One thing that as always bothered me is that antenna is not balanced
with that lop-sided feed. Further-the-more that assumed 300-Ohm
impedance has a pretty hefty reactive component. As baluns like
resistive loads I would question just how well the balun works under
that condition?

I too have modeled the antenna and agree that the claims made
regarding feedline's radiation contributing to a strong vertical
component is over stated to say the least.

Good luck with your cut and try.

73
Danny, K6MHE



On Sat, 30 Aug 2003 02:02:46 GMT, "Charles Wittnam"
wrote:


That is probably what I am going to do. I became a
little more anxious after I read L.B. Cebik's articles on OCF dipole
antennas, "From the Ground Up." Things are a little bit more complicated
than I thought. Most baluns used in OCFs are 4/1. I have have included a
graph from his article .First of all, the calculations used to design OCFs
are supposed to estimate the 300 ohm impedance point. These are always a
rough estimate since as you get further from the center of the antenna it
goes up exponentially. When you factor in nearby objects, your radiating
feedline, and antenna height the calculated 300 ohm point can be in a
different area altogther. A 50% error in impedance result with a 5%
shift in the 300 ohm point, which on a 40 meter OCF would be about three
feet. More recently some of the 40meter OCF builders ( I think the 40
meter "Carolina Windom" would be an example), have tried to hit the
flatter part of the curve by using a different offset calculation. Instead
of D3/L=.167 for the 300 ohm point they used D1/L=.38 and D2/L= .62 for
a 100ohm point. They are using a 2:1 balun. In the inclosed info that came
with the antenna it says it is using a 1:1 balun which I sincerely doubt.
So I measured my antenna and the offset is closer to the second formula,
than the first so I was confused.. If I cut it up and it did not work well
and I knew what the balun was I would know which part of the curve ( and
the antenna) to head for! . So you can understand my intial reluctance to
act. Perhaps this is being overly cautious. You get that way when you are
given a "1:1" balun placed in the position where you would put 2:1 in an
antenna that usually uses a 4:1 AND the company does not answer questions
about their product.

I would highly recommend Cebik's article series on OCF antennas, in
which he debunks a lot of their claims, especially the "vertical radiator"
aspect of the feedline and the "yagi" performance. He basically argues
that these claims are being backed up by overly simplified assumptions about
the antenna being plugged into antenna modeling programs. He believes they
are no different than dipoles really. For my purposes I wanted an antenna
system with an offset feed because I can hide the feedline coax easier
given my available tree locations. (and keep the XYL happier...) This
antenna has a black balun and black insulated flexible wire which really
blends in well on my heavily wooded lot. Right now I am trying to salvage
the situation. This is the first time I have run into an amateur supply
company that has behaved so poorly. It was a rude awakening. I may just do
what you suggested. I hope I have the 4 to balun since I have other uses
for it. If the down sizing doesn't work then I will either just swap out
baluns or try a different approach, such as using a parallel feedline and
not using the balun. We will see.





"Dan Richardson" wrote in message
.. .
On Fri, 29 Aug 2003 21:37:00 GMT, "Charles Wittnam"
wrote:

I am trying to salvage the situation before going
to
war with Antennas and More with my credit card company. They shipped the
wrong antenna to me and have NOT answered numerous calls or emails. I
was
looking for a 40 meter OCF dipole and they sent me a 80 meter OCF, which
was not what I ordered because of space limitations.
If I could cut the thing down to use on 40 meters then I can save the
inevitable hassle of dealing with this outfit and going to war with them
via
my credit card company.. However, my understanding is that a lot of the
commercial builders of the OCF dipoles use a 2/1 balun for their 40 meter
antennas since they are trying to match a different impedance spot on the
shorter antenna., hence it is not necessarily the case that you can just
cut
them down. Any thoughts?


KD7HU


Why not just scale the antenna? That's is to say, in your case, reduce
the length of the wires by a factor of ½.

Danny, K6MHE