There’s a joke in the world of radio that all you need for a HF antenna is a piece of wet string, but the truth is that rudimentary antennas rarely perform well. Random pieces of wire may pull in some signal, but along with it comes a ton of unwanted interference and noise. It’s thus worth putting in the effort to make a better antenna, and if you’re not fortunate enough to have a lot of space, your best choice may be a magnetic loop. [Robert Hart] takes us through the design of a receive-only coaxial loop. It’s referred to as a Moebius loop because the conductor takes a “twist” path between the inner and outer halfway around.
The idea of a loop antenna is simple enough. It’s an inductor intended to respond to the magnetic portion of the wave rather than the electric part. They’re normally made of a single turn of wire in a loop of diameter well below half a wavelength, and, in their transmitting versions, they are often tuned to resonance by an air-spaced variable capacitor. Coaxial loops like this one provide enhanced resistance to electrical noise. He’s using some rather expensive Andrews coax for its rigidity, but the less well-heeled can use cheaper stuff without penalty. The result, when put on a frame of PVC pipe and a speaker stand, is an excellent portable receiving antenna, and if we’re being honest, something we might also consider in our own shack.
This isn’t by any means the first magnetic loop we’ve brought you. Have a look at this cleverly concealed one. We recently looked at the YouLoop, a similar Moebius loop antenna.
Rediscovery of PA0FRI antenna? Now add an active element to it. https://www.pa0fri.com/Ant/Active%20antenna/Active%20receiving%20%20loop%20antenna%20eng.htm
Looks more like a youloop antenna: https://airspy.com/youloop/
I’m more worried here about mentioning concealing them. The hostility towards (receiving) ham operators here is spreading as well. People are aggressive and even threatened me. Blame migraines on it and said they would sue me. It is just receiving. I only do a few SDR and sat projects.
U.S.A, right?
add some plastic and make it a peace sign
The U.S. Government has not done its fair job in protecting amateur radio operators from zoning an HOAs. Ham radio operators are our last line of communication.
Also, HAM radio operators help push the tech forward.
https://en.wikipedia.org/wiki/Radio_Amateur_Civil_Emergency_Service
http://arrl.org/ares
I would not be able to play nicely with my neighbors there lol. I would be telling them to eff off and huff wind..
Can’t you put the antenna between two layers of sheetrock so they can’t see the antenna?
Main reason to hide them is to get around HOA rules for exposed antennas. The FCC doesn’t protect our hobby enough
If looking at transmitting, beware: the tuning capacitor forms a resonant circuit, and therefore has SIGNIFICANT RF voltage.
You are correct, but someone transmitting at say, 4 W, really dont need to worry about it.
The eyes might in danger, though, due to being heated by RF easily.
That’s because of blood circulation being lower in the eyes than elsewhere.
So even when doing low power, like 4W from a CB radio, it’s better too keep a magnetic loop at a distance.
Say being 2 or 3 meters away while transmitting. Anyway, just saying. Magnetic loops aren’t being fully understood yet. Long time studies are missing.
Umm, what? Magnetic loops were making transmissions a century ago. And I promise you, my tissues are heated up more by standing in the beam of directional microwave internet equipment that’s running a test cycle than by having a piddly little CB radio next to my face, and yet I don’t suffer anything from either one. Nor does anything particularly harmful happen near a leaky high power transformer or moving near a MRI or a large electric field gradient, though to be fair I haven’t got a pacemaker or anything and a static electric field can at least end up raising hairs and such obviously.
The CB hardly even gets absorbed by flesh compared to the microwaves – and those could actually have a wavelength to noticeably heat small areas. But I would expect to feel that, and when the heat isn’t localized to the retinal cells like a laser, it should take a significant amount to disturb the eye temperature more than normal conditions do. I mean, you can feel your eyes can get cold if you’re in freezing windy conditions without goggles, and they do get warm in very hot conditions especially if you don’t wear sunglasses. But they actually do regulate their temperature, and even though blood flow is limited it’s still significant as a part of the equation. You can figure that you’d have to have an unusual amount and duration of heating to be equivalent to being outside in harsh sun without glasses for the amounts of your life that it generally takes for early cataracts to become a concern.
Unless you are sticking your head inside the loop, I wouldn’t worry about it
This is a receive-only design that does not have any place for a tuning capacitor to go.
Challenge accepted!
B^)
Ugh lol it won’t be the same design if you do; the loops like this one are not meant to be resonant or tuned, they are just meant to produce a weak but clean receive signal that a sensitive wideband amplifier or receiver can make arbitrary use of without needing to do anything to the loop. Being tuned would be worse, because then you can’t receive everything at once anymore.
So if your receiver is something like a ham radio or an SDR, you can skip the 1:1 balun and just go right into the unbalanced 50 ohm input? It looks like with the two halves of the loop being out of phase, that is where the noise canceling is taking place and the balun is just to give you an unbalanced output. Or am I seeing this wrong?
Well, you’re wrong in that “just to give you a balanced output” is a lot more important than you think. Without the balun the feedline can pick up noise that gets fed back to the loop and then coupled to the receiver. Given that the whole purpose of the loop is to discriminate against noise from directions other than the desired signal, that’s a pretty big deal.
Back in the 80’s we had to purchase additional box to our old TVs to receive channels above 12. That box used the same antenna loop plugged in the back of it.
https://www.kn34pc.com/sch/sch_device/ptp_216.html
The loop antenna is not shown there but the connector on back is visible.
Lovely summary Jenny List thank you :) I’ve build many other loops in the past, I whipped this one up from stuff I had laying around for a workshop I was running on SDR receivers. Any 50R coax would work. I’ve built other split coax loop antenna in the past, but I shared as I was just surprised how well it worked across the whole HF Band, just guessed lengths and very low noise compare to a wire antenna. I say the hardest is wingding the balun, but this also could be wound on a smaller core with a smaller gauge wire. I’ve tried with a LNA and then an antenna tuner with very little difference other than raise the noise floor.
Robert – Adelaide, South Australia