[Elecraft] Balun at input or output of tuner

[Jim Brown]

On 12/12/2011 2:06 PM, Paul Christensen wrote:
> However, the bifilar winding is not really like a wide
> conductor-separated transmission line so, perhaps mismatch losses are less
> than the same length of RG-213, but more than open or balanced lines.

I have not attempted to measure the Zo of the bifilar wound chokes I've 
built using #12 and #14 THHN, but Jerry Sevick, in the last of his 
books, did wind some using exactly that method and that wire, and he 
says the Zo of those he wound were about 100 ohms. As I often do, I 
wound mine and studied them before seeing Jerry's work, so it's sort of 
like independent researchers coming to similar conclusions. What I have 
not seen (so far) in Jerry's work is the concept of common mode Z and 
resonance as a useful thing.

The REALLY NICE thing about the bifilar chokes I've wound is that they 
hit the target 5K ohm CM impedance over the entire HF ham bands -- that 
is, 80M to 10M, or with a few more turns, 160M to 17M -- with a single 
#31 core and very inexpensive wire.  The secret sauce is simply how the 
the dielectric constant of the insulation meshes with the rest of the 
design.  Another nice thing about these chokes is that they can handle a 
lot of voltage and current.  I've patched them in line with my 1.5kW 
Titans and the antenna tuner (that is, matched load, no common mode 
voltage), run them keydown for a while, and they barely get warm to the 
touch.

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.  
The primary issue to deal with is how to rig them so that they are 
secure mechanically.  I've not played with that yet, and mechanical 
design is not my strong suit. :)  There are photos of the chokes I 
measured in the RFI tutorial.

73,