Jack Belrose, VE2CV did considerable research and experimenting with OCF
dipoles some years ago. Some of his work was published in QST. One of
the conclusions he reached was that it's awfully hard to keep the common
mode RF off the feedline to the shack.
Roy Lewallen, W7EL
Charles Wittnam wrote:
Danny,
I will keep a look out for RF Gremlins. I don't plan to run more than
100 watts. My rig will be the first to let me know since it is a Ten Tec
Jupiter, which I suspect would not like a lot of RF in the shack. You know
if you start randomly switching on the touch lights in all the bedrooms in
your neighborhood while transmitting with your OCF dipole in the evenings
you would probably would give a while new meaning to "interfering with
vertical polarization." You would also become very popular. g
73,
Charlie
KD7HU
"Dan Richardson" wrote in message
...
Charles,
Looks like you've got a good handle on things. I have used an OFC, fed
with ladder line. Nothing particularly outstanding in its performance.
However, as to common mode current: running around 100 watts I
experienced no problems, but when I bumped up the power to about
600-watts all hell broke loose. My computer, the TV, stereo, touch
lights and just about any thing else electronic went wild. And I use a
pretty good single point RF ground system here. So be prepared that
some "strange things" may happen. g
Good luck, nice talking with you.
73,
Danny, K6MHE
On Sat, 30 Aug 2003 16:20:10 GMT, "Charles Wittnam"
wrote:
Danny:
I think the you have raised a valid concern about
the
balun. This is something I did not fully appreciate before I made my
decision to try this. Cebik makes the same point and suggests if you
use
this antenna you might be better off using a parallel feed line with a
tuner, and no balun. I will make the cuts and scale this antenna down
preserving the same ratio of offset. Before I hang this permanently I
will
need to get a better feel for its performance and limitations. I may do
a
head to head test with another antenna set up and see. I was intrigued
by
W5DXP's "completed inverted L", which is a full wave loop variant.
The major benefit of this whole experience for me has been the amount of
research it has instigated. I also have a better understanding of
antenna
options. I have come to the following conclusions about commercial
"windoms".( which really are folded OCF dipole antennas).
1. The claims about performance are probably overblown due to:
a. Faulty assumptions about the degree the "vertical
radiation" of the feedline plays in modifying performance.
b. These incorrect assumptions being placed into antenna
simulation programs and printed as truth.
c. Placebo effect: I suspect some of the testimonials
are due to expecting better performance. On some occassions the
improvement may actually be due to hanging these antennas at "recommended
heights". Of course your dipole would do better up there too.
2. The unbalanced feedline makes RF in the shack a common problem since
your "vertical radiator" can be coming into your shack, depending upon
whether you have taken the necessary precautions to prevent this.
3. You are almost always better off making your own antenna.
4. I do NOT recommend dealing with any amateur supply company that
routinely will not return inquiry calls.
5. Read up on OCF dipoles as much as possible before building or buying
one.
I will keep you informed of my
progress. I have truly appreciated the input. I hope this discussion
helps
some other hams who are contemplating purchasing commercially made
"Windoms."
Charlie KD7HU
"Dan Richardson" wrote in message
...
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
om...
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
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