[Elecraft] Balun suggestions

[Jim Brown]

On Thu,2/4/2016 5:35 PM, Wes (N7WS) wrote:
> Even though we often refer to the output of the KAT500 (or any other 
> device that normally connects to a coaxial line, the currents on the 
> output should be equal and opposite - which is the definition of 
> balanced.

Sorry, that is NOT the definition of "balance." Balance is defined by 
equal potential to the reference plane, which for most land-based 
antennas is either the earth or the chassis of the transmitter that 
feeds it, which is usually ground referenced.

> Consider the currents between the center conductor and the inside of 
> the shield for coax.  They must be equal and opposite.  It is the 
> common mode current on the outside of the feedline that produces any 
> unbalance. 

No, the common mode current is the RESULT of imbalance. AND -- common 
mode current is present in two-wire feedlines when the antenna or the 
transmitter is unbalanced.  In those 2-wire feedlines, common mode 
current is the DIFFERENCE between the current in the two conductors.

Note also that feedlines have wavelength, and both voltage and current 
vary along the line. The DIFFERENTIAL voltage and current vary as a 
function of the behavior of the line as a transmission line, with Vf 
being that of the line. The COMMON MODE voltage and current vary along 
the line as a function of the line's behavior as an ANTENNA, where Vf is 
determined by the insulation on the outer conductor(s), so  is 0.98 - 0.99.

I laugh at those who attempt to measure current in both sides of a 
2-wire line and think that what they have measured is the current at 
every point on the line. It is not -- it's only the current at the point 
you measured it, and it's only right there if your measurement technique 
is without error. Which ain't easy.

73, Jim K9YC