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Re: Helix Match



Jean-Louis,
I have seen several versions of the "quad helix" on the I-net that all seem 
to go back to an ancient 70's QST article by a W6 (sorry can't find it right 
now for a proper reference).  The original author did use a bare wire above 
(and behind) a common reflector screen as an impedance transformer to get 50 
Ohms at a centralized connector.  Very crafty design in the matching section!

Keep in mind, you can modify the input impedance of the basic helical design 
by varying the lambda and the pitch angle (but not the number of turns).  It 
is quite easy to get a 200 Ohm helix design (it will be about 15 % bigger 
than a more classic 140 Ohm design) just by dimensional arrangement.  You 
could then use 200 Ohm transmission lines of any (but equal) length to a 
common connector to get a "perfect" 50 Ohm feed.  This concept eliminates the 
tricky transmission line design.  

Going the other way, you could design a pair 100 Ohm helicals and feed them 
with RG-62 to a common connector.  I considered this design for a while, but 
ultimately figured the reduced size (and gain) of the 100 Ohm models and the 
losses in the coax matching scheme would eat up the potential 3 dB gain from 
the second helix.

73 et les meilleurs voeux pendant la nouvelle année,
Jerry, K5OE

  > In an Engineer's HandBook, I've seen a description a 4 helix 
>  antenna sharing a common reflector and common connector.
>  A common coax connector is installed right in the middle of the
>  reflector, and a wire runs from this connector to each helix.
>  Each wire is NOT parallel to the reflector and is supposed 
>  to convert the wxyz ohms of each helix to a 200 ohms impedance
>  (200/4 = 50 ohms at the connector level).
>  This seems to be a very clever theoretical solution.
>  But what about real world ? So many parameters are interacting :+))
>  
>  73 de Jean-Louis F6AGR
>  
>  
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