[Elecraft] OCF antennas and baluns??

[Don Wilhelm]

Chas,
I am aware of the questions you ask, but unfortunately I don't have the
answers.  I believe that is territory that has not been thoroughly explored,
and furthermore, I don't think one could adequately simulate the situation
with any of the antenna simulation packages because we don't know enough
about it to provide the simulation program with the correct inputs.

This sounds like good territory for some rather sophisticated open field
testing to determine what the real radiation pattern and relative field
strength of an OCF really is with the different feedline/balun arrangements.
Anyone have the space and capability to do such near field and far field
measurements? (or could be done at UHF in a good anechonic chamber - anyone
have one available?).  Sorry, but casual reports on the air are not adequate
for such a study - we do know that an OCF antenna works (very well in some
instances), but I don't think enough is known about it to determine how to
modify it to make it 'better' (better radiation patern, less feedline
radiation, etc.).  All I really know is that with the conditions presented
by the OCF antenna, if it is coupled with a balun, the RF 'fights with
itself', and that will create either real loss due to heat or radiation in
some unintended direction (that is not total system loss, but may be
considered as loss if a particular radiation pattern is being sought).

73,
Don W3FPR

----- Original Message -----
>
> You bring up some interesting points concerning the use of a balun in an
> OCF antenna.    I have had mostly good experiences with the OCF fed with a
> 4:1 current balun at the top; not that it is working optimally.  Some
> questions for you to ponder.  If the OCF antenna is fed with ladder line,
> the current in the ladder line is unbalanced, and thus will radiate.  So
> which is better, an OCF antenna fed with ladder line or with a balun at
the
> top?  I have used both, and it seems the ones fed with coax with the 4:1
> balun work better.  Concerning the balun, as the current at the antennas
> ends is zero and a current balun is trying to force them to be equal, it
> looks like to me that it is going to fail to do that.  How about a 4:1
> voltage balun, which does not try to force the currents to be equal?  What
> are the differences?  If I use a ladder line down to the ground, and place
> a 4:1 current balun at that point and a coax the rest of the way to the
> antenna tuner, the current at the balun is going to be unbalanced due to
> the source, and even though the balun is trying to force the current to be
> equal, how is it going to happen?  Possibly the unbalance in current is
> going to be reflected back into the coax, which is unbalanced anyway.  The
> balun should keep current common mode current off the coax shield, or does
> it in this situation?  Perhaps a 4:1 current balun followed by a 1:1
> balun?  Maybe just a 4:1 voltage balun?
>