"Owen Duffy" wrote in message
...

"Wayne" wrote in
:

I found a 50 year old pencil drawn schematic that was done by an
Collins Radio engineer. So....anyone ever use a coax balun as
described below?

Wind a 3.5 inch diameter with 7 turns of RG-58. Space the turns such
that the coil is 4 inches long.
Connect the output of an antenna tuner to one of the balanced feedline
conductors. At one end of the coax coil connect the braid to the
antenna tuner output, and the center connector of the coax to ground.

The other end of the coiled coax: connect the center conductor to the
other balanced feedline conductor. Connect the braid at that end of
the coil to ground.

Comments?


Wayne, the device as you describe it is a form of balun (being a device
that facilitates a transition between states of balance).

Considering the ATU output terminal to have voltage V, V is applied
directly to one feedline conductor, and if the common mode impedance of
the coaxial choke is sufficiently high and its length sufficiently
short, approximately -V to the other conductor... so it implements a 1:4
impedance transformation.

The problem is with the "sufficiently" qualifications above. The common
mode impedance of a coil of coax like this has been well explored, it
has a very high impedance over a narrow range near resonance, and
mediocre to low away from resonance, and so not well suited to broadband
use.

Nevertheless, these types of things have appeal to those who are
prejudiced against ferrite, or who just do things on the cheap.

I hasten to add that I have used just such a coil of coax as a single
band 1:1 current balun, but it was carefully resonated for the band in
use, and it measured much higher common mode impedance than practical
ferrite cored baluns.

Mentioning current baluns, you describe a balun that could not be
categorised as an effective current balun, as if you were to ground the
side of the load direct connected to the ATU, the whole thing fails.

I know that to many there is a magic to all things that came from
Collins, but you don't see this widely done, and for good reason.

Owen
*******************
Thanks for the comments Owen. I was curious about the schematic, but tend
to stick with my "on hand" ferrite baluns.
--Wayne