[TenTec] Built in SWR meter bannans

[Steve Hunt]

If you wind a 4:1 Guanella balun as two identical 1:1 chokes on a common 
core, you force the CM voltages across the two chokes to be the same 
because they share the same flux. If we call the input voltage V, the 
balun then drives the output terminals to +3V/2 and -V/2 with respect to 
ground. If you had a perfectly balanced load that would force 3 times 
the current into one leg compared to the other. Analysis here:

http://www.karinya.net/g3txq/temp/4%20to%201%20current%20balun.png

and here's the experimental demonstration on a 'scope using a perfectly 
balanced 200 Ohm load:

http://www.karinya.net/g3txq/temp/bal1.jpg
http://www.karinya.net/g3txq/temp/bal2.jpg

Because of Sevick's pre-occupation with differential-mode performance he 
never spotted that effect. In fact he made lots of DM measurements - DM 
impedance, phase delays etc - but with a *floating* load; andas we all 
know, if you have a floating load you don't need a balun to maintain 
current balance :)

The simple fact is that a 4:1 Guanella balun wound as two 1:1 chokes on 
a common core will force voltage and current *imbalance* with any 
non-floating load; I don't call that "working" !

For a 4:1 Guanella balun to "work" properly and drive equal currents 
into any load, the CM voltages of the two chokes must be able to take up 
different values; they can't do that if they share the same magnetic 
circuit.

73,
Steve G3TXQ


On 24/04/2013 21:30, Rick - DJ0IP / NJ0IP wrote:
> Steve, Sevick is right.  You "can" wind a 4:1 Guanella balun on a single
> core.  I guess the question is, how well does it work?
> I have tried it both ways.  In fact I was using dual core's for a couple of
> years until I read that there is very poor tolerance in the cores... up to
> 20 % variance. Since I have no way to measure this and don't know where to
> buy matched pairs, I begin to wonder how do I maintain symmetry when there
> is 20% variance.  So I can't decide whether it's better to use 1x FT-240 or
> 2x FT-140.  Any ideas on this?
>
> 73
> Rick, DJ0IP
>
>