[Elecraft] Balun at input or output of tuner

[Jim Brown]

On 12/12/2011 5:10 PM, Jim Brown wrote:
> Yes, chokes like these would be a great choice for use at the feedpoint
> of ANY HF antenna, whether fed with coax or parallel conductor line.

One VERY important exception that I forgot to mention is that common 
mode chokes can be destructively overheated by running high power into 
badly unbalanced antennas. Off-center fed antennas like Windoms can 
place VERY high common mode voltages across common mode chokes. The only 
solution I know of is to use multiple chokes in series on the feedline 
IMO, this sort of antenna is a poor choice in today's world, where local 
RF noise is made worse by pickup on the feedline.

A few years ago, I investigated this by  modeling the common mode 
voltage, and the resulting heat dissipation, in a common mode choke at 
the feedpoint of a 40M dipole whose feedpoint was moved off center in 
increments of 3 ft, at 1.5kW, fed by a half wavelength of coax (67 ft).  
At this worst case feedline length, you don't have to go very far off 
center to produce a lot of heat in the choke.  The results are 
summarized in a table in a Power Point for a presentation I've done for 
several ham clubs.  One of these bifilar chokes would be OK on a Windom 
at the 600W level produced by the KPA500, but could fail at max legal 
power. See page 43 of

http://audiosystemsgroup.com/CoaxChokesPPT.pdf

73, Jim Brown K9YC