[Elecraft] K3 ATU Balun question

[Bob Cunnings]

You brought back an old memory.

Many years ago I made a bifilar wound air-core 1:1 balun. I was
inspired by the 1974 ARRL Antenna Book, which described it on pg. 102,
fig. 3-43(B). Figure 3-43(A) presented a 4:1 balun. On the same page
is a photograph of a broadband unit, made of airdux type coil stock in
an aluminum housing. I'll quote from the book, which fortunately I
still have:

"In practice, a single set of coils can be designed to work over the
3.5 to 30 MHz range. Design is complicated because there is mutual
coupling between turns, which modifies the characteristic impedance.
However, suitable units are available commercially (B&W 3975)."

I wound the 1:1 on an old phenolic coil form I had and put it in an
upside down pickle jar attached to a post at the feedpoint of a 40 m
delta loop. Not a very challenging application - single band and all.
The antenna was already matched to a 50 ohm feedline pretty well with
a quarter wave section of RG-59, but I wanted to limit currents on the
outside of the coax. It didn't seem to make the antenna any worse!

Air-core baluns don't get mentioned much anymore, but as you suspected
they can be made to work, within certain limits.

Bob NW8L

On Sun, Mar 16, 2008 at 12:21 PM, Mike Scott <mike at paxsen.com> wrote:
> 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
>
>