"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.
-
Along the lines of trying to understand this, would it be possible to use
the
same circuit with the coil of coax replaced by a 1/4 wave length of open
wire line?


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.

-
Could you state this a different way, as I don't follow the grounded
output reference.

snip

thanks
Wayne

"Wayne" wrote in
:

"Owen Duffy" wrote in message
...

....

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.

-
Could you state this a different way, as I don't follow the grounded
output reference.

I have written some notes about baluns in antenna systems at
http://www.vk1od.net/balun/concept/cm/index.htm . The notes explain why
a current balun is most suited to 'general purpose' antenna application.

In an ideal current balun, the current in each output wire should be
independent of whether one terminal or the other is grounded.

In the case of the balun describe, grounding one or other output
terminal has a large effect on output current in each of the wires.

Proponents of some balun designs insist that they work perfectly into an
isolated load. Of course, it the load is truly isolated, ie there is not
current path to ground from the load, then they are correct, but you
don't need a balun in that case because since the load is properly
represented by a two terminal network, the current into one terminal
MUST equal the current out of the other terminal. Problem is that real
antennas aren't necessarily will represented by a two terminal network
(though we often talk as if they are).

Owen