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Tuesday, September 4, 2012

Build your own HF balun

A balun is a MUST for dipoles or similar antennas when they are feed with coaxial cables. Many hams connect the center conductor of the coaxial cable to one side of the dipole, and the shield to the other. Wrong!

From the RF point of view, the shield can be modeled as two conductors, the internal shield (the real shield, this is, ground) and the external shield, who is really far to be ground. In this way, your dipole has 3 arms, the two from the dipole and the coaxial cable shield (external face).

Many times the coaxial is too long that represents a high impedance and a low effect on the dipole, but others, it presents a low impedance and radiates. This can cause serious RF feedback in the shack, TVI problems, among really poor lobules and directivity in the dipole pattern.

To prevent all these effects, a balun is needed. A balun is only a RF transformer. You can build your own balun, is quite easy. Use a ferrite bar from a AM receiver or better, a toroid. Turn all the coils at the same time and connect them properly as you can see in the above diagram. The most common balun for a dipole is the 1:1 ( the one at the left ), but a 4:1 balun can be useful also for wire antennas.

Make six turns of wire around the toroid and keep all wires parallel between them. It's an easy process that can be done in a few minutes. Use a wire according with the power you will use. Pay attention to the toroid you use. Use only a good RF toroid. Many surplus toroids are from switching power supplies or telephony equipment and are not usable in the HF range. In case of doubt, test the toroid. Use a dummy load (50 ohm for 1:1 and 200 ohm for 4:1 balun) and connect it to the output.

1:1 balun wiring diagram

Connect your transceiver at the input with a SWR meter and check the balun on the bands you will use it for. You should see a clear 1:1 SWR on all bands. If you don't see 1:1, is possible the toroid is not suitable for HF use, or you has made wrong connections between coils. The asterisks at the drawing represent the start of the coil. They all must be turned in this precise direction or the balun will not work.
4:1 balun wiring diagram

I can't assure you the balun will work with all the power you will use. For up to 100 watts, there must no be too much problem with any good made balun, but higher power will require another kind of toroids or ferrite bars. If you made your balun with a ferrite bar, remember all coils are turned on the same bar! A stripped ribbon cable can be quite useful for QRP, and cooper insulated wire for higher power levels.

Playing with my very first balun I learned a HF dipole NEED a balun. If you never used a balun, build one and play with it, you will be as surprised as I was!

12 comments:

  1. This is interesting, but I find the description a little unclear. I would really like to see a physical rather than electrical drawing of the set-up you are describing.

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    1. I agree a physical setup is much more easy for a new ham .... me ..... I understand your electrical description, seeing both and the materials you use for construction will help. Thank you for taking the time to explain the process and share your knowledge. :) N2WAG DEE

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  2. You are absolutely right. I added a pair of wiring diagrams.

    Thank you for your comment!

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  3. You English is excellent and makes the use of your web site that much easier!

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  4. Jim SV7JLZ - SA0BGIAugust 17, 2013 at 5:00 PM

    Well done!!!!

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  5. Jim SV7JLZ - SA0BGIAugust 17, 2013 at 5:02 PM

    Well done , easy to understand.

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  6. I wish I had read this article earlier than this. It is excellent and would have saved me hours of work in adverse conditions.Many thanks.

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  7. Hola,
    Leyendo tu articulo sobre la construccion de un 'balun', el cual he encontrado muy informativo, le agradeceria me aclarase una duda, cuando se refiere a la barrita de ferrita no indica cuantas vueltas recomiendas, mientras si lo hace al refiere al toroide.

    En otros articulolos sobre 'balun' por regla general al referirse a las vueltas del cable de cobre sobre la ferrita suelen indicar unas 13 vueltas.

    Le agradecere me indique si es asi o debo de darle tan solo 6 vueltas, o sea, igual que formando la bobina sobre el toroide.

    73,

    Jimmy, ZB2BL.
    www.facebook.com/zb2bl

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    1. Un buen punto de partida es utilizar el mismo número de espiras tanto en el toroide como en la barra de ferrita. Sin embargo utilizando una barra de ferrita, dependiendo de su tipo, puede ser necesario añadir más espiras. Es un poco un juego de prueba-error.

      Para comprobar si el número de espiras es el correcto, hay que comprobar el funcionamiento del balum en la banda más baja en la que se pretenda utilizar, normalmente 80 ó 160 metros, con la ayuda de una carga o resistencia adecuada.

      73!

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  8. Hi Miguel,

    Your schematic and wiring diagram of the 1:1 balun is very helpful. Another one by 2E0KGV has an error and I built the balun using his info. I rebuilt it after finding your article and now it works OK but I may have too many turns.

    Thanks.

    Gene

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