[Amps] RF in the Audio

[Jim Brown]

On 9/24/2011 5:20 PM, Peter Voelpel wrote:
> But the question concerned that perfectly! balanced open feeder which
> certainly carries no common mode current.

Ah, but it DOES, because the feedline is part of a complete circuit. If 
the rest of the circuit is unbalanced, it causes current in the feedline 
to be unbalanced. That's why I do not use the words "balanced feedline."

Follow this simple logic. If the current in the left side of an antenna 
is 1.0A and the current in the other side is 0.9A, the current in the 
side of the feedline connected to the left side MUST be 1.0A, and the 
current in the wire connected to the right side MUST be 0.9A.  When you 
add a common mode choke to the feedline, you're adding a high impedance 
in series with the common mode circuit, which forces the current in the 
two sides of the antenna to be more nearly equal.

That's another shortcoming of unbalanced antennas like Windoms -- they 
are noisy on RX.

73, Jim K9YC