> element then a taper for a short distance then parrell..Has any one got
> any idea wot it should be?? es size?. manual says 1/4 by 18 inches where
> 20m uses 30 inch long hair pin loop de vk3pa
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Allan,
I presume you are missing the two rods as well as the shorting clamp for
the beta match.
The physical size of the beta match is determined by the amount of
inductive reactance needed to
transform the feedpoint Z to 50 ohms.
The bracket spaces the rods 8" apart. The "taper" at the feedpoint
end of the rods adjusts the
spacing to 8" as close to the boom/element mount as possible without
shorting to it.
There are any number of sizes and shapes that will provide the necessary
reactance for the match. A couple of substitutes are:
A loop approximately the size of the original beta match made from #8 or
10 wire will work as a replacement...simply
adjust the overall length for a match...no short to the boom
A simple and very effective replacement for the beta rods/shorting
bracket is simply a coil. Make it from 1/4"
dia copper tubing or even enameled #8 or 10 wire. A coil 2 1/2" ID and
6 turns will work. Simply adjust the
length of the coil for a 50 ohm match. The center does not have to be
shorted to the boom.
I used this matching method several years ago on a homebrew 20M yagi .
If the feedpoint cannot be reached from
the tower, stand the antenna on the reflector end and adjust it on the
ground.
73, Bob, W5AH
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