amdx wrote:


"Paul Keinanen" wrote in message
...
On Sat, 14 Nov 2009 12:05:21 -0600, "amdx" wrote:

Hi All,
I'm having a disagreement about the use of this folded dipole
calculator
impedance calculator.
http://www.k7mem.150m.com/Electronic...ed_dipole.html
We are using it to calculate the impedance of folded dipoles to
drive a
yagi.

Excuse me, but what does such simple dipole calculator have to do
with the calculations for a Yagi feed element ?

I think it all started with the MFJ-1800 that uses a folded dipole,
but
there
are many yagis that use a folded dipole as the driven element.
But, I think (now) I understand enough to know why you ask the
question.
I think your point is the impedance of dipole feed on a yagi is highly
modified
by the reflector and directors.

Even assuming a single element feed element antenna, playing with the
tube diameter (upper and lower bars) or the number of bars in a
folded dipole will radically change the feed impedance.

Paul


I excerpted this from my latest email to my friend I'm having the
disagreement with. (Might
not even be a disagreement, it could be a miscommunication.)
Excerpt;
"The online calculator takes the (Impedance of a dipole x Ratio).
This
gives you the impedance
after folding the dipole.
We still have no idea what the impedance of a dipole is when
surrounded by
the reflector and
director. We do know how to change the transformation ratio once we
know what the impedance
of dipole would be in that surrounding."
Mike

The nominal impedance of a dipole in free space is approximately 72
ohms. If it is part of a system its impedance will be influenced by
that system causing it to change from nominal.

Folding the dipole allows it to behave as a transformer ! Thus the
nominal dipole impedance can be altered to a value that can more easily
be matched to the feed line.

Commonly the impedance of a dipole that is part of a system. ie "Yagi"
falls to a much lower value. Under these conditions the folded dipole
can be used to raise the impedance seen at the feed point to a more
usable value.

Someone mentioned "Gama match". Whilst symmetrical gamma matches can be
used, single ended is popular because it is inherently unbalanced and
can be easily matched to a co-axial feedline.

--
Best Regards:
Baron.