On 9/14/2014 11:47 AM, Don Jones wrote:
But I would like to try the OCF method mentioned by Rick DJ0IP.
Sounds like for a 44 ft OCF antenna the feed point would be 19ft/25ft.
Correct?
Hoping I can get 10M to play better and would like better efficiency
as compared to a 34 doublet on 30M.
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I have been tracking and following Rick's development of his OCF dipole
for a LONG time, and he has tested that puppy sixteen ways to Sunday and
back, in addition to using it on the air in various contests. I, too,
am interested in OCF dipole designs because I wanted to get a 40 m OCF
dipole to also play on 15 m, which is usually not possible. I cut mine
at different non-typical feed point 25% to 75% and got 15 meters to
play on a 40 meter OCF Dipole - but at some expense to the 6 meter band,
which usually would be better (mine has about 5 : 1 SWR on 6 meters,
and it would usually be lower, if I did not adjust it to make 15 play
better at about 3.5 SWR) - I think Rick and I are both interested in
the same thing - i.e., to experiment with various feed points and
determine how many bands one can work on one wire.
Another reason Rick's work is interesting, is because these things are,
as Jim Brown strongly observes, more subject to problems from Common
Mode Current than other designs, which can add unwanted electrical
noise and it can also adversely affect impedance mismatch, as Rick as
demonstrated in in dozens of tests. The goal is, of course, to get as
many workable bands, with the lowest mix of SWR and mismatch, as
possible. MY goal was simply to get 15 m to play on a 40 m OCF dipole
- which it usually does not. I understand Rick's goal was more
ambitious, to make the most of a single wire antenna.
What did I learn from his work, and my own research... ?
1) they ARE subject to CMC if you are not careful;
2) not all baluns are created equal,
3) it is often better to wind the transformer bit separate from the
choke bit, and both are important in their own right,
4) OCF Dipoles generally work on all "even" harmonic bands, but with
some tweaking of the feed point location, they CAN be made to work on
both "even" and at least some "odd" harmonic bands,
5) Whilst they work best on the lower bands - closest to the one for
which they are cut - you see simpler, fuller lobes on those lower
bands, and get more, and less uniform, lobes, on higher bands, and the
more the thing exceeds a wavelength of the frequency of interest, the
more this occurs,
6) similar differences in take off angle seem to occur as you move to
higher frequency bands,
7) hanging your OCF horizontally is better than as an inverted-V as
this helps avoid or minimize the CMC,m
8) take your feed line away from the elements in a perpendicular
direction. Rick's tests indicate leaning the feed line toward one side
or the other alters the CMC/SWR result with measurable impact, and
9) the length and type of transmission line may have a substantial
impact. You can see this if you play with the ARRL TLW (Transmission
Line for Windows) program to analyze the impact of different lengths of
transmission line. Rick can tell you a LOT more about this than I, but
as he mentioned, above, a quarter wave length may have a substantially
different result compared to a half wave length, and various points in
between.
A web site is dedicated to OCF dipoles, and there is data on many other
feed point splits - which is where I got the idea to move mine. He has
a chart of data on several different OCF feed point splits, which is
very instructive.
See here:
http://hamwaves.com/cl-ocfd/en/index.html
I seem to recall W8JI has a nice piece on splitting one 80/20%.
Just MY take...
--------------------------- K8JHR -----------------------------
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