All good stuff, Stuart.
I'm a great fan of the vertical dipole and you prune it exactly as you
suggested.
I think this is one of our hobby's best kept secrets.
The OCFD is a highly misunderstood antenna.
The textbooks are very sparse on the topic and as a result, most people's
knowledge is very limited.
One of the things I have been playing with is different splits than what the
textbooks describe.
What I have observed is, the farther away from the middle of a dipole you
place the feedpoint, the worse the problem with CMC is.
What I have also found (by myself, through modeling - then building and
testing) is that by moving just 7% off of the center of the dipole, you
dramatically lower the impedance on bands which had very high impedance, but
the increase in CMC is negligible.
So when people cry "common mode current" when we speak of OCFD, they are
basing their opinion on text book examples IN WHICH CASE THEY ARE RIGHT, but
there is another, better way to do it. I'm not saying it is the holy grail
but it is certainly a GREAT compromise.
By moving just 7% off center, you can still feed a dipole with openwire but
its second harmonic will have only a few hundred ohms rather than 2000+
ohms. This greatly reduces the demand on the tuner. In fact, most
transceiver's built-in tuner can match this.
Today I erected ODFD no. 13 and ran several tests. Pleasing results. So
far it was only tested with an analyzer. Need to haul a transmitter up on
the hill and run more power through it, then measure the CMC. Great
expectations for this one. Will probably commercialize it.
73
Rick, DJ0IP
-----Original Message-----
From: TenTec [mailto:tentec-bounces@contesting.com] On Behalf Of Stuart
Rohre
Sent: Thursday, July 11, 2013 9:24 PM
To: tentec@contesting.com
Subject: Re: [TenTec] OCF antennas evolution
Some of us have also had good results with variations of the G5RV, using a
stub of parallel llne and transitioning to Coax without a balun at that
point. Varney in his original design, ran parallel line all the way to the
shack. Is going to coax after the stub unbalanced?, sure.
Did it work? With 60 feet of coax from the stub to the rig, it worked very
well, usually without a tuner on 40 and 20. The angle of take off or
efficiency seemed suspect on 15; but it accounted well for itself on
favorite bands. It was necessary to have the stub come off horizontally from
its junction with the feed point, since the trees holding the antenna up
were not over 40 feet high at the attachment points. Thus, a
40 foot stub could not hang vertically, with the house under it.
One can be very happy with an antenna that does the bands you prefer.
The flat top for my version of the G5 was a variation of the ZS6BKW design
that tries to provide for WARC bands as well. About 92 feet as I recall.
The main point of discussion is it seems easiest to put up symmetric
antennas to preserve balance in a dipole. If you don't feed in the center
or with balanced line, you may find some issues. You then deal with them,
and since I come at things from a minimalist approach, if I don't have RF in
the shack from feeding a balanced system with coax, that is OK. If I had RF
in the shack, I would rearrange the antenna as a system to correct that, or
at least find the cause, and do something to deal with it.
The OCFD solves some very real problems for hams. One is lay of the land,
where you might have difficulty center feeding a long dipole in your
available space.
In a vertical dipole, one end is coupling to earth by proximity, while the
upper end does it to a lesser degree. To restore symmetry there, you can
make one end longer than the other, especially when making a multi band stub
decoupled dipole. Making it an OCFD eases the provision of multiple bands on
one vertical system.
In the end, the success of any antenna is can you match it conveniently, and
does it produce contacts?
-Stuart Rohre
K5KVH
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