Topband: Binocular Cores

[Jim Brown]

On Mon,6/8/2015 5:05 PM, Tom W8JI wrote:
> Think about choke baluns where they have 10,000 ohms impedance, 
> dominantly resistive. If you applied a 500 ohm load and 1000 watts you 
> would have 2000 V/2 =1000 volts in an ideal balance condition, which 
> is 100 watts in the cores. This is why core selection generally has to 
> shift to a dominantly reactive impedance, rather than resistive, at 
> high power. 

I'm with you up to this point, Tom, but here, you're mistaken. An 
effective common mode choke is dominantly resistive in the frequency 
range where it is to be used. The reactive component of the choke can be 
cancelled a length of feedline that is capacitive at the operating 
frequency, which increases the dissipation and makes the choke 
ineffective. The key to not having it fry is to make the Z high enough 
that the common mode current is small.

Furthermore, the common mode circuit is NOT a simple voltage divider 
when the choke is added to the feedline of an antenna. Rather, it is 
part of an antenna system that includes the "intentional" antenna and 
the "unintentional" antenna (the feedline), and it is the lengths of 
those components, and the position of the choke, that will determine the 
voltage across the choke and the resulting current. To find the 
dissipation, we must add the equivalent circuit of the choke to an NEC 
model of the antenna with the choke and the feedline (as a single wire), 
including it's connection to ground. NEC will tell us the current in the 
choke, and I squared R is the dissipation. An antenna with severe 
imbalance can easily fry a very good choke at high power.

73, Jim K9YC