[Antennas] Some thoughts on baluns

[Charles Greene]

Kees,

Some comments below.  Sorry for the delay; I was in FL and just got back today.

At 10:19 AM 1/8/2003 -0600, Sandy and Kees Talen wrote:
>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.

The problem with baluns using Type 43 material at lower freqs is that no 
one uses enough.  It will take 30 to 50.  The type 7x have higher loss but 
have a higher permeability and require fewer beads (for a choke balun).  So 
people make baluns using the same number beads as the type 7x, that is, 8 
to 10, using type 43, and then say it doesn't work well.  The best way to 
test the balun for efficiency is to see if it gets warm during operation 
and if so, use more beads of a lower permeability and loss.  ON4UN in one 
installation recommended using type 61 beads nearest the antenna, followed 
by type 43, for a total of 50.  The type 61 has an even lower permeability 
than the type 43, but lower loss too.

I tested some baluns to see if they got warm by using a Johnson KW Matchbox 
hooked up in reverse to a KW dummy load, and tuned the balanced output for 
700 ohms, hooked up the one side of the balun to the balanced output and 
the other side to a transmitter through a tuner.  This was so I could 
control the impedance, and also test off-air.  Baluns made of the type 43 
material got a little warm but not real hot.

You can measure the inductance of a balun using an LCR meter.  The inside 
conductor and the outside shield both should measure about the same 
inductance.  The value you should strive for on 80 meters is 88 uhy which 
will give you an impedance of about 1000 ohms.  Then use twice this 
impedance for 160, 1/2 for 40 and so on.


>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


73, Chas, W1CG
K2 #462