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[TowerTalk] Use of 450&200 ohm baluns with open wire line

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Subject: [TowerTalk] Use of 450&200 ohm baluns with open wire line
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Date: Tue, 16 Nov 1999 10:49:01 EST
In a message dated 16.11.99 07:33:13 Pacific Standard Time, K7GCO writes:

    Using a 450 ohm balun connected to a 450 ohm open wire line seldom works 
as    does the use of a 4:1 balun.  You would have to just luck out with the 
right length of open wire line.  It assumes the Z at whatever length the open 
wire line is--is 450 or 200 ohms resistive which it frequently isn't.  With 
the typical SWR's the Z's change on the length from high to low every 1/4 WL. 
 It will be great if your antenna load is 450 or 200 ohms and the feedline a 
1/2 WL multiple.  You can get any R value of 3-5000 ohms down to 50 or less 
and it's usually reactive.  I use a "magic length" of 1/2 WL multiples on all 
bands.  That assures the least reactance if not zero on certain frequencies.  
It's handy if you know how to GD open wire line for a particular resonant 
"magic" length--which is easy to do.
      Fortunately the Johnson MB will tune out a fair amount of reactance but 
its best to have the least you can.  I'd find a 1 KW MB or the 275W model, 
take it all apart and inspect everything for arcs.  Remake all connections.  
I use them without the case. They use the same tap on one side for 15&10M 
which unbalances the tuner some on 10M.  It will work on 30, 18 & 12M also. 
Make provisions for using other than just 2 turns in the link if needed.  Use 
RF ammeters in the output or shunted light bulbs.  If you can get 1:1 SWR, 
maximum transfer of energy occurs--most of the time.  Did you see the bit I 
had on the MB in TT and the dual differential modification or reversal for 
certain odd loads.  I sometimes use a variable Xc in the link but it's not 
needed most of the time.  I use 100 ohm balanced coax into it a lot (2 RG-8's 
connected in series).  Add another SO-239.  I have made my own mono-MB with 
commercial plug in 75,150, 500 or 1KW coils or made my own for 20-2M with 
silver plated 1/4" tubing.

<< Hello K7GCO,
  I am in the throws of putting up a 132' dipole fed with 450 ohm line.  I am 
 to try to bring the line into the shack through a 9:1 balun made of 10 turns 
 RG8 on a bleach bottle (about 6" in diameter) and a pig tail to the shack 
and to
 the back of the Drake MN2700 tuner.  I will have about 200' of twin lead in 
 run.  Have you ever tried the coax method outlined above?  Any help you can 
be is
 tnx es 73 de n8csp k
 << >>

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