[TowerTalk] Force 12 B-1/B-1S Baluns

[Jim Thomson]

Date: Mon, 13 May 2013 13:53:15 +0100
From: Steve Hunt <steve at karinya.net>
Cc: towertalk at contesting.com
Subject: Re: [TowerTalk] Force 12 B-1/B-1S Baluns

If the choke is primarily reactive, and the coax braid impedance is also 
reactive but the opposite sign, adding the choke may *reduce* the net CM 
path impedance and *increase* the braid current. Making the choke 
resistive avoids that problem.

The choke resistance doesn't need to be "sky high" in this application - 
it simply needs to be adequate! 1500W into a balanced 50 Ohm load means 
137v worst-case across the choke. Keep the choke resistance to 2k Ohms 
or more and the worst-case dissipation is a modest 9W. 2k Ohms is very 
easily realised over a wide frequency range.

73,
Steve G3TXQ

##  OK, I get it.  And if the coax braid  Z, and is reactive..but is XL instead of XC, the
net CM path Z  could also increase ? 

## I did the maths, if you double the power, the diss will double....so diss is proportional to power,
this is good to know.   Then I suppose you have to factor in the weight of the ferrite used. 

##  I played around   with some of jim browns designs with large diam loops, but they are tough to use in some
applications..esp with 40m yagis...and 80m rotary dipoles.   With the 40m yagi, u have the boom to deal with.
The 80m dipole,  u have the mast to deal with.   U gotta get those coax loops well away from metal or the stray C
from coax loops to metal creates more problems.  Close wound loops on  4-8 type x 31/43  2.4” OD torroid 
cores  seems to solve most of the stray C issues.   Then the entire mess goes inside a gasketed nema box. 

## 213 u  or  393 teflon coax..silver soldered, seems to work pretty good so far. 

##  The  1/4  wave  or  3/4  wave coax...  minus 2%  also works good...but that’s a monoband device. 

later........ Jim   VE7RF