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The following question is regarding the ability of a transformer, UnUn or BalUn, to be efficient, if the transformer (UnUn in my case), is feeding a load with reactance, and what can be done about the reactive load, on the transformer's input (primary):

  1. Is the reactance on the output of a transformer, that is connected to a reactive load, seen on the input of the transformer?
  2. If the transformer does transfer the reactive component, can that reactance be tuned out on the input side?
  3. I'm trying to ascertain if reactance of an antenna, can be tuned-out on the input of an UnUn, to conserve the UnUns efficiency, or make it as efficient, as it would be if the UnUn were feeding a purely resistive load?
  4. If reactance carries through to the primary, in the case of an UnUn, does reactance also carry through in the case of transformers that have no ports in common, i.e., transformers where the primary and secondary terminals are not physically connected? Would that type of transformer present a purely resistive, higher impedance, at the primary?

Schematic of question example:

Transmitter|Coax|Antenna_Tuner|UnUn|Antenna_with_reactance

I know I've asked the question several ways; so as not to be misconstrued.

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Is the reactance on the output of a transformer, that is connected to a reactive load, seen on the input of the transformer?

Yes (transformed by the transformer's impedance ratio, and modified by any non-ideality of the transformer).

If the transformer does transfer the reactive component, can that reactance be tuned out on the input side?

Yes. We do this all the time in amateur radio. You might have a folded dipole connected to a 4:1 balun. On almost every frequency, that antenna will have some reactance, but that reactance can be matched by a tuner on the input side.

I'm trying to ascertain if reactance of an antenna, can be tuned-out on the input of an UnUn, to conserve the UnUns efficiency, or make it as efficient, as it would be if the UnUn were feeding a purely resistive load?

Not quite. If the load is reactive, and the matching is on the "other side" of the transformer, then that necessarily means that there is reactive power flowing through the transformer, and that reactive power is eligible to be turned into heat by any losses in the transformer, while not doing any useful work.

If reactance carries through to the primary, in the case of an UnUn, does reactance also carry through in the case of transformers that have no ports in common, i.e., transformers where the primary and secondary terminals are not physically connected?

Yes, it doesn't particularly matter whether it's an isolation transformer (a "transformer with no ports in common"), an autotransformer (unun), or a transmission-line transformer (many current baluns). All of them will ideally transform an impedance in the same way, although their non-idealities are probably different.

Would that type of transformer present a purely resistive, higher impedance, at the primary?

No. A complex impedance is transformed into another complex impedance.

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    $\begingroup$ Very good @hobbs - KC2G. Your answer confirmed a few things I suspected. My current HF antenna is adjustable in length, so I can tune the antenna to resonance, just about anywhere, but I was considering eliminating that feature for mechanical simplicity; but it may be useful to be able to change the antenna length, along with using a tuner at the UnUn feed-point, to take care of any impedance mismatch. $\endgroup$ Commented May 22 at 2:57
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Don't lose sight of the purpose of a balun or unun.

While baluns generally can be combined with impedance transformers, that's really a separate function that just happens to be combined into the same device.

Unun's typically do not include impedance transformers and are 1:1.

The main purpose of a balun or unun is to force the line to be balanced to reduce common mode current. (If this wasn't the main purpose, a regular transformer would be used without the balacing part.)

By forcing the line to stay balanced, common mode currents are reduced. Common mode currents can cause the transmission line to become part of the antenna, and unless it happens to be a tuned length, this will also cause an additional impedance mismatch and accompanying higher SWR.

The resulting higher SWR translates to more power in the line as it reflects back and forth, and magnifying any losses (and accompanying heating) in the coax and the transformer.

So an ideal 1:1 unun on a well balanced line does not actually add any impedance to the transmission line at all. If your line has imbalances that the unun is actually trying to fix, then (net) reactance may be added to half of the transmission line, with the goal of attempting to force the line closer to balance.

To put it another way, if the transmission line is balanced and working correctly, magnetic fields within the line cancel out and there is no energy left over to reach the balun. But if the line is imbalanced, there will be leakage from the line which the balun will impede, resulting in a more balanced line on the other side of the balun. (However, this impedance results in energy absorbed by the balun, and thus heating. But hopefully this will result in less energy being absorbed or leaked by the remainder of the transmission line.)

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  • $\begingroup$ Good info for those who are reading this thread. I am very particular about impedance matching at load, for the very reason you explain. I am not a fan of a radiating transmission line. In my case, I have a 49:1 UnUn for my adjustable EFHW Inverted L, and I was considering eliminating the UnUn for a purely LC lumped impedance match, but it means fabricating it myself since commercial tuners don't have the range needed in the lower HF bands, but they do if using an UnUn, but I wasn't sure if this would be as efficient, it looks like it isn't, but for now, it's the easier path. $\endgroup$ Commented May 22 at 11:23
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    $\begingroup$ This is also why you should try to geometrically balance your feed line against the antenna -- so the balun doesn't have to do quite so much work to fix the balance. It also helps to put the unun closer to the antenna (or right directly on it), although there are other nodal locations where it is most effective and minimally lossy too. And ther's nothing wrong with a radiating transmission line as long as you like omni and it's doing it in a tuned controlled intentional way (and not along the whole length into the shack). $\endgroup$
    – user10489
    Commented May 23 at 5:10

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