[TowerTalk] Yagi Matches

[D. Rodman, MD]

On Tue, 8 Sep 1998, Ralph Fedor wrote:

> 
> A question:
> 
> If a beam antenna is properly designed -- let's say a 6 element 20 meter
> monoband Yagi -- can one change FROM a T-match with a half-wave coaxial
> phasing section TO a hairpin match without a change in any or all element
> lengths or a change in element spacing?
> 
> Thanks in advance.

Ralph, the answer is a definite maybe but not likely.  The hairpin match
with balun has a frequency sensitive component in the balun.  That is to
say there is a point where the coil of the balun ends and the lead of the
antenna starts.  This alone means that the driven element MIGHT  have to
be changed to accomodate the leads going to the hairpin.  This has to be
determined for each balun.  It can be modeled, but you need to initially
measure it.  That is the first consideration.  Secondly, the
characteristic feed impedance of the T match might be out of the range of
the hairpin parameters you would like to use.  This would require
recalculation of the antenna with computer modeling to refigure the match.

As you can see, it is possible to get everything just right coincidentally
and exchange matches.  It is not likely.

To do this, model your antenna with the exact dimensions for the element
and match.  Determine the feed impedance and then put in your hairpin
values.  If redesign is necessary, you then have a chance to really tune
the antenna up.  Once you do this, the antenna should be placed in a
convenient place to experiment with the tip sections of the driven element
before putting it back on the tower.  

Good luck.


> 
> 73-
> Ralph - K0IR
> 
> 
> 
> 
> 
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> 

David J. Rodman, MD 
Assistant Clinical Professor 
Department of Ophthalmology 
State University of New York at Buffalo

FAX 716-859-4565 
Office 716-854-1137

e-mail: rodman@acsu.buffalo.edu


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