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Antenna (Dipole) balun - what is it for?

  1. Oct 25, 2016 #1
    hi,

    my question is in regard of a printed (microstrip) dipole antenna.
    I know that a balun acts as an unbalanced to balanced line transformer.
    (in my case of the dipole antenna: the coaxial feed is unbalanced and the half-wave diple antenna is balanced, so i need a balun.)
    But what exactly does "unbalanced" and "balanced" mean in this case?
    Im a bit confused as on many sites the balun seems to be seen as an imedance transformer (like a quarter wavelength transformer) only.
    So how exactly does it work and what exactly is it needed for in my case?

    Im happy about every answer that sheds some light into my dipole-darkness! :D

    thank you!
     
  2. jcsd
  3. Oct 25, 2016 #2
    I think you have that backwards.

    A balanced transmission line has two conductors. The fields are tightly bound when you have two conductors running in parallel. An unbalanced transmission line uses one conductor. The unbalanced transmission line uses the environment as a ground so the field is spread out. The Balun couples the conductor-conductor wave to the antenna-environment transmission.
     
  4. Oct 25, 2016 #3

    Averagesupernova

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    Since when is coax balanced transmission line?
     
  5. Oct 25, 2016 #4

    davenn

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    I agree with @Averagesupernova, this is incorrect

    coax cable is a 2 conductor transmission line and it is UNBALANCED
     
  6. Oct 25, 2016 #5

    davenn

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    A BALUN can be used for one or both of 2 situations

    1) a balanced feedpoint, like with your dipole, to an unbalanced transmission line like coax cable
    in the case of a straight dipole, it's feedpoint is around 75 Ohms and a 1:1 BALUN can be used to 75 or 50 Ohm coax
    The difference between 75 and 50 Ohms is neither here nor there in regards to tuning and SWR

    2) But maybe your dipole is a folded dipole and it has a feedpoint impedance of around 300 Ohms
    NOW the BALUN must be a impedance transformer as well as doing a balanced to unbalanced match
    This time you use a 4:1 BALUN to match the feed point to the coax cable



    Dave
     
    Last edited: Oct 25, 2016
  7. Oct 25, 2016 #6

    davenn

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    here's a 1:1 BALUN I have drawn
    the open end at the top presents a high Impedance to RF currents at the frequency of use
    The sleeve is 1/4 wavelength at the freq of use

    upload_2016-10-26_15-16-37.png


    will draw a 4:1 BALUN when I get time

    can't be bothered drawing one .... here's someone else's drawing

    balun.gif

    again for this BALUN, the loop is 1/2 wavelength at the freq of use
    Ohhh I must state too, to save confusion, the lengths mentioned ... the 1/4 sleeve and the 1/2 wave loop ... these are an electrical 1/4 and 1/2 wave for the coax used. If a freespace 1/4 and 1/2 wave are used, the lengths will be too long

    An example
    freespace 1/2 wavelength at the 144MHz Amateur band calc .....
    1 wavelength in metres = 300 / freq (in MHz) = 300 / 144 = 2.08 metres
    1/2 wave/length = 2.08 / 2 = 1.04 metres
    Electrical 1/2 wave = 1.04 x the Vf ( velocity factor of the cable)
    1/4 inch diameter RG58U has a Vf of about 0.66 therefore
    1.04 * 0.66 = 0.6864 metres (68.64 centimetres)


    Dave
     
    Last edited: Oct 25, 2016
  8. Oct 26, 2016 #7

    sophiecentaur

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    It seems that no one has actually answered the question in the title.
    The following assumes a transmitting antenna but a similar argument can be applied to a receiving antenna - it just involves one more step in the description.
    The purpose of a balun is to eliminate currents flowing down the outside of a coaxial feeder. If you go to lots of trouble to design, say, a dipole, that design relies on the fact that there are specified currents flowing in the two arms. (Equal magnitudes if the dipole is centre fed). If you just hook it up to the end of a coax line with one element to the inner and the other element to the outer, the current flowing on the inside of the outer braid will be divided up between the dipole element and the outside of the coax. Those parasitic currents in the outer will contribute to the radiation pattern of the dipole and need to be eliminated if you want to rely on your design. That current on the outside of the feeder could be zero or even more than the element current, depending on the length of the feeder and any other conductors it goes past. You need to block that unwanted current by presenting a high impedance path to the outer.
    One very easy way of suppressing current in the outer is to thread a few turns of the whole coax through a toroidal ferrite (choke balun). Quick (for some operating frequencies) and dirty and there is no impedance transformation but that may not matter. The 'sleeve balun' in Dave's post works by presenting an open circuit to current down the outside; the quarter wave sleeve transforms the short circuit at the far end to an open circuit at the connection. The quarter wave trick is very smart and the principle is used in all microwave oven door seals, where you cannot rely on a good metallic connection between door and oven. It turns a deliberate physical open circuit into a good short circuit path for all currents circulating inside the oven - so none leak to the outside surfaces.
    I strongly recommend that you Google terms like Balun, sleeve balun, pawsey stub, choke balun in Images. There are hundreds of pictures of different styles - take your pick of articles that suit your level.
     
  9. Oct 26, 2016 #8
    Thank you all for your input!
    Especially the last two posts contributed greatly to my understanding.

    Thank you very much! :)

    PS: Im just a little confused, can one of you please tell me why there has to be a 180 degree phase shift between both dipole arm feeds?
     
  10. Oct 26, 2016 #9

    sophiecentaur

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    That's eezy peezy (but confusing at the same time) and you will get it as soon as you read this: For the dipole to radiate, you need current flowing in the same direction in each arm i.e. up and up or down and down. i.e. the currents need to be in phase along the axis of the dipoles. So it's as if the transmitter wasn't there. But , to make this happen, you have to 'blow' in one arm and 'suck' from the other arm. Relative to zero, the two driving volts need to be 180° out of phase (push pull). Wave your arms about and it will make sense. :smile:

    PS Look at this rather ancient link. It has several pictures of different baluns.
    PPS If your transmitter is very small compared with the wavelength, battery operated (no power lead) and situated right at the feed point, you don't need a balun.
     
  11. Oct 26, 2016 #10
    ok, this makes perfect sense now!
    Always amazed how quite complex stuff can be broken down into eezy-peezy-bits by some people. ;)

    Thanks a lot, now i can continue working on my antenna designs in good conscience of actually "knowing" whats going on!
    Thanks a bunch! :D
     
  12. Mar 13, 2017 #11
    I would try to explain you the real concept of Balun in simple words.

    BALUN - BALanced-to-UNbalanced Converter acts like a simple Transformer. Just like the transformer, Balun too has its Primary and Secondary windings around the ferrite core material. The only small difference between both of them is that the transformer in Balun is basically a current transformer. Not the actual current transformer, but it is sort of device whose resistance changes with the variation in the number of windings on the core. So Balun is also called as an Impedance Matching Network.
    By using Balun between two distinct points of the circuit, you can have their Impedances matched, perfectly.
    With such kind of Balun in your case (Dipole antenna), you can match the ends of the Antenna to any of the inputs of measuring devices. As the antenna will be perfectly matched with the device using this Balun, you will observe far better results, because there would be no reflections from the match point.
    This helps you getting more accurate results while testing as well as practically implementing the working of Antenna.

    Hope you are clear with the basic idea of Balun from this answer.
     
  13. Mar 13, 2017 #12

    sophiecentaur

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    There are many designs of Balun that do not involve wound components, of course but the name describes the essential function, however it's achieved (as in the OP).
    Any impedance transformation is a side issue and sometimes you need a transforming elements in addition to the Balun function itself.
     
  14. Mar 13, 2017 #13

    davenn

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    indeed

    @Shubham Jaydeokar ...... for antenna conditions as what the OP was talking about, your statement is far from the truth and only occasionally seen

    please review my post ... post#6 .... of 2 very common examples of no primary or secondary windings, no ferrite cores

    3 main purposes of baluns ....
    1 -- to stop RF currents flowing on the coax braid ... as this will radiate and cause all sorts of problems with the stray RF
    2 -- as the name suggests to match unbalanced feeders to balanced antenna feed points
    3 -- impedance transformation 50 Ohm to 200 Ohm, 75 Ohm to 300Ohm etc


    Dave
     
  15. Mar 14, 2017 #14
    That was really informative, Dave!
    Would always appreciate your help for letting us (including me) know about the latest detailed knowledge about a particular subject.
    Thanks' Dave.:thumbup:
     
  16. Mar 14, 2017 #15
    Thanks' @sophiecentaur for adding some of the important points in this discussion.
    That was really helpful.
     
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