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I thought maybe this was a 4:1 balun, but a number of 4:1 single core balun diagrams that I found online had different connections. N1HFX's balun page and the bottom of G4WPW's page show a connection of one wire directly from the balanced to the unbalanced side. The pictured balun does not have such a connection.

There is only one core and the connector at the bottom is an SO-239. I may get a chance to test it later on with a 50 ohm load and an impedance tester, but at the moment this balun is not in front of me.

Is this a current balun or a voltage balun?

What should the impedance ratio be?

unknown HF transmitting balun

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  • $\begingroup$ One way to check the impedance ratio: connect it to a VSWR meter of some sort (on the SO239 side) and then try a range of resistors on the "balanced" side and see which gives the lowest SWR. Start at ~7 MHz, try higher and lower too. Small 1/4 W carbon resistors are fine. $\endgroup$
    – tomnexus
    Nov 30, 2016 at 21:03
  • $\begingroup$ Hard to tell exactly what's going on at the top there. $\endgroup$
    – Phil Frost - W8II
    Dec 2, 2016 at 15:59

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Looks like it might be a clever way of winding a 4:1 voltage balun to simplify the wire routing (or just straight up incorrectly wired). I'd have to see the connections at the top more clearly to be sure.

Regardless, seems pretty safe to assume it's a voltage balun. The quick way to tell would be to check it for a DC short between the terminals on the unbalanced (or balanced) side. If they're shorted, it's a voltage balun, and as such, not much of a balun at all. The choking impedance from these single core 4:1 voltage baluns is so low (usually <500 ohms) as to be nearly useless unless your antenna has low enough impedance that yo don't need the 4:1 to begin with. W8JI and K9YC have both done good tear downs showing just how awful they are for most applications. They should be considered an impedance transformer and not much else.

EDIT: drawing out the windings, it appears it is an attempt to shoe-horn a 2 core Guanella 4:1 current balun on to a single core. Figure 2 here shows how it would be accomplished with two cores, and the builder appears to have just decided to do a split winding on a single core, perhaps in an attempt to get the best of both worlds, good common mode isolation and a 4:1 transformation on a single core. In reality, I would guess this actually causes it to exhibit less than a 4:1 transformation, in addition to having little common mode suppression. It appears it would exhibit considerable shunt capacitance between windings, and depending on the core material, potentially a low impedance shunt path through the core.

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  • $\begingroup$ From my experience, these types of baluns are almost always used as an impedance transformer rather than a choke balun. I decided to open up my 1:1 balun to check the windings for similarity. Unfortunately I can't be completely certain but if I had to guess, I would say it was a 1:1 balun. I have a 4:1 balun too but it is installed doing its job linking ladder line to coax on my Dipole antenna. $\endgroup$
    – K7PEH
    Dec 1, 2016 at 19:23
  • $\begingroup$ And, if you are looking for a good choke balun, I also recommend the K9YC designs (coax and ferrite based). I have made several of his choke baluns with average choke impedance at operating frequencies of at least 5000 ohms. $\endgroup$
    – K7PEH
    Dec 1, 2016 at 19:25
  • $\begingroup$ A voltage balun is a perfectly legitimate kind of balun. It's just not also a choke. $\endgroup$
    – Phil Frost - W8II
    Dec 2, 2016 at 15:57
  • $\begingroup$ Phil, a voltage balun is indeed a legitimate type of balun, when it is constructed and applied properly. This is rarely the case in ham radio applications. In many cases they will actually increase common mode current on balanced line by introducing a phase difference between the output terminals, and they are not capable of providing sufficient common mode isolation in most cases where an antenna is non-resonant. They are almost never appropriate for the most common ham radio applications, and can actually make RFI issues worse in some cases. $\endgroup$
    – Hamsterdave
    Dec 3, 2016 at 15:51

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