I am in the process of designing a small receiving loop antenna for the lower HF bands. By small I mean a circumference of less than $0.2\lambda$. Thus, a uniform current distribution can rightly be assumed. If we forget about impedance matching and tuning at the moment, would we really need a balun for this antenna? It seems to me that there are no paths for a common mode current to flow. Because of the small electrical size and hence uniform current distribution of the loop, KCL applies, and the current flowing on the inner coax conductor has to equal the current flowing on the (inner side of the) shield. No current on the outside of the shield should exist, because where would it come from?

Am I right?

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    $\begingroup$ Welcome to Ham SE, Troels, and thanks for your question. How is the situation of your loop different from, e.g., a half-wave dipole? The dipole should not have current flowing on the outside of the coax shield, because the current on the center conductor and the current on the inside of the shield should be the same. But experience shows that they are not the same, owing to a variety of factors: proximity of other conductive structures, currents induced on the outside of the feedline by "stray" RF, etc. Doesn't your loop face the same set of issues, which would affect its directivity? $\endgroup$ – Brian K1LI May 8 at 13:42

You may be overlooking that the feedline is itself an antenna.

If signals came only from the loop, then sure, there would be no need for a balun. But the feedline is also effectively a big vertical antenna in the common-mode, and is going to pick up signals as such. Again by the KCL, the currents picked up by this vertical, at the end of the coax where the loop is attached, can travel right around the loop and enter the center conductor where we know they will affect the receiver.

The purpose of the balun is to insert a large impedance at this point so that can't happen. To retain the desired radiation pattern of the loop and to avoid having the radiation pattern of a vertical superimposed upon it, you must show that common-mode currents from the feedline can't couple into the differential-mode.

Alternatively, you can consider what happens to the loop in transmitting when the coax is represented as a superposition of its differential and common modes:


simulate this circuit – Schematic created using CircuitLab

$V_{dm}$ is the differential-mode voltage generated by the transmitter. $Z_{ant}$ is the antenna impedance. And $Z_{cm}$ is the common-mode impedance which is unknown: it depends on the length of the feedline and the environment.

From this it should be clear that unless $Z_{cm}$ is very large, than the current in $Z_{cm}$ due to the voltage source is significant, and therefore there exist common-mode currents on the feedline which will radiate. And by reciprocity, you can conclude the feedline also acts as an antenna on receive.

The addition of a balun makes $Z_{cm}$ very large, thus reducing the common-mode currents through impedance divider action to a negligible amount.

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  • $\begingroup$ I should have included the feedline-as-antenna in my answer. Sometimes I do that on purpose because the loop is too directional and I want less of a null. But then you need an un-un in the right place on the feedline so the radiating part is resonant or your SWR is bad. That's how the carolina windom radiates on 10m. $\endgroup$ – user10489 May 9 at 5:06

There are two purposes for a balun: impedance matching transformer, and blocking common mode current.

A small loop shouldn't need impedance matching, as the built in tuning capacitor should handle that.

Would a balun be needed on any antenna, including a small loop? Adding balun typically doesn't hurt if it isn't needed, especially if it's just a few loops of coax ("air balun"). Common mode current isn't guaranteed even when a balun is recommended, so you can frequently leave it out.

Where would current on the shield of the coax come from? Usually it is induced by coupling with the antenna due to the geometry between the coax and the antenna and has little to do with any electrical path within the antenna. Sometimes you can eliminate the need for a balun just by making sure this geometry is symmetrical. Or, the balun can help make this geometry less critical.

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