[Elecraft] how did I calculate the loss?

[Charles Greene]

Don,

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?

At 05:15 PM 12/16/2003, Don Wilhelm wrote:
>----- Original Message -----
>
> > Don,
> > Does my 4-1 balun at the input of my K2 introduce losses and if so will
>more power
> > get to the antenna without the balun?
> >
>
>If the balun is doing it's proper job, it is not likely that additional
>losses are introduced by its presence - it will convert the unbalanced
>output of your tuner to a balanced condition.
>I did not mean to imply that baluns are lossy - actually they are a good
>antenna system to employ.
>The point I was trying to inject is that the antenna system itself (and not
>the balun) will determine whether the feedline currents are equal and
>opposite - yes baluns do properly force their output currents to be equal
>and opposite, but the antenna configuration is the real determining factor
>in the overall balance question.
>
>Part of the problem is that we usually think of balanced and unbalanced in
>only two situations - perfectly balanced, and totally unbalanced.  In
>reality, antenna systems can (and do) present situations that are somewhere
>in the middle.  Unless the antenna itself is inherently balanced, or
>inherently totally unbalanced, a balun may not be the best answer.
>
>As an extreme example, consider an off center fed antenna (where the
>feedpoint is neither a current maximum nor a current minimum).  If you
>follow the antenna and feedline currents (remembering that 'mother nature'
>demands that the current at the antenna ends be zero), there is no way that
>the feedline currents can be made equal and opposite.  In that case, the
>feedline will radiate over its length - if a balun is introduced along the
>feedline to force the currents to be equal and opposite, the length of
>feedline that will radiate is that between the balun and the antenna
>feedpoint.  OTOH, if the balun is moved to the antenna feedpoint, the
>currents introduced (launched) onto the antenna wires will be equal and
>opposite and the antenna current pattern will be distorted because the
>currents at the antenna ends must be zero - and the sum of all that
>translates into loss somewhere in the system (that loss may or may not be at
>a balun that might be present).
>
>I have had many questions about this in the past, and challenges on a few
>'fine points', but no one has been able to substantiate that it is
>incorrect.  It has always worked out for me when considering antenna system
>balance - and I have not yet found an effective way to 'force' the currents
>to be balanced unless the antenna configuration itself permits that balance.
>
>If anyone has further questions, I recommend we take it off the reflector.
>While this is an interesting topic, it is off topic for the Elecraft
>reflector.  There are plenty of other forums willing to discuss this subject
>at length.
>
>73,
>Don W3FPR
>
>
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73, Chas,  W1CG
K2 462b, 3571