In article ,
Wayne wrote:
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?
Many people have. It's also often referred to as a "common mode choke".
One nice characteristic of them is that they don't contain ferrite or
iron cores which could saturate (and create distortion and harmonics)
at high RF current levels.
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.
I'm not sure why he specified hooking the braid to the tuner output
and the center conductor to ground. Most people do it the other way
around (it's easier to connect to a PL-259 that way).
Doing it his way brings the "hot" side of the tuner output out closer
to where one might touch it... there may be more of a risk of a shock
or flashover-to-ground situation here, if the SWR on the line is high
and the tuner is looking into a high impedance at the input of the
choke.
This may not be an issue in your particular deployment.
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?
http://www.hamuniverse.com/balun.html shows numerous variations on
this same basic idea, and there are about a zillion others out there
on the Net and in the literature.
There are numerous variations:
- Single-layer solenoid-wound (as in the one you described), either
close-wound or spaced.
- Multiple-layer solenoid-wound.
- Scramble-wound.
Different winding style will give different amounts of inductance, and
different (sometimes very different) amounts of distributed
capacitance.
If I understand the situation correctly, a winding style with low
distributed capacitance (e.g. single-layer solenoid-wound with space
turns) functions primarily as a common-mode inductive choke... fairly
broad-band, *if* you have enough turns to give you the necessary
amount of common-mode inductance.
A choke which is multi-layer (perhaps even scramble-wound) can have
more distributed capacitance, and it can act as a parallel-resonant
trap at certain frequencies... giving a nice, high choking impedance
at those frequencies. Above and below that resonance, though, its
impedance can drop off fairly rapidly, and it may not choke the line
effectively. Coaxial chokes of this sort are (I believe) often wound
to give optimal performance on one or two bands.
[I could well be wrong about these details... it's been a while
since I looked at this subject in any detail.]
--
Dave Platt AE6EO
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