[Moon] VHF antenna matching

[Jerry Kleker]

Hi Hakan,

I have been using "folded" dipoles for all of my Yagi antennas ever
since I was a youngster (a long time ago).

With short Yagi antennas the element spacing would be adjusted until
the FD impedance would drop from 300 ohms to 200 ohms. Then a standard
4 to 1 balun would match 50 ohm coax to the antenna perfectly.

Now comes the "long Yagi" with high gain and if you adjust the director
spacing for maximum gain, you can get the feedpoint impedance down to as
low as 20 ohms (and lower) at the dipole driven element (the dipole
driven element is about 75 ohms in free space).

The Gamma match approach works OK except that the dipole "resonance"
and desired impedance do not work together. The folded dipole becomes
inductive and you have to adjust the element lengths until resonance
at the desired frequency is achieved again. Then the feedpoint impedance
changes and then you have to adjust the Gamma match again. It becomes
a "vicious circle".

The solution is to use the "impedance step-up" folded dipole approach
where The top element is chosen (such as 1/2 inch diameter (12.7 mm)).
Then the element where the feedline is connected is a smaller diameter
which determines the impedance "step-up" ratio that is needed. I use
this approach up to 1296 MHz. You might desire an impedance "step-up"
of as much as 10 where the conventional folded dipole has a step up of
only 4.

I have a program that can be used to find the correct diameters of the
folded dipole and the spacing of the folded dipole elements. I wrote
the program in Basic about 30 years ago and a good friend recently
cleaned it up and compiled it for me.

If you are interested in the program let me know and I will send you a
copy. Or you can tell me the impedance that you are trying to match and
the type of feedline that you want to connect to the "step-up folded
dipole". Then I can give you the ratio of the diameters of the folded
dipole conductors and their spacing.


73 es GL,

Jerry   W7QX    DM44ar




Hello Moonbouncers!

(i Guess this is yesterdays news to most of you)


I have today been trying different DE for a possible future EME antenna.   I 
have a good antenna design using just a splitdipol as DE that i have been 
thinking on.

Something i find strange is when i mounted the DE only and listened on a 
beacon some 200km away it drops down in the noise but when i try with two 
different Gamma matched DE i hear and see a big difference on the waterfall 
(as i clearly hear the difference it should be over 1dB i guess.)


I have been doing the tests with the DE 5 meter above ground mounted on a 
pipe and all DE on the same place. No tower preamp was used.


Is it possible the gamma match is more sensitive compared to the splitdipol? 
it will not feel so good to build on that one if so.  I have not tried the 
folded dipole but will do coming weekend. i guess this is the winner as i 
see it on many good antennas.


73  Hakan SM7WSJ

PS: i know there are some stations that have built with splitdipol and good 
success and the gammamatch are not so popular this days as it seems more 
sensitive to WX

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