K2RIW:Re: [Moon-net] [Moon] VHF antenna matching.

[Richard Knadle W7QX es SM7WSJ]

W7QX es SM7WSJ de K2RIW  05/07/07

Dear Jerry and Hakan,

  There is more than one solution to the problem of achieving a "perfect
impedance match" of the Driven Element (DE) of a Yagi.

  You do not have to change the mechanics, or the way the DE is fed -- such as 
using the "impedance step-up" folded dipole approach.  You only need to 
change the length of the DE, then (usually) add a shunt capacitor.

  Just changing the DE length will allow you to adjust the Resistive Component 
of the impedance anywhere from 1 ohm, up to more than 1,000 ohms.  Then, 
simply compensate for the Reactive Component of the impedance that was 
created by the length change.

  At first this rarely-described principle may seem to be too simple.  Also, 
there are many amateurs who misunderstand the length relationship between the 
DE and the Reflector.

  You will be pleasantly surprised when you find out how well this principle 
works -- almost every time.

  There is a web site that will give you more information.  Go to Dr. Tom 
Williams MICROWAVE AMATEUR RADIO site:
<http://www.wa1mba.org/>

  Near the bottom of the page click on:
"Yagi Impedance and Matching info - and element lengths for the Jr. Boomer".

  Then click on "concepts of Yagi matching", "The Monopole Resistance Chart", 
and "The Monopole Reactance Chart".

  Let me know if you have any further questions.

  73 es Good VHF/UHF/EME/Yagi DX,
  Dick, K2RIW

  ===============
On Sunday 06 May 2007 11:12 pm, Jerry Kleker wrote:
> 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
>