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I'm a new radio amateur setting up a half-wave 10-meter inverted vee antenna fed by 50 ohm coax. I understand that the inverted vee lowers the normal 72 ohm dipole closer to 50 ohms, so I only need to choke the common mode signal from feeding back on the outside of the coax. I obtained an old Hy-Gain BN-86 1:1 3-30 MHz balun, which Hy-Gain literature calls a current balun (notated in their literature below).

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However, opening the enclosure, the wiring turns out identical to references of a Ruthroff 1:1 voltage balun (found on a several websites).

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The diagram above from Marki Microwave shows that with one additional winding, a current balun with separate series windings (which seems proper for my use) becomes a voltage balun (an autotransformer).

Q: Will the third winding in my balun somehow help my inverted vee dipole or would an improvement be to remove the winding (rewind the balun) to convert this to a standard 2-winding current balun as a choke? I also wonder if the 30 MHz high limit is too close to 10 meters for this balun to be useful.

enter image description here

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  • $\begingroup$ Hello Peter, and welcome to this site! In almost all cases, voltage baluns are meant for end-fed antennas and not for dipoles like yours. $\endgroup$
    – Mike Waters
    Jul 23, 2019 at 22:57

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You could rewind it, but the result may not necessarily be better. This is because the core must be optimized for different needs in each balun design.

In what you're calling a "current balun", the idea is to maximize the impedance in the common mode. Ideally, that impedance is mostly resistive, meaning high loss. Saturation current isn't so important, because as long as the balun is working and common-mode current is low, the core won't saturate.

In a "voltage balun", the objective for the core is to provide an efficient and effective coupling between windings. Unlike the current balun, the windings are such that the differential mode contributes a net magnetic flux to the core. Thus, the core must have minimal loss, since any loss would degrade antenna efficiency and contribute to heating the core. Moreover, the core must not saturate at the current expected at maximum operating power.

A single core design can not likely satisfy both use cases.

You'll note that many current balun designs use ferrite materials and a closed toroid core. This closed magnetic circuit and the material properties of ferrite maximize impedance, but also offer a low saturation current.

The balun you picture uses a rod core, which affords much less impedance but a much higher saturation current. It may also use a powdered iron rather than ferrite material. While these design choices allow the core to handle more power as a voltage balun, they would also make it ineffective as a current balun.

I suggest you use the balun as-is. Voltage baluns aren't necessarily bad, and provided the dipole, feedline, and surroundings are reasonably symmetrical, a voltage balun could work just fine. You can give it a try and see how well it works:

If it doesn't work so well then you can try adding or replacing with a current balun, or perhaps you can make some adjustments to your installation to make it more symmetrical, giving the voltage balun a better chance to work.

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  • $\begingroup$ Your answer clearly explains the objectives between current and voltage baluns and how different core types and designs control them. I'll follow your suggestion to first try the balun as-is. My concern started when all electrical terminals measured zero ohms to each other. Curiosity made me open it to see what was really inside. It seems Hy-Gain might have used the same enclosure for many baluns, so this may not even be a model BN-86. $\endgroup$
    – Peter Buxton
    Jul 24, 2019 at 22:06
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    $\begingroup$ My untuned half-wave 10 meter inverted vee antenna (56 feet of feed coax) with no balun had SWR 1.5 at 26.8 MHz (needs to be trimmed shorter). Then I tried the stock balun and the SWR was infinity (needle pegged) at any frequency between 25 and 29 MHz. I clipped the ends of the winding colored blue in my photo above, leaving only the series "current" windings on each leg. Then the SWR was 1.5 at 27.3 MHz (slightly closer to 10 meters). Does that indicate that the stock balun is wrong for this dipole? $\endgroup$
    – Peter Buxton
    Jul 28, 2019 at 21:52
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    $\begingroup$ @PeterBuxton Could mean all kinds of things. It could mean the balun is defective. Or it could mean the dipole is only a good match if you let the feedline radiate, and the balun is working perfectly. $\endgroup$
    – Phil Frost - W8II
    Jul 28, 2019 at 22:50
  • $\begingroup$ @PeterBuxton You said "My concern started when all electrical terminals measured zero ohms to each other." You should measure zero ohms in this case. You are measuring the DC resistance, but you need to measure the AC (RF) impedance with the balun connected to a proper load, which could be a non-inductive resistor or dummy load. An antenna analyzer is needed here, not your ohmmeter. :-) $\endgroup$
    – Mike Waters
    May 14, 2020 at 21:46
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    $\begingroup$ @MikeWaters Measuring DC resistance was only a black box test of a supposed current balun to verify that the two windings were not connected together. When I found that they were, I opened it to find that it was really a voltage balun. Then I posted to ask how a voltage balun is used vs. a current balun. I appreciated your first comment that in almost all cases voltage baluns are for end-fed antennas. I since then have made my own current baluns for my dipoles. $\endgroup$
    – Peter Buxton
    May 16, 2020 at 14:45
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Yes, it does appear that re-winding that balun would be a good idea, as the primary purpose of a 1:1 choke balun is to cut off common-mode currents on the transmission line.

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  • $\begingroup$ In leu of rewinding the balun, would it work to just clip the ends of the winding that is not needed (colored blue in the added photo), or should the winding actually be removed? And what do you think about 30 MHz working with this ferrite rod? $\endgroup$
    – Peter Buxton
    Jul 24, 2019 at 15:58
  • $\begingroup$ I'm hoping this post will help others too, not just me. $\endgroup$
    – Peter Buxton
    Jul 24, 2019 at 17:40
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    $\begingroup$ I am loathe to make a recommendation because I don't know the rod material. G3TXQ (SK) wrote a helpful article on this subject. $\endgroup$
    – Brian K1LI
    Jul 24, 2019 at 20:04
  • $\begingroup$ I cannot seem to find any known data on the rod material. The article you listed by G3TXQ does indeed look helpful on this subject $\endgroup$
    – Peter Buxton
    Jul 24, 2019 at 21:46
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Peter Buxton, you are right. The ferrite rod used here is most probably mix #43, the one commonly found in the BC Radio receivers. The one with the 3 windings is an 1:1 voltage BalUn, the center (blue) winding being the magnetising winding. If you removed this Blue winding you get 1:1 current balun or CMC choke that is what you require to suppress the CMC that tends to flow on the outer surface of the coax braid. Phil Frost has mistaken the core material as iron carbonyl mix #2 but whatever he said about the #2 is correct; and it is used mainly as high Q inductor as in harmonics filter below 10 MHz. But the CMC has nothing to do with the SWR you happen to observe in your antenna. SWR arises due to mismatch of impedance at the feed point of the dipole and the coax cable.

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    $\begingroup$ Hello and welcome to ham.stackexchange.com! $\endgroup$
    – rclocher3
    May 10, 2020 at 19:01
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    $\begingroup$ Thanks for your input. Phil Frost did mention the core as being a rod core but only surmised that the core material could be powdered iron. He did not state that it is iron carbonyl mix #2. $\endgroup$
    – Peter Buxton
    May 12, 2020 at 14:24
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There is an electrical discontinuity at the center point of the dipole; where the dipole is "balanced", and the feed coax is "unbalanced". This discontinuity causes some disruption in the flow of energy, a reflection of some amount of it.

Specifically in the center-fed resonant dipole, this engineer recommends a 1:1 voltage balun (aka transformer). It will overcome the discontinuity while also greatly reducing any "feedback" RF energy from traveling back down the coax.

KK4IB

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  • $\begingroup$ Hello Sparks, and welcome to ham.stackexchange.com! $\endgroup$
    – rclocher3
    Jan 14, 2021 at 23:59

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