Current Balun vs Voltage Balun

Discussion in 'General Technical Questions and Answers' started by KC9QQ, Oct 2, 2009.

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  1. KC9QQ

    KC9QQ Platinum Subscriber QRZ Page

    I am building a 135' dipole which I would like to feed with 450 ohm ladder line. However, I don't want to bring the ladder line into the shack so I would like to use a 4:1 balun to transition to RG-8X for the last 15-20 feet. I am unclear on when to use a Voltage vs Current balun. Also, is there a length of Coax that I should avoid?


    Fred, KC9QQ
  2. G4ALA

    G4ALA Ham Member QRZ Page

    Voltage Balun

    A 4-1 voltage balun would seem to be appropriate, with the high impedance facing the line and the low impedance side facing the co-ax.


  3. WB2WIK

    WB2WIK Premium Subscriber QRZ Page

    A 4:1 current balun is most appropriate. See for their "Remote Balun" product, which is intended for specifically the application you have. It's 4:1, current balun, with a hook to hang it outside under the eaves or something, and terminals for the balanced line, plus an SO-239 for the coax connection.
  4. WD0GOF

    WD0GOF Ham Member QRZ Page

    I too would have to vote for the current balun. The current balun will force equal currents in the coax feed. This will reduce noise and rf in the shack. Check out these sites for plain language discussions on the baluns.{3E5220F7-2D0F-45B5-85F7-3B654F804C4F}

    73, Walt
  5. WA4OTD

    WA4OTD XML Subscriber QRZ Page

    Hello Martinsville from Carmel, IN!

    For matched antenna the 4:1 voltage balun is good. In unmatched I think you need the current balun. So depends on how you will operate the antenna.

    This probably explains it where anyone can understand.,17,FAQ’s
  6. G4ALA

    G4ALA Ham Member QRZ Page


    I made a fundemantal error in my first reply.

    The voltage transformation should, of course, be 3:1, giving an impedance transformation of 9:1, thereby matching the 50 Ohms with the 450 Ohms. Therefore, use a 9:1 voltage balun to match the impedance of the ribbon to the impedance of the coax.

    There is nothing wrong at all with also including a 1:1 "current" balun to achieve common mode rejection and promote balance. However, the impedance transformation is a necessity. A "current" balun on its own will not work.

    Sorry for the confusion

  7. AL7N

    AL7N Ham Member QRZ Page

    Trouble is, the "450 Ohm" line is not going to present 450 ohms at the shack end when used to feed a 135 foot antenna operated over several bands.

    The impedance at the feed end will be some other reactive value over a wide range depending on the frequency you operate the feedline/antenna system at.

    "450 Ohm" line will only present 450 ohms at the feed end when it is terminated in it's characteristic impedance at the load end...450 ohms....Resistive.

    Don't worry about the impedance transformation, just use a 1:1 current balun to couple the coax to the balanced feeders. It will work fine.
  8. K9ZMD

    K9ZMD Subscriber QRZ Page

    Current Balun is the Answer

    Heftier 4:1 voltage baluns might handle the freight, but I can tell you that my LDG 4:1 voltage balun got very crispy inside when I was "multi-banding" an 80 meter doublet fed with window line. Not major power either, just attempting to find a match on 20 meters with less than 35 watts output. Fractured the toroid & burned through the enameled wire.
  9. KA5S

    KA5S Subscriber QRZ Page

    I was able to get rid of most RF in the shack from my 200 ft center fed antenna with a 6:1 balun on the window-line, and a 1:1 current balun inboard of that facing the tuner. Of course neither is actually seeing the impedance it is designed for, and it's possible I'd do as well or better putting the 1:1 between rig and tuner.

    I've had a "KW" 1:1 balun get hot enough to boil spit at 100 watts. Misuse of the device, sure. But instructive.

  10. G3TXQ

    G3TXQ Ham Member QRZ Page

    Use a current balun when you want to force equal currents, and a voltage balun when you want to force equal voltages. In this application we usually want to prevent feedline radiation - that requires equal currents.

    The original poster didn't tell us what length of ladderline he is using, so the impedance, just on 80m, could be anything between 50 ohms and 3200 ohms; given that tuners have an easier time with a high impedance than a low one, why would we want to step the impedance down?

    Given the information that we have, the most appropriate choice is a 1:1 Current Balun.

    Check out the excellent advice on the DX Engineering web site:{3E5220F7-2D0F-45B5-85F7-3B654F804C4F} which includes this quote:
    Length of the coax? As short as possible - it'll be handling a high VSWR if you use the antenna on several bands.

    Steve G3TXQ
  11. W5DXP

    W5DXP Ham Member QRZ Page

    With Z0=450 ladder-line feeding an 80m dipole, the only impedance guaranteed never to exist at the balun is 450 ohms. Hint: 450 ohms is the center of any SWR circle on a Smith Chart and not on any of those SWR circles. The only time the impedance seen by the balun is 450 ohms is when the SWR is 1:1 on the Z0=450 ohm ladder-line which it never is when feeding an 80m dipole. The balun needed is NOT dictated by the Z0 of the ladder-line. Such is a common misconception.

    Here is an 80m dipole fed with 450 ohm ladder-line that requires a 1:1 balun because the impedance at the balun is always between 25 ohms and 100 ohms. No impedance transformation necessary.

    If a 9:1 balun was used with this antenna, one would have to add a tuner to the system in order to overcome the mismatch introduced by the 9:1 balun and the transformed impedances would be between 3 ohms and 11 ohms, a lossy condition for any ordinary tuner.

    A 1:1 current-choke-balun, like the w2du balun, doesn't care what the differential impedance is. It functions by reducing the common-mode current. If the differential impedance is completely unknown, we are usually better off with a 1:1 current-choke-balun.
    Last edited: Oct 3, 2009
  12. KC9QQ

    KC9QQ Platinum Subscriber QRZ Page

    Thanks for all of the great information!

    Thanks to all the replied and provided recommendations and links. i am ready to place an order so that I can get the antenna back in the air before winter.

  13. K4AX

    K4AX XML Subscriber QRZ Page

    1:1 current balun is the way to go if your doing multiband with a 135 dipole.
  14. KA5S

    KA5S Subscriber QRZ Page

    Here's a picture of my two-balun assembly.


    This replaced a 1:1 balun that got hot with only 100 watts in my application. I do NOT blame the balun, by the way; I misused it. I hope the ferrite is still effective, because I'll have a use for it on a different antenna.

  15. K4AX

    K4AX XML Subscriber QRZ Page

    Hello Cortland, how did you come up with the 6:1 ratio, is that what simply works out best on more bands with that length of antenna and feedline?
  16. KA5S

    KA5S Subscriber QRZ Page

    Because I had one? No, seriously, I thought 300 ohms would offer a decent compromise for MARS between 2 and 14 MHz with my 200 ft dipole and uncertain length of window line (need to move the antenna again). I have not, in fact, MEASURED the feed point impedance, or I might have done differently.

    One thing I thought about is using a 100 ohm Balun to feed two LDG tuners, one for each conductor in window/ladder line. That would put all the impedance matching outboard of the Balun. It might be overkill, as I could just float one tuner after a 1:1 balun with about the same result.

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