Is this a Current Balun or a Voltage Balun?

Discussion in 'Antennas, Feedlines, Towers & Rotors' started by 2E0CIT, Feb 28, 2019.

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  1. K2XT

    K2XT Premium Subscriber QRZ Page

    This is not a thorough answer, but assuming the device is sealed up so you can't see inside you might be able to make an educated guess. If the housing is a long tube (say 6 inches ling, 1 or 1.5 inch dia) then it is likely to be a piece of coax with ferrite beads on it, a current balun.

    No sooner did I write the above than I realize Murphy is involved in my posting and it could also be a ferrite rod like a W2AU which is a voltage type. Oh well.
  2. 2E0CIT

    2E0CIT Ham Member QRZ Page

    Ha Ha! probs, I did have the thing open many months ago when troubleshooting an antenna problem and noted two binocular cores...but already having assumed it was a voltage balun for some reason, I just weathersealed it and forgot to make a diagram of the wiring...duuuh!

    It would help if Palstar and others labelled their products more descriptively or responded to emails.

    All's well that ends well, -my newly-learned testing procedure (THANKS TO ALL!) shows it's a current balun with a very reasonable insertion loss across the HF range. All I've got to do now is rebuild the rest of the doublet and evict the foxes.....
    73 Jeff
    K2XT likes this.
  3. AA4BQ

    AA4BQ Ham Member QRZ Page

    Very interesting discussion guys. Thanks.

    So, bottom line then: Is there a way to modify the Palstar BT-1500A to cure its deficient imbalanced output? I, too, compared this tuner to JMB tuners and came to the the same conclusion. JMBs are better it seems. Imagine that. Thanks for your insight on this.

  4. KD2PII

    KD2PII Ham Member QRZ Page

    I asked about Voltage vs Current baluns on stack exchange and it seems the consensus is to always go with a current balun. Current also only act on the common mode current. I believe Voltage Baluns works against differential mode current and are thus more likely to saturate or heat up. But I could be mistaken.
  5. 2E0CIT

    2E0CIT Ham Member QRZ Page

    Not a simple choice of voltage balun = bad, current balun = good!
    My simple take on this as a perpetual beginner to this amazing branch of science is that the things we call 'voltage baluns' are in fact transformers, i.e. 4:1, 6:1, 9:1, 49:1, etc, necessary for feeding certain antennas but they do little to limit common mode current on the coax shield whereas current baluns /chokes impede the common mode current......and where one uses a voltage balun/ transformer in circumstances of imbalance (antenna in its environment) one should also use a current balun / choke .....and depending on the antenna type it's important to know exactly where to place them!
    But one thing that's very clear to me is that with all the advice freely available on QRZ it's a lot cheaper and much more fun to make balun$/transformer$/choke$ ..than to buy them! :)
    73 , Jeff
    PU2OZT likes this.
  6. KD2PII

    KD2PII Ham Member QRZ Page

    It seems that when it comes to most antenna applications, the suppression of common mode current is important. The answer to that stack exchange question made a good point that voltage baluns have a place, but mainly within circuit designs and less likely at the antenna feed point. For some impedance ratios, the only type of balun seems to be voltage type. There's the 4:1 guanella, but that's about it.

    I am still confused on why we would ever want to put a 1:1 Voltage balun at the feedpoint of a center fed half wave resonant dipole. If the currents are already balanced, then there's shouldn't be any left over current to flow down the outside of the braid. And the voltages should already be balanced as well. In the real world there is always some imbalance. A Voltage balun would force the voltages to be equal and opposite by allowing the current in each leg to change. But that's the opposite of what we want isn't it? A 1:1 current balun would seem to be the better choice.

    What kind of antenna application would call for a 1:1 voltage balun? Some kind of loop or something? Or maybe on the low impedance side of a double L network?
  7. WA7ARK

    WA7ARK Ham Member QRZ Page

    What you want on a coax-fed, center-fed dipole is a high impedance in-series with the coax shield close to the dipole's feedpoint. The goal is to make the impedance down the coax shield much higher than the impedance looking along the dipole half leg wired in parallel with the coax shield.

    This is nicely illustrated by @GM3SEK 's diagram:
    You want to impede I3 so all of I2 becomes I5.

    Ideally, that impedance is both resistive and inductive (has a low Q) over the range of frequencies where the dipole is used. A pure inductance (ugly balun) is not ideal because the inductance you randomly choose might just resonate with existing parasitic capacitance, making the CM worse. A pure inductance may work on a single band, but if the antenna is used harmonically on multiple bands, chances are a random coil of coax will resonate on one of the harmonic bands...

    Here are five ways of hooking coax to a dipole. Only lazy old-farts that don't know any better like the BAD method. UGLY is not much better and if used at all, do it only on a single band. The VOLTAGE MODE version blocks CM (I3) due to Galvanic isolation between the windings, but there are high losses in the core because all the RF power is being coupled through the ferrite. The last two are both CURRENT MODE. There is low loss in the ferrite (and almost no heating) because if the Zcm is high enough, I3 (or the difference between I1 and I2 in the W2DU method) is tiny. The magnetizing flux in the ferrite core is proportional to I3 in the COAX case or (I1-I2) in the W2DU case...

    I like the COAX method; simple and it works...
    2E0CIT likes this.
  8. KD2PII

    KD2PII Ham Member QRZ Page

    @WA7ARK based on your last post- it seems that a current mode balun is superior. It's more efficient, less lossy, and doesn't heat up as fast as the Voltage mode. Am I missing something here? Why would anyone choose the voltage solution over current?
  9. WA7ARK

    WA7ARK Ham Member QRZ Page

    In the past, I have used BAD and UGLY. Knowing what I have learned in the past twenty years, I use COAX or the 1:4 Guanella version (with two independent cores).

    The whole point of my posting is that CURRENT MODE is the only way to go...
    PU2OZT likes this.
  10. WR2E

    WR2E Ham Member QRZ Page

    Because they don't know any better.

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