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[TowerTalk] Re: Question on Spacing for Ladder Line--and Match Box Tuner

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Subject: [TowerTalk] Re: Question on Spacing for Ladder Line--and Match Box Tuners
From: (
Date: Mon, 30 Apr 2001 21:15:30 EDT
 In a message dated 4/30/01 5:03:33 PM Pacific Daylight Time, writes: << 
   I wonder if Steve ever got his question answered, about the IDEAL WIDTH 
(of open wire spacing)?
  Jim WA9FPT

 Jim that is a very good question and that will be answered also.  This is 
the 51st e- mail today I've sent.  
 Typical spacings are 4-6" which gives a range of about 400-600 ohms Zo 
depending on the wire diameter.  The formula is Zo = 276 log S/r.  S = wire 
spacing and r =wire radius.  Spacings beyond 6" raises the Zo very little.  
As it turns out the width and Zo are not all that important as is the tuners 
ability to match the Z at the end of the feedline.  This is determined by the 
Zo of the line and antenna Z.  
  Since the MB has a wide range, transfer of energy to the feedline still 
occurs when adjusted properly. The longevity of the spacers is "very 
important as is the length of the feedline."  I sill have one open wire 
feedline over 60 years old.  This is what causes the "big problems" and tuner 
designs that don't work well.  Don't use wooden dowels boiled in wax--they 
make a "RF Crackeling Sound".  

I have one open wire feedline that's in the shape of a sine wave.  Would you 
believe it was run staight originally but had a very high SWR on it and it 
changed it's shape.  That's an absolutely true story--I just made it up!   I 
have one made up that way using 1/4" copper tubing and can get a lot of hams 
to beleive it when I can keep a straight face.  That also appeared in QST as 
a Jeeves cartoon.  k7gco 

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