[Antennas] Some thoughts on baluns

2003年1月11日 20:03:37 -0500

Correction below:
At 07:41 PM 1/11/2003 -0500, Charles Greene wrote:
>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.

Change previous sentence to read:
Then use twice this inductance 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
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73, Chas, W1CG
K2 #462