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I was investigating about magnetic loop antennas. In a paper about them I found some bold claims:

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and

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This almost sounds too good to be true, considering a lot of us hams are surronded by city noise and this paper claims the noise would be much lower with this design.

What is the real performance of this kind of antennas, then?

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Under some circumstances, this can be true. What Leigh seems to be talking about is nulling out a local noise source, which a rotatable loop excels at.

Between the loop and a vertical or dipole, in my experience the loop will often do what he says. But a loop compared to a beam? Not so much.

I have always respected the author, Leigh Turner VK5KLT for his technical knowledge. However, unless a rotatable beam (such as a tribander) is quite low (say, under 40 or 50 feet), I take exception to his statement that a loop will nearly always hear better than an HF beam.

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I've been using magnetic loops for 20 years

A magnetic loop can work well if it has two qualities: enough power handling (a 5 kV vacuum variable will handle 100 Watts, 15kV will do a kW); and a decent remote tuning setup. tuning is critical, as Eham reviews show the main dfficulty is getting the SWR properly dipped. Thanks to the popularity of robotics, low cost stepper motors are available, and do the job.

While size isn't critical, efficiency goes rapidly downhill at diameters less than than about a 20th of a wavelength. 1/2" or 3/4" copper tubing is self supporting, low loss, and is easily flattened and drilled for low loss connections. Paint it a dark color for stealth, and to avoid corrosion.

Dealing with high voltages and extreme narrow bandwidth are the price paid for an antenna that makes a magnetic near field, which seems to penetrate nearby conductors, such as trees, the ground, house wiring, powerlines, etc. almost as if they weren't there, because induced currents are in phase with the antenna's field. Near field losses are reduced by an S unit in a typical urban location, when compared with a wire antenna or a vertical whose electrical near field induces out of phase re-radiation.

The statements about quiet receive are approximately correct. In my experience, fixed local terrestrial noise sources (a pole pig with nesting squirrels, an LED streetlight)can be reduced typically by two S units (10 dB) by a vertically hung loop that is tied with a side line to keep the null lined up. Unlike stations using an antenna tuner at the radio, received and transmitted RFI does not come from the loop's feedline because its matched all the way to the antenna, where tuning and matching is done.

Further details are at www.x44.cc, or by googling my ham callsign, K1QAR

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    $\begingroup$ Could you, please, describe how matching the feedline at the antenna (instead of only at the radio) reduces radiation and pickup? This is a very interesting idea. $\endgroup$ – Brian K1LI Jun 12 at 11:19
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    $\begingroup$ Ok, there are at least two methods of feeder matching a magloop. One uses a smaller loop, between 1/5 and 1/8 the size of the main (radiating) loop, connected across the coax, located opposite the tuning condenser. Varying the size of this loop, or tilting it varys the coupling (in the old days these were called variometers) and hence varying the impedance the coax sees. The second method is series capacitive feed, with a capacitor between each side of the coax and each side of the tuning capacitor. More on that later. $\endgroup$ – Ted Jun 12 at 13:02
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    $\begingroup$ The loop capacitor feed, which originally appeared on a Russian antenna website, has the advantage of keeping both feed and control lines on the same side of the loop. There doesn't seem to be any deterioration in antenna Q if doorknob capacitors are used. Equal capacitance is used on each side of the feed, and these capacitors need to have at least half the voltage rating of the tuning capacitor, as they are connecting to a high voltage part of the antenna. I use a series string of several 100pF 3 to 6kV doorknobs on each side to make up capacitances ranging between 20 and 60 pF $\endgroup$ – Ted Jun 12 at 13:18
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    $\begingroup$ Thanks for describing the feed system options. I may be thick, because I still don't see how that relates to radiation from or pickup of noise by the feedline. $\endgroup$ – Brian K1LI Jun 12 at 16:27
  • $\begingroup$ I hope you don't mind, but I took the liberty of adding your callsign to your profile. And I see on QRZ that you have two nice loops! Which one did you mean? You could put one or both photos in your answer if you'd like to. :-) $\endgroup$ – Mike Waters Jun 12 at 18:15
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I use a magnetic loop as my main antenna since I am a renter. It is outdoors about 10 feet from the building and about 10 feet above ground level. It is approximately 1 meter in diameter.

Performance:

usable frequency range: 40 meters through 15 meters.

15 meters similar to dipole you can rotate.

20 meters similar to dipole you can rotate.

30 meters about 3 to 6 db worse than dipole not very directional.

40 meters about 6 to 9 db worse than dipole not very directional.

It is not usable at all on 80 meters , not even for receive of local strong signals.

Overall mag loops have one significant problem: Extremely low usable bandwith (very high Q factor). This is also the reason they work so well so this problem cannot be fixed, you have to work around it. If you want to change frequency more than 1 kHz (not MHz) you will need to re-tune. For this to be practical you will need a remote tuning device of some kind.

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In practice there are a lot of other factors to consider. For instance you are not using a magnetic loop in the free space, it probably will be placed on the balcony. Thus you should consider the proximity of metal objects and the fact that half of the horizon will be closed by the building itself.

Also beside of receiving a signal you should consider the transmission. Magnetic loops usually have power rating about 25W SSB 10W CW, thus the possibility to be heard by someone decreases.

Finally magnetic loops are directional and very narrowband. This means that you should be near the antenna and tune / rotate it all the time. Which is not only inconvenient but also reduces the time you TX or RX.

I have an experience of using magnetic loop in the city. A 23m long wire antenna performs much better in the same conditions despite the fact that it's much more noisy than a dipole or a magnetic loop.

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  • $\begingroup$ Could you please describe your magnetic loop? Sounds like it was quite lossy. Also, many magnetic loops are remotely tuned. Even remote rotation can be done, exactly like a beam antenna on top of a tower. $\endgroup$ – Mike Waters Jun 12 at 18:26
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    $\begingroup$ Chameleon F-Loop 2.0 $\endgroup$ – Aleksander Alekseev - R2AUK Jun 13 at 8:53

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