[Elecraft] K3 ATU Balun question

[Mike Scott]

We had a prior discussion about the use of the K3 ATU driving an antenna on
multiple bands fed with open wire feed line with a short length of coax and
a Balun on the transceiver end. The discussion mentioned that Baluns have
losses when SWR is high and that the proper Balun was 1:1. 

The Balun losses lead me to think air core and whether the losses could be
avoided...

Baluns do three things: Transform impedances, transform unbalanced to
balanced, force currents to be equal and opposite (current Balun). We don't
need impedance transformation as that is what the ATU does, I am not sure we
absolutely need common mode choking of the current Balun; it depends on
whether asymmetry has been introduced in the doublet or feed line.

Is there any problem with using an un-tuned 1:1 air coil transformer with
unbalanced primary and balanced secondary? I am thinking back to link
coupled transmitter days. Many tuners used a topology like this but they
tuned the primary or secondary for impedance matching purposes. Since the
ATU is there to match impedances the minimum we need is low-loss unbalanced
to balanced conversion. If the Balun core is the primary mechanism for loss
generation at high SWR, why not get rid of it?

I make my own Airdux style coils. I could wind an "Airdux style" coil in
bifilar fashion (equal number of turns) and feed one winding unbalanced and
leave the other winding balanced to connect to the open wire feed line. I am
not sure if I am going to get efficient 160M to 6M coupling out of one coil
but what is wrong with this picture?

Similar construction with different hookup could take the same bifilar coil
arrangement and turn it into a 1:1 current Balun if that was a better
solution.

The coil could go into an outside the window weather proof box.

Mike Scott - AE6WA
Tarzana, CA (DM04 / near LA)
K3-100 #508/ KX1  #1311