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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!

40 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. Hola, Miguel.
      I'm thinking of a 50Ω to 75Ω balun to feed a dipole. A turns ratio of 5:6 yields an impedance ratio of 50:72 which is close to ideal. Looking at the first drawing at the top of the page I imagine that a left-hand winding of 5 turns and a middle and right-hand winding of six turns each would work. But frankly I'm not sure about the middle winding. Can you confirm/comment?
      ¡Muchas grácias!

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    3. The first drawing is for 1:1 balum, and the second one for a 4:1 balum. Other relations will have other topologies.

      If you still want to experiment with the topology of the 1:1 balum I recommend you to use a cheap nanoVNA or similar device and see what happens with the middle winding in a 5:6 balum. Honestly, I have no idea :o)

      73!

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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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    Replies
    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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  9. Ya very easy. i will try tks vu2vid

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  10. I would like to see the diagram how to stack the two together,
    to make a 1:1 Balun on the coax feed side, and a 4:1 output to the antenna.
    Tnx es 73, Lance K8LCS

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    Replies
    1. In that case I used to place a RF choke before the 4:1 balun. I've never tried to place both baluns in series. Something to test in a rainy weekend.

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  11. The 2nd drawing 4:1 is wrong. Just check it again.

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    Replies
    1. Oops... You are right!

      I modified the drawing with the right one. Thank you!

      Delete
  12. Hello Miguel. May i know if you tried to use an HF dipole antenna without a balun? May i know the SWR meter reading? DV9HWO

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  13. SWR readings do not has to be better or worse with or without a balun, but usually they are. The balun blocks, or isolate, the RF from flowing by the outter side of the coaxial shield, so you are actually tunning a two arm dipole, not a three arm dipole.

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  14. HI, is the sketch and the diagram corrected now, the one for 1 to 1 ??

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  15. VERY GOOD ARTICLE INDEED WHAT ABOUT A BALUN 4/1 FOR DELTA LOOP ANYENNAS IS IT THE SAME AS ABOVE

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  16. Good first article on Balun design.

    Curious - why 6 turns? Where did this info. come from? What happens if more turns used?

    Also, do you have any SWR data for the 1:1 balun design that you have proposed?

    Also, it seems that there are other designs for 1:1 baluns i.e. bifilar, and and different wiring designs as below. Any thoughts on the pros / cons of these alternatives?

    Cheers!

    http://cdn3.bigcommerce.com/s-4q7cv/products/167/images/72/1113__68940.1408661116.1280.1280.jpg?c=2

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    1. The number of turns determines the working bands. Too few turns and the balum will fail at low bands. Too much and the stray capacitance will kill the upper bands. You will need to find a good compromise for the desired bands.

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  17. I'm a newbie with toroids. what size should it be?

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    Replies
    1. More important than size is the material: it should work at the desired frequencies.

      About the size, large enough to withstand the applied power without dissipate too much heat. Heat = losses

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  18. Hola Miguel, muy bueno el artículo.
    Enhorabuena.
    Yo también he estado jugando con un balun hace poco:
    http://www.ea1ddo.es/Balun_Guanella_4_1.php

    Nor vemos.
    73, Máximo - EA1DDO

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  19. Finally a Clearly Written explanation of how to build a simple Balun. Is it possible to get this for a 1:1.5 for 50 to 75 ohm cables???

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    1. It can be done but they are not called balun since 75 and 50 ohms coaxial cables are unbalanced transmission lines: no balanced - unbalanced conversion.

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    2. Good day Miguel
      I agree...they are called an UnUn or Unbalanced to Unbalenced.
      I ask for a drawing since i have an abundance 75 Ohm 3/4" (19mm) direct burial cable....plus the connectors.....
      Just need one of yout Great Drawings....They are easy to understand.
      Thank You in advance.

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    3. January 2021 QST has info on the 50 ohm to 75 ohm unun. They have 5 turns to antenna and and antenna tap at one turn from transceiver. You may have to experiment with the number of turns and cores.

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  20. if you transmitting
    1.5:1 is efficiency of 95%, you might want just stop there, coax loses now plays higher part.

    If you receiving - it does not matter at all

    also, be aware that if you are trying to match a dipole to your 75ohm tv cable, dipole does not have 50 ohm impedance. Not in practice. It depends on the ground and heigh above the ground.

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  21. Hi Miguel
    What size wire should I use to handle 1kw ?
    Thanks
    VE6AQF, V21C

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  22. What about teflon (PTFE) coated and silver plated #18 stranded wire on an FT 240/43 core?

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  23. The designs shown look like voltage baluns (Ruthroff). I was under the impression that current baluns (Guanella) were preferred for most applications because they also provide common mode current isolation, which voltage baluns do not.

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