What exactly is an "Unun"?

Discussion in 'Antennas, Feedlines, Towers & Rotors' started by KF4ZHL, Aug 9, 2010.

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  1. KF4ZHL

    KF4ZHL Ham Member QRZ Page

    This is the first time I've heard of an "unun". I saw a picture in another thread. If I may ask, what exact purpose does an "unun" serve? It appears to be simply base loading the antenna with a coil (or it could also be a transformer of some sort). If it's a coil, it would move the center resonant frequency of the long wire down in frequency. I'm guessing it would also narrow the bandwidth. Would it also cause a sharper attenuation at the bandwidth ends? Is this the same concept as putting a wide band pass filter on the antenna system filtering out the most common frequencies of noise generating sources at the antenna? Would it be best when making one of these to actually calculate the coil dimensions and wire length resonant frequency to the center of the bands one plans to use?

    If it's a transformer, I assume it's purpose would be to bleed DC static charges on the long wire to ground, isolating the reciever (would that simply make it a 1:1 transformer)? I'm guessing a transformer could also do all which I mentioned up above as well. If it's a transformer, do the windings need to be made a certain way (Each coil winding resonant), or is that unimportant when using a tuner anyway?
  2. G3TXQ

    G3TXQ Ham Member QRZ Page

    Its function is to act as an impedance transformer between an unbalanced load and an unbalanced feedline; the transformation ratio is most usually 4:1 or 9:1.

    They are often used with "untuned" verticals to limit the extreme VSWRs that would otherwise occur on the coax feedline - and therefore the cable losses - on some bands. A properly designed UnUn should do nothing other than alter the antenna impedance by a fixed ratio.

    Steve G3TXQ
    Last edited: Aug 9, 2010
  3. KF4ZHL

    KF4ZHL Ham Member QRZ Page

    OK, it greatly increases the impedance of the feedline in comparison to the radiating element. A balun does something similar then. It increases the impedance of the shield braid only though. Does an unun increase the impedance of the center conductor and the shield braid then, or just the center conductor?
  4. G3TXQ

    G3TXQ Ham Member QRZ Page

    Don't confuse 'impedance transformation' with 'driving balance'; they are separate functions.

    A UnUn provides an impedance transformation, but does not drive balance. A BalUn primarily drives balance (voltage or current), although to confuse matters some designs also incorporate an impedance transformation function.

    Impedance transformation relates to the differential-mode load on the coax - "centre-conductor/braid" in your terminology. Balance is primarily about increasing the common-mode impedance of the feedline - "braid only" in your terminology.

    Steve G3TXQ
  5. W8JI

    W8JI Ham Member QRZ Page

    Un Un is an acronym for UNbalanced to UNbalanced.

    It can be a 1:1 ratio or a million-to-one turns ratio.

    As a matter of fact, the technically correct term for what is usually called an unun is an autotransformer.

    A good properly working current balun serves perfectly as an unun and can be correctly called an unun. This is because any true current balun has no fixed voltage balance to ground at either end, and as such has no balance gender.

    From an article I am writing:

    "To understand how a balun operates and why a balun is needed, we must understand balance. We tend to think of balance only in the amount of current in each conductor of a transmission line, but that thinking can mislead or confuse us. Perfectly balanced lines and perfectly unbalanced lines alike have equal and opposite currents entering and leaving the conductors at each end!

    What then defines an unbalanced line, source, or load? The answer lies in the voltage or electrical potential between line conductors and the environment around the line. In the ideal balanced line, the electric potential of each conductor is equal and opposite in relationship to the environment surrounding the line including the chassis or cabinets of our equipment. In the ideal coaxial line, the outside of the shield has no electrical potential difference to the environment around the line, including the chassis or cabinets of our equipment. The shield of our coaxial cables, as we commonly accept and understand, is at ground potential. We say the shield is “grounded”.

    With real-world antennas, the coaxial shield connection point almost never has zero electrical potential to the environment around the shield or points further along the cable’s length. Being a less-than-ideal zero-potential-to-earth termination shields almost always have common mode current, even if a small percentage of differential (normal transmission line mode) current. For example, the four radials of a groundplane antenna, no matter how configured or tuned, are never truly at the same electrical potential as the environment around the antenna or shield potential further down the feedline. Experimenting with a groundplane antenna, we find the feedpoint is mostly but not perfectly unbalanced. The shield is not connected to an electrically zero point. Significant current can and often does excite the outside of the shield on a groundplane antenna, with outside shield current 20% or more of antenna base current under some feedline grounding and lengths! We consider the groundplane antenna “unbalanced” and it is definitely not balanced, but it is not perfectly unbalanced."

    So you see, a perfect current balun can be used as an un-un. They are functionally the same. The term "unun" simply means unbalanced to unbalanced and requires no specific impedance ratio. On the other hand an autotransformer is an unun also, and can have an impedance transformation depending on tap point.

    73 Tom
  6. AI3V

    AI3V Ham Member QRZ Page

    While a autotransformer is ONE way to make a unun, it is not the only way.

    You are confusing several concepts here.

    Specifically the concepts of balance, unbalance, and impedance(turns) ratio.

    Irregardless of balance or unbalance, or combination thereof, you WILL have a impedance ratio.


    To the O/P:

    A UnUn is simply a transformer that connects 2 unbalanced (often coaxial) devices together.

    The UnUn may also provide a impedance ratio other than 1:1, either a step up, or a step down.

    The UnUn may be wired as a autotransformer, or it may take the form of 2 seperate coils.

    It is most commonly seen as a tapped base loading coil when matching a short Marconi antenna to a 50Ω coax.

  7. W8JI

    W8JI Ham Member QRZ Page

    I'm not confusing a thing Rege, but you may be. :)
  8. KL7AJ

    KL7AJ Premium Subscriber QRZ Page

    An un-un can also be nothing more than an isolation transformer. In the TV broadcast business we used to use a lot of 1:1 video transformers to get rid of ground loops and such.

    The isolation function and the impedance transformation functions can be combined in an un-un, but they aren't the same thing.

    Some hams believe that inserting a broadband transformer, like a bifilar un-un into an antenna system will make the antenna syatem broadbanded. It will not. Most transmission line transformers aren't designed for reactive loads.

  9. W8JI

    W8JI Ham Member QRZ Page

    That's right. An UnUn can be a lot of things, but Hams often wrongly classify it as an impedance transformation autotransformer because a certain book called autotransformers "unun's".

    Un un really just means unbalanced to unbalanced, and balance is determined by the voltage to "ground" on the transmission line conductors.

    A properly working balanced two wire line has equal and opposite flowing currents, and equal and opposite conductor potentials to earth.

    A properly working coaxial line also has equal and opposite flowing currents in each conductor, but one conductor (the shield) must have zero potential to "earth" or the environment surrounding the line.

    A good current balun, because it has high common mode impedance, works in either balanced or unbalanced applications. It serves perfectly as an unun or a balun.

    It can increase bandwidth, because VSWR bandwith often improves with a high line impedance. The unun can step 50 ohm line impedance up to 200 ohms, and that can and does improve VSWR bandwidth. Model or measure an antenna and you will see this is often true.

    73 Tom
  10. KD0CAC

    KD0CAC Ham Member QRZ Page

    Another good discussion to watch .
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