Hi Ian

A folded dipole in free space is as you say close to 300 ohms. A folded
dipole in a parasitic (eg yagi) array is reduced in Z in the same way
that a "normal" dipole does in the same place. ie a 75 ohm FS dipole
might get to 8 ohms with close spaced parasitic elements around it. In
the same place a folded dipole would be reduced by the same factor. This
means the feed Z is now around 36 ohms. You can also play with the
respective diameters of the elements that go to make up the folded
dipole to get a different Z transformation. I once built a 78Mhz device
out of 3/4" vs 1/2" Cu pipe to get a feed Z of around 200 ohms - for a
4:1 balun to match...

You may also like to look at a complex impedance meter that uses tuned
coax lengths. Gordon VK2ZAB (Now VK3 something) posted an updated design
to Carl SM6MOM's original article - http://www.grantronics.com.au/docs
Kind of hard to work out what's what. Just grab all the PDF's! It will
tell you the whole feedpoint equation rather than having to fiddle with
loose coupling a GDO.

Cheers Bob VK2YQA.

wrote:


Bob,
Thank you for esponding to my questions, and apologis for my
delay in responding.

GDOs have their limitation as the coupling between the
oscillator and resonant circuit
causes the frequency to be pulled - a GDO with a frequency counter
might be a very useful instrument.

My main question about the use of a GDO is how high in
frequency can a practical GDO get
to in frequency and still oscillate, couple to a similar frequency
resonant system and be able to observe a
"dip".
Your advice on coupling and matching to the antenna are
excellent help, what I didn't understand
was the impedence value at the folded driven element. Probably
something from long ago, but I believed the
impedence of a folded element was 300 ohms???

Could you please explain what you meant by :-

"or maybe increase the number of
driven elements folded dipole style to get the Z up to 200 ohms. A yagi
feedpoint Z is going to be maybe 10-20r. One extra driven element
(simple folded dipole) will multiply that by 4 and a third element
another 4 times."

In physical term I don't know what this means.

An article that caught my attention recently that I will share
with you is a cheap method
for measuring complex impedance, it is at:-
http://web.ukonline.co.uk/g3ldo/aegextra.htm

Once more thanks for your help.

Regards,
Ian