AW: [Elecraft] PlugIn Balun for K2/Kat1 ATU

[Vic Rosenthal]

"George, W5YR" wrote:
> 
> Maybe you could restate that a different way for me in another posting.

Consider a parallel line with RF ammeters in each leg.  If the currents are
unbalanced, this is equivalent to equal and opposite line currents with a
common-mode current superimposed.  If the differential-mode impedance at the
balun is high, then the differential-mode current at that point would be low.  

The balun's job is to reduce the common-mode current relative to the
differential-mode current.  If the differntial impedance is high, then for a
given amount of common-mode current (as you correctly point out, the common-mode
impedance is unrelated to the differential), a balun would need to attenuate the
common-mode current more in order to obtain the same ratio of differential to
common currents.  Therefore a balun needs to be designed to display a higher
impedance to common-mode currents in order to work as well at a high-impedance
point in the system.  

> Its ability to reduce or limit CM current is determined by the impedance
> (largely inductive-reactive but with some real loss) which it presents to
> the CM path: the outer braid of coax. 

I agree.

> This property is independent of the
> load or "system" impedance or of the magnitude of the CM current.

I agree again, but the effectiveness of the balun in reducing imbalance depends
on the ratio between the CM and differential currents where it is installed.

Maybe you should reply off-list; I bet Eric is getting tired of this!

Vic K2VCO