[Antennas] Some thoughts on baluns

Wed, 8 Jan 2003 10:19:35 -0600

I've been trying a few things with ferrite choke current baluns,
similar to the bead balun first covered by W2DU.
All balanced loads being fed with an unbalanced line need a
balun of some sort to prevent currents on the OUTSIDE of 
the shield (the current on the outside is in phase with the 
center conductor). If ALL the current flows in the center 
conductor and the inside of the shield, you will present a 
"balanced line" (equal and opposite current flow) at the 
end of the coax, ready to connect to your "balanced load". 
You may have just connected the coax to the antenna and 
have made contacts all over the place, but I can pretty well 
guarantee that the radiation pattern is not what you think it 
is and you may be communicating with your neighbor's 
intercom. Current on the outside of the shield WILL radiate.
Electrons always follow "path of least resistance" so if you
add impedance to the outside, you will force that current to
the inside, hence the ferrite choke current balun. Saturation
is not a problem because you are creating a "blocking path"
forcing the inside of the shield to carry all the current.
The average coax choke balun on a balanced beam is 
typically 6 turns of coax 6" in dia (or around that). 
Testing shows this to make a very effective choke balun 
for 20/15/10m. Coils for 80m and 160m are too large and 
pose other problems, so we use large, high u (2000 and 
up), ferrite "beads" for the choke baluns. The materials 
which appear to work best are #75 followed by #73 
and #77. The #43 material won't work well at the lower 
frequencies.Those materials are found in many of the 
large clamp-on split and non split "beads" used by the 
computer industry on monitor, keyboard, and power cables. 
Use the ones with a 1/2" hole so you can wind 4 turns of 
RG/58U (4 turns roughly increases the effectiveness by 
the square ....or x16). These are pretty easy to find and 
Mouser has some if you can't. 
How do you know what material you have ? Take your 
MFJ Antenna Analyzer and run a single turn through the 
bead. The performance will be worst at the lower 
frequencies, so on 80m measure the shorted loop without 
the bead, should be <<5ohms. With #75 material it will
measure around 70-80ohms, with #73 or #77 material 
it will measure around 40-50 ohms. #43 material will
measure around 10 ohms or lower.
What does this mean ? A 4 turn 4" dia coil of RG/58U
with two #75 or three #73 or #77 large "beads" makes
a very effective choke balun for 80m - 10m and a pretty
effective 160m - 10m choke balun. Both sides of the
balanced line looked about equal on the scope. You can 
maybe back off one of the beads and still be OK if you 
have tested the it. A 6 turn 6" dia coax coil by itself 
makes a very effective current choke balun for 20 - 10m. 
Relative to the 4" coils, don't make the bending radius 
too small or the center conductor will migrate, especially 
with softer foam coaxes and it creates necessary 
stresses on the "snap-on" bead mechanism. The two 
halves must completely touch.
To take this one step further it's probably a good idea 
to put one of these choke baluns in the coax as it exits the 
transmitter, on coax(s) (used or not) as they exit the 
tuner, maybe one or two along the coax transmission 
line ....may not do anything, can't hurt anything, probably
more important with cheaper (minimum shield) coax. All 
you are adding, from the transmission line view is a few 
extra feet of coax. This is in addition to the choke balun
at the balanced antenna load.
In fact, a few turn coil in the power cable, as it exits the 
transmitter, might help a 20m - 10m radiation problem 
if that was the source.
Fire away.
73 Kees K5BCQ 