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Author Topic: Balun design  (Read 765 times)
KA7QOR
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« on: May 02, 2004, 07:38:02 AM »

I need to construct a 2:1 balanced to unbalanced choke/current type balun. I have an antenna that is 22ohms balanced at the feed point and I wish to feed it with 50 ohm coax (RG-142). I would like to be able to use this balun for 200W. I have three antennas the same; one ant is 10M/20M (22/19 ohm), another for 6m (36ohm) and another for 70cm (32ohm). So it would be nice to try building baluns for HF/VHF/UHF.
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K0IZ
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« Reply #1 on: May 02, 2004, 08:44:55 AM »

If you want to do it right - buy/borrow a copy of Jerry Sevick (W2FMI) book on transmission line transformers.  Lots of good info on building all types of baluns and ununs.  A balun converts unbalanced to balanced.  A unun is unbalanced to unbalanced, but has impedance transformation.  So a balun plus a unun converts unbal to balanced AND impedance change.

To stepdown in impedance, you need a 1:25 unun connected to a 1:1 balun.  This will take two toroids.  The 1:25 will convert the 50 ohms unbalanced (the 2:25 side) to 22 ohms unbalanced (the 1: side).  It will have a trifilar winding.  The 1:1 balun will have a bifilar winding and converts the 22 ohm unbalanced to 22 ohm balanced (your antenna).

(If you were converting 50 ohms to a higher impedance, you would connect the 50 ohms to the 1:1 balun and connect the antenna to the 2:25 side of the unun.  That would give a step-up impedance transformation)

These are described in Jerry's book, figures 7-14 and 9-3.  The size of the cores and wire size will be dependent upon how much power you want to run.  2.40" #61 toroids with 14ga wire will handle at least 2kw.   For lesser power you could use smaller toroids and maybe 16 to 20ga wire.

SInce you are converting from 50 ohms downward, you would hook the 2:25 side of the unun to 50 ohms.  The other side of the unun is connected to one side of the 1:1.  Then the second side of the 1:1 goes to your 22 ohm balanced load.  

Sounds like a lot of trouble vs some kind of single core balun, but this is the best way to go.  The combination will have extremely flat SWR and virtually no loss.   John.

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K0IZ
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« Reply #2 on: May 02, 2004, 09:05:02 AM »

My suggestion in the note above will work for your 10/20 meter needs.  You could use a 1:1.5 unun instead of a 2:25 for the 6 meter application (see Jerry's book).  However for 6 meters and for 70cms, since the antennas are single band, a better choice might be quarter-wave transformers (coax).  I'm no expert on these, but perhaps two 1/4 wavelength pieces of 75 ohm coax in parallel (to give 1/2 of 75 ohm impedance).  Maybe someone else will post better info.  Regards.  John.
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K0BG
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« Reply #3 on: May 02, 2004, 09:23:50 AM »

John's correct about Jerry's book, however, a shorter version is available from the ARRL and contains more "how to" information. It is called "Building and Using Baluns and Ununs" and can be purchased on-line.

The appendixes contain additional information on short verticals and they're worth the price just by themselves.

You might want to look over Tom Rauch's (W8JI) site for additional information: http://www.w8ji.com

Alan, KØBG
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AC5E
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« Reply #4 on: May 02, 2004, 09:51:46 AM »

I have not found a conventional "wire wound" balun that works well at VHF.

Personally I would probably construct a quarter wave section of 36 - 37 ohm concentric (coaxial) line from rigid copper tubing, and use another and larger section of tubing outside that for a sleeve balun. Teflon rod is readily available and you should only need a spacer at the ends of the quarter wave section, and at the open end of the sleeve. Of course, you will have to take care with weatherproofing the assembly.

If that proved difficult I would most likely parallel two sections of 75 ohm line to obtain a matching section.

And if, for some reason that proved impracticable I would most likely change the antenna's feed system to provide a 50 ohm impedance.

73  Pete Allen  AC5E
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K0ZN
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« Reply #5 on: May 02, 2004, 10:04:43 PM »


Hi,
I will put in my two cents and say that I agree fully with all the foregoing comments. W2FMI, Dr.Jerry Sevic, is "THE" guru on transmission line transformers. Follow his book or books based on his designs and you can't go wrong.

FYI: Amidon Associates use to make and sell good quality (I haven't checked lately) Baluns and UNUN's based on Dr.Sevic's designs.

I would also agree that at VHF and UHF conventional wire baluns don't work very well...they can be difficult to make and can be lossy, particularly at 432 Mhz. The stray capacitance is very high at these frequencies. Stick with linear devices or "hardline" sections as the losses are much lower.

Check out the ARRL antenna book or other good VHF books.

73,  K0ZN
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