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There seem to be lots of descriptions and/or articles on using multiple turns of conducting element in small magnetic loop antennas (STLs). But I haven't found any information on full size full wave loops with more than one turn.

What happens if one uses multiple turns in a larger full wave in circumference loop antenna? (e.g. using integer multiples of one wavelength of wire around a loop support system or lot only big enough for one wavelength in circumference)?

Are there no descriptions of large multi-turn loops because they don't match or radiate? Or because it's just a waste of wire?

hotpaw2
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  • I thought that your question was a duplicate of [this question](https://ham.stackexchange.com/questions/9013/loop-antenna-configuration/9015#9015) at first, so briefly I closed it (sorry!). However, that was more about the smaller coupling loop in a magnetic-loop antenna. Having said that, [my answer there](https://ham.stackexchange.com/a/9015/8717) directly answers your question. – Mike Waters Sep 19 '20 at 18:47
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    I don't think the answer for STLs applies at all. In a full size full wave loop, all adjacent loop turns (if any) will be in-phase (when run at a resonant frequency). – hotpaw2 Sep 19 '20 at 18:56
  • [That's](https://ham.stackexchange.com/questions/17321/multi-turn-large-full-wave-loop-antennas?noredirect=1#comment28704_17321) an interesting point! Maybe someone can model it. – Mike Waters Sep 19 '20 at 18:58
  • How far apart would these turns be, and how high above the ground? – Mike Waters Sep 19 '20 at 19:00
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    Methinks it may look like a shorted transmission line, – glen_geek Sep 19 '20 at 21:40

2 Answers2

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Small loops and full-wave loops are very different antennas.

There isn't a well-defined distinction between "small loops" and "large loops", but a typical rule of thumb is a loop is "small" when its diameter is less than 1/10th of a wavelength. At such a size, the phase delay around the loop is negligible and thus the current can be considered equal at any instant for the entire loop. This significantly simplifies the analysis of the antenna.

A full-wave loop is electrically not at all like a small loop. Rather, it is similar to a folded dipole. RMS current will be at a maximum at the feedpoint and directly opposite it, and at 90 degrees to those points RMS current will be at a minimum, and voltage at a maximum. Just as in a dipole.

In a small loop with multiple turns, the turns are tightly coupled through their shared magnetic field. The tight coupling follows from the small (compared to wavelength) size of the antenna. Putting more turns on a small loop is like putting more turns on a transformer. (Though it's usually better to increase the diameter if possible, if the objective is to encircle the maximum magnetic flux with the least amount of material and lowest resistive loss.)

An antenna which was one wavelength is diameter with more than 2 turns would be a very odd antenna indeed. I'm going to guess it's a waste of wire, by this reasoning:

The two turns form a balanced transmission line. If you cut this transmission line and unroll it, you get:

schematic

simulate this circuit – Schematic created using CircuitLab

The feedpoint is N1 and N4: these would be on the same side of the antenna had we not "unrolled" it.

What then is going on at N3 and N2? Well, the entire transmission line is 1 wavelength long, so the differential voltage at N1 and N2 must equal the same at N3 and N4.

Furthermore, N3 and N2 are actually the same node, because when the loop is put back together this is where the two turns connect.

Thus, N1 = N2, and the voltage source is effectively driving both sides of the transmission line in parallel. But the current is split across the two turns, so the feedpoint impedance is quadrupled. (Thanks to Brian K1LI for pointing this out.) While this might be useful in some circumstances, a 4:1 balun might be an easier way to achieve the same effect.

If the electrical length of the transmission line isn't exactly 1 wavelength, then there will be some reactive power circulating in this transmission line. Maybe someone will do some NEC modelling to know for sure, but my intuition says this doesn't do anything especially useful.

Phil Frost - W8II
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Well put, Phil. According to a simulation comparing a 1-turn full-wave loop to 2-turns of the same loop, the radiation pattern is the same while the feedpoint impedance increases by a factor of four:

enter image description here

These values are for an antenna less than a half-wave above the ground. Raising it to a half-wave above ground does move the maximum radiation to lower angles, but the impedance increases by the same ratio.

Brian K1LI
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    Ah yes, that would make sense: because the voltage source is "mirrored" at the point where the turns connect, the voltage is doubled and so impedance must be quadrupled. Similar to a folded dipole. – Phil Frost - W8II Sep 21 '20 at 16:36
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    It might be interesting to see if there's any impact to SWR bandwidth, especially if the turns are spaced some distance apart. – Phil Frost - W8II Sep 21 '20 at 16:46
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    Not having a lot of time to spend on this, I ran SWR plots on the 1-turn and 2-turn loops shown above. The 2:1 SWR bandwidth, at the respective resonant feedpoint resistances, increased from 1-MHz (a lot!) to 4-MHz (a whole lot!) by virtue of a second dip introduced where the side length increased from 0.27$\lambda$ to 0.32$\lambda$. Gain is unaffected This behavior depends substantially on feedpoint placement. – Brian K1LI Sep 21 '20 at 18:06
  • Interesting. A single-loop full-wave loop usually requires a transformer or coax balun. So adding turns increases the required balun ratio, but in turn provides greater SWR bandwidth? Depending on wire cost, might be a useful trade-off in some situations. 4 MHz is almost enough for a 17M+20M dual bander. – hotpaw2 Sep 22 '20 at 04:24