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[TowerTalk] Baluns...again.

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Subject: [TowerTalk] Baluns...again.
From: "Jim Thomson" <>
Date: Tue, 28 Dec 2010 07:04:13 -0800
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Looking through some old notes  plus the info on Jim Brown's  site.... there 
was  a question about using
an electrical   1/4 wave or electrical 3/4 wave  of coax.   The  1/4 [3/4]  
wave of coax  was run between the
DE  of the yagi  and the tower... with the braid of the coax  bonded to the top 
of the tower.  

The theory is... if a  1/4  [3/4]  wave of coax has it's braid bonded to the 
top of the tower... the Z  at the  DE
should be sky high..and there should be no requirement for a balun.    
Apparently there  was an article out
about doing just that. The ant was a quad... and individual  1/4 wave [ and 3/4 
wave]  pieces  of 50 ohm coax
were  brought to a remote switch box at the top of the tower..where of course 
all the braids are bonded to the tower
top.  The author claimed that no baluns were needed on the various loops  used 
on each band.   I have not seen the article,
but it was  brought to my attention by a local ham. 

W8JI  also mentioned the use of  1/4  and 3/4 wave lines  from DE  to tower 
top...and bonding the braids to tower top.  Seems
to me that W8JI  mentioned that if a balun was used in conjunction with the  
1/4 [3/4]  wave coax, that the balun would makes things 
worse.   There was some discussion on one of the other refelctor's  years ago  
about the  1/4 and 3/4 wave concept.  It had something do do
with the height of the tower.   If the tower was not the 'correct height'.... 
the concept would not work.    Does anybody remember this ?  

In one of Jim browns write ups.... there is a depiction  showing  feedlines  of 
various  lengths... from  0  up to... but not  quite  1/4 wave.  Then
the chart starts up again... just a bit longer than a 1/4 wave..... and up 
to... but not  quite  a  3/4 wave.    Then it starts up again.. just past a 3/4
wave.  OK, what gives?   Is an  electrical 1/4  wave  and odd  quarter waves  
the real answer to a high Z  on  one band ?    In the case of 
something like  monoband yagis.... this may prove to be a huge benefit... if 
the theory  actually works.    If I remember correctly, the  main coax
braid was bonded to the base of the tower as well.   IF the theory does work... 
and a high Z  is presented to the DE... how high is this Z?    Is is
mainly reactive... mainly resistive..or a bit of both ?      Why would a bead 
balun or torroid balun, when used in conjuntion with a odd qtr wave
of coax makes things worse ? 

later........ Jim   VE7RF    

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