[Elecraft] Balun at input or output of tuner

[Jim Brown]

On 12/14/2011 4:59 PM, Don Wilhelm wrote:
> The balun has the same loss no
> matter where it is placed, so lets assume it is placed at the output.

There we go with that nasty word again (balun) -- we're talking about a 
common mode choke, right? :)

The problem is more complex than it looks.  Let's say we have a matched 
200 ft line with loss of 3dB (to make the arithmetic simple), the choke 
is wound with the same Z as the rest of the line, and we drive the line 
with 1kW. If we put the choke at the transmitter, the differential loss 
in the choke will be half of what it would be at the transmitter end.

Now, let's say we have a mismatched line that we're driving with that 
same 1kW, and the Zo of the choke is still the same as the Zo of the 
rest of the line,  To compute the differential loss in the choke, we 
must determine the current distribution along the line so that we know 
the current in the choke. Now, the TOTAL differential loss in the line 
will be the same no matter where the choke is, but since the coax going 
through the choke is coiled up, its dissipation is more concentrated 
than the rest of the line, and it's thermally coupled to the core, and 
maybe it's also in an enclosure.

The real world may be even more complicated -- perhaps  the Zo of the 
choke different from the Zo of the rest of the line.  Or perhaps there's 
some sort of impedance transformer, not a simple common mode choke.  
Again, we've got to work through the transmission line problem(s) and 
find the current in the choke.

And we haven't even talked about common mode dissipation yet. :)

73, Jim K9YC