Many thanks to SWLing Post contributor, Alan, who writes:
Thomas,
You have had quite a few posts on loop antennas. I found a technical article about how to accurately test them.
I have just come across this article which you may find interesting. It comes from the BBC Research and Development:
https://www.bbc.co.uk/rd/publications/measurement-magnetic-loop-antenna-performance
It is written by a real Broadcast Engineer.
Alan
Thanks for the tip, Alan!
Interesting aside.
we tried an HF loop at a club meeting inside a substantial building.
the room was partition with a wired glass frame with the loop parallel to the glass
moving the loop away seemed to indicate an amount of reflected energy, optimised about 1 metre apart
never followed it up but always wondered
G0DOR
Did the BBC ever design a loop antenna based on this testing paradigm?
Thanks for posting this.
I was baffled by the math, but . . . did anyone notice this comment from the conclusions:
“Polarisation Illustrations of the antenna usually show it mounted upright; that is, with
its axis parallel to the ground. This is fine for the reception of LF and MF transmissions, which
are vertically polarised, but most HF transmissions are horizontally polarised. ”
As soon as practical, I am going to experiment with mounting my MFJ 1886 loop horizontally to see what kind of difference that makes in receiving HF signals.”
Cheers, Jock
> “did anyone notice this comment from the conclusions: …”
That’s covered somewhat a couple of sentences later: “In practice, the situation is unlikely to be as bad as that, since distant transmissions arrive courtesy of disorderly reflections from the ionosphere.”
In other words, for anything other than LoS / groundwave reception of HF, you can’t assume that the signal is horizontally polarised, because polarisation varies with the indeterminate effects of reflection from and refraction within the ionosphere.
Something else to keep in mind is that, if the antenna is mounted horizontally, the reception null is up-down – which will affect skywave reception, particularly at low angles of incidence (i.e the closer the incoming signal is to vertical, the less sensitive the antenna will be to it). Doubly so if the antenna is mounted horizontally close to ground, as that proximity will tend to widen the lobe of the null pointing away from the ground i.e. signals at higher angles of incidence will also see increased attenuation.
Something else that the paper points out that’s often overlooked in discussion of loop antennas: the supposed “magnetic” properties (sensitivity to H-field and insensitivity to E-field signals) of loop antennas is almost entirely dependent on the termination impedance. Open-circuit / high impedance, and the antenna is sensitive to the signal’s E-field (electric) and relatively insensitive to the H-field (magnetic). Short-circuit / low impedance, and the behaviour reverses – it’s sensitive to the H-field and relatively insensitive to the E-field.
(And, no, in this case “50 ohms” is not low impedance – that’s several 10’s or 100’s of times the inherent impedance of the antenna. In this case, milliohms, or maybe up to a few ohms, is “low impedance”.)
Most people experimenting with loops assume all loops are magnetic; the paper explains why that’s not the case. It also has important implications if you’re using/experimenting with tuned loops e.g. parallel vs series tuning.
Ron,
So flipping my loop to horizontal is likely not a great idea?
Cheers, Jock
Sorry, didn’t mean to give that impression at all. By all means try it, especially if you can get it a few loop diameters above the ground. Definitely a “suck it and see” situation.
I was more trying to temper expectations, Like any antenna in practice, performance vs orientation is going to be affected by a bunch of local and personal variables e.g. propagation mode of the signal as received at your location, specific antenna characteristics (IIRC the MFJ1886 isn’t exactly a low input impedance design anyway, so will be somewhat more responsive to the E-field than a ‘true’ ‘magnetic’ loop), preferred stations, etc.
In general, small loop antennas work best in the vertical orientation – but that’s not a guarantee!. And remember, the white paper is all about _small_ loop antennas; the characteristics start changing once loop size increases to appreciable fractions of wavelength and beyond, to the point that they start behaving in almost the opposite fashion to small loops.
Ron,
Thanks for the kind and informative reply
I’ll give it a try and see.
Cheers, Jock
Thanks Alan!
to read carefully, not easy, but very interesting.