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Oop, looks like I done hit de tar baby.
Gene Fuller wrote: I have read your comment about the requirement for two cores for many years, and I am still puzzled. In the ideal case the balun action is performed by transmission line transformers, and the flux induced in the core by the differential current is minimal. The core primarily affects frequency response and common mode choking action. It's true that the flux induced by the differential current is minimal in a transmission line transformer. If, in fact, the balun is wound with coax as I usually make them, the flux induced by the differential current is zero because the differential field is entirely inside the shield. And it's true that the core affects common mode choking action. The common mode choking action is exactly what the function of a balun is -- that's how it effects equal and opposite (purely differential mode) currents on the two transmission line conductors. In other words, that's how it produces balance and prevents feedline radiation. So the flux from the common mode component of the current is the very thing we need to look at when we evaluate baluns. According to his books, Jerry Sevick has designed, analyzed, built, and measured many single core 4:1 current baluns. He never mentions the problem you allude to. I don't know which edition you have. I have only the first and second; I believe there was a third. In the first edition, it was clear that Sevick didn't understand the purpose of baluns or how they accomplished it. So I discount anything he wrote about baluns in the first edition. But he figured it out some time between the first and second editions, so there's some credible information in the second. A number of the 4:1 baluns he shows are indeed wound on a single core. And he dutifully reports their frequency responses when terminated in a correct impedance load (seldom the case in a real antenna application). But nowhere do I see any measurement of their ability to do the primary job of a balun: to balance the feedline currents. So there's no way of knowing whether any or all of the many "current" baluns he constructed actually perform the function of a current balun. A way to qualitatively check to see if a balun is really acting like a current balun is to connect a load across the output. Short one terminal of the output to the "ground" terminal of the input, and measure the impedance looking in. Then disconnect that short and short the other output terminal to the "ground" terminal of the input and measure it again. You can do this with an antenna analyzer or impedance bridge. A perfect current balun will show the same, correct impedance in both tests. A perfect voltage balun will show very different impedances. There should be a way to get a quantitative measure of the longitudinal (common mode) impedance from this test, but I don't have time to dig it out just now. I'd like to add that not long ago I encountered a claim of a 4:1 current balun on a single core. It was indeed -- it was a conventional, as opposed to a transmission line, transformer, although it was wound in a way that made it resemble a transmission line transformer. And it had a fairly decent bandwidth, although a transmission line transformer is inherently much broader and doesn't couple differential flux into the core. So a 4:1 current balun can be constructed on a single core. But I don't know how a 4:1 transmission line current balun can. What is happening here? A whole lot of things could be happening. If an antenna is perfectly symmetrical, a voltage balun will do the same job as a current balun. For an explanation why, see http://eznec.com/Amateur/Articles/Baluns.pdf. So a lot of the "current baluns" might well be voltage baluns. I seldom see anyone actually test the current balancing properties of baluns, and without this, there's no telling what's been constructed. I haven't looked carefully at Sevick's single core 4:1 "Guanella" baluns. They might be voltage baluns, or they might be conventional transformers -- bifilar windings don't automatically create a transmission line transformer. I don't see any test he ran to verify that they're functioning as current transformers or for that matter as transmission line transformers. In a transmission line transformer, the differential current always has a transmission line path to follow. Some connections can split the wires carrying a differential current apart, in which case you no longer have a transmission line transformer. If you know of a topology which produces a transmission line 4:1 current balun, I'd be very interested. You could use the same simple analysis method used in the balun article to show that it really is what you think. Mike Coslo wrote: I'm not sure myself, Gene. I've wound single core 4:1's, I've bought at least 2 of them. I've also been told variously they don't work, and can't even be made. Just what IS happening here? You probably made 4:1 voltage baluns, which worked well enough in your applications. Where a voltage balun is really a bad idea is when your antenna system isn't symmetrical. In an effort to balance the voltages on the two output terminals, it creates unbalanced currents. The difference between them becomes a common mode feedline current. This is also explained in the balun article. .. . . Here Uncle Remus paused, and drew a two-pound yam out of the ashes. "Did the fox eat the rabbit?" asked the little boy to whom the story had been told. "Dat's all de fur de tale goes," replied the old man. "He mout, an den agin he moutent. Some say Judge B'ar come 'long en loosed 'im - some say he didn't. I hear Miss Sally callin'. You better run 'long." Roy Lewallen, W7EL |
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