[Elecraft] Balanced solution for KAT500 tuner?

[Jim Brown]

On 9/30/2012 12:02 AM, Jim Brown wrote:
> YES!  That is EXACTLY what I have repeated, over and over again.  It has
> NOTHING to do with coax.  Coax, if it is not decoupled by a common mode
> choke, simply ADDS to the imbalance that is already present.  EVERY
> transmission line needs a common mode choe at the feedpoint to decouple
> the line.

I think about this as an engineer, and parts of the logic are so 
engrained my brain that I leave them out when explaining how things 
work.  So, let me add this.

In a perfectly balanced antenna fed by parallel wire line, and 
terminated in a balanced circuit at the transmitter, the current on the 
left wire is exactly equal to the current on the right wire at every 
point on the line (but  varies along the line, because it's a 
transmission line), and at every point, the current are out of polarity, 
so they cancel.  That line does not radiate, and it does not receive.

But if ANY part of the system is out of balance -- the antenna, the 
line, the TX end -- the two currents are NOT equal.  -- the DIFFERENCE 
between the two currents is the COMMON MODE CURRENT, and that line DOES 
radiate, and it DOES receive.   That common mode current goes from the 
antenna to wherever the line is connected.

A COMMON MODE CHOKE adds some impedance in series with that common mode 
circuit. If it is a GOOD choke, it is a LARGE RESISTIVE impedance. It is 
"brute force" solution -- by adding a high resistance in series with the 
common mode path, we reduce the common mode current to a very small 
value, which causes (almost) all of the antenna current to flow from the 
left side of the antenna to the left side.

Please don't force me to  burden the entire list with an extended 
discussion of all of this that I have carefully prepared and posted as 
part of my RFI tutorial, and as part of a Power Point on coax chokes.  
Go study that.  THEN ask questions if you have any.

73, Jim K9YC