Generally, the biggest problem a new ham radio operator will come across when starting out on the high frequency (HF) bands is finding physical space for the antennas. For a quick example, a dipole antenna for the 20 m band will need around 10 m of wire, and the lower frequencies like 80 m need about four times as much linear space. But if you’re willing to trade a large space requirement for a high voltage hazard instead, a magnetic loop antenna might be just the ticket.
Loop antennas like these are typically used only for receiving, but in a pinch they can be used to transmit as well. To tune the antennas, which are much shorter than a standard vertical or dipole, a capacitor is soldered onto the ends, which electrically lengthens the antenna. [OM0ET] is using two loops of coax cable for the antenna, with each end soldered to one half of a dual variable capacitor which allows this antenna to tune from the 30 m bands to the 10 m bands, although he is using it mostly for WSPR on 20 m. His project also includes the use of an openWSPR module, meaning that he doesn’t have to dedicate an entire computer to run this mode.
The main downsides of antennas like these is that they are not omnidirectional, are not particularly good at transmitting, and develop a significantly high voltage across the capacitor as this similar mag loop antenna project demonstrated. But for those with extreme limitations on space or who, like [OM0ET] want a simple, small setup for running low-power applications like WSPR they can really excel. In fact, WSPR is a great mode for getting on the air at an absolute minimum of cost.
With a MagLoop antenna I was able to talk SSB (voice) from Washington State to Japan with only 5 watts of power. Mostly 40 meter and 20 meter bands. I had to time it for the gray line, so there was about a 1-2 hour window where is was optimum to do that. A lot of fun.
Magloops are great for noisy environments, because they’re deaf to man made noise that’s more electric.
They also have a preselector built-in, which is good for any receiver.
However, they should be used with care when sending signals.
High voltages may build up (=don’t touch while xmit), the operator’s eyes and other body parts with low blood circulation may heat up by being physically close to the antenna.
So it’s better to keep a distance of a few meters. Also applies to equipment, such as computers. Better safe than sorry. By comparison, magloops are still not deeply understood in practice.
The feed loop is 1 turn, and the antenna is 2 turns (or 4 turns in the optional configuration) so the transformer voltage will be 2x the feed voltage.
However, the L and C form a resonant circuit, so the voltage will be the transform voltage multiplied by the Q-factor of the circuit. Air capacitors have a very high Q, so the Q of the circuit is limited by the Q of the inductor.
A single wire loop probably has a fairly high Q, probably more than 50 and possibly as high as 200. The voltage seen on the loop will be this Q value times 2 times the feed voltage, which can be quite high as the parent poster noted.
The voltage will be AC at the transmit frequency, and over about 400 Hz the skin effect becomes dominant and so the RF energy will run across the surface of your skin. You can get burned by touching the resonant circuit.
I would attach a “chicken stick” to the capacitor and tune it from a distance.
Also, maybe some non-conductive shielding to prevent accidentally brushing up against it.
All antennas can build up “high voltages” during transmit. The ends of a half-wave dipole will have more than 500 Volt potential with 100 Watt input. Don’t touch them when transmitting. Magloops are usually used with much less power. [OM0ET] is only using a maximum of 5 Watts.
“…the operator’s eyes and other body parts with low blood circulation may heat up by being physically close to the antenna.” Reasonable precaution, but in HF bands at these power levels, completely a non-issue.
73
Electrically small antennas build way higher voltage of course than big antennas.
Hi, yes. I just said this because of the variable capacitor. It shouldn’t be touched while “tuning”, despite the temptation to do otherwise. It’s safer to rather change it in small steps and then let it go while actually testing the antenna (pressing PTT, reading VSWR meter). Ideally, most of the “tuning” works by listening already. The preselector effect helps here.
Mag loops are not deaf to man made noise “because it’s mostly electric”. A well made loop can have lower noise levels because the directivity pattern has deep nulls where the antenna is insensitive to noise. Being able to rotate a loop is very useful to be able to peak wanted signals and null unwanted signals.
Voltages don’t ‘build up’ they are are a direct result of the operation of an electrically short antenna. It’s not unusual for 5kV to be present across the capacitor ‘key down”.
Mag loops are very well understood – and have been for decades, it’s simply that how a ‘mag loop’ – a misnomer – operates has been drowned in myth, in opposition to the settled physics.
Like every antenna type, there’s a whole bunch of mythology and testosterone (mega wonder stealth super viper tactical cobra penetrator) as well as a whole pile of lies that make frontier snake oil salesmen seem downright honest.
Plus, in the absence of understanding, it is common to cargo-cult… or to “conserve the bullshit”, if you will. The explanation doesn’t have to make sense, if it’s the only explantion someone has heard, if they don’t really want to figure it all out from first principles.
“The main downsides of antennas like these is that they are not omnidirectional,…”
Directionality is a very useful feature of loop antennas: they have a sharp null in the response pattern that can be used to reduce reception of noise, by rotating the antenna to put the noise source in the null. Harder to do if you choose to attach the loop to your wall, but that’s a user issue.
And if you really want omnidirectional response, then just make the loop horizontal.
I have a loop antenna on the roof for receiving and never knew this!!
I’m off to design a 3D printed turntable for it now ;)
I brought a YouLoop to go with a Retekess TR110, and it definitely improved reception indoors over an external simple wire antenna, which what better than the built in antenna that basically left it deaf for HF/SW. I only have messed with it a bit so far, but could pick up CW on HF front inside the house. I have a small battery powered receiver amp to try with it next. When it is sunny I will get round to putting up a long wire on some sotabeam poles I have.
I can definitely agree though that as a new ham, the antenna setup has been the biggest drag on me getting round to using the my transceiver. Not sure about having high voltage inside or anywhere it could be touched with this loop described in the article though!
So, as a new ham, I would be looking to RSGB to address “What’s the quickest and easiest way to usefully on HF with 10-20W?” if they hope to achieve their strategic aim of getting more people into the hobby.
“What’s the quickest and easiest way to usefully get on HF with 10-20W?”
I wish there was an edit button.
What part of “put up a dipole” is difficult to understand?
Hint: dipoles don’t need to be straight. Another hint: There are many HF bands. A 10m band dipole is only 5m long.
Not sure why this is RSGB’s responsibility to work out
Thank you for kind responce, I believe you are trying to offer assistance.
It’s a fair point, although I have a have an EFHW for 10-80M, my post wasn’t really about me.
My thinking goes along the lines they [RSGB] want more people in the hobby, and space is a consideration for a lot of younger people. So making it feel more accessible would probably help, thus going some way to achieving their stated strategic goals, IMHO.
So seriously, I think your hints could well be part of the advice they might choose to give people, making the options known and not letting the perfect be the enemy of the good would probably help some folk…
:-)
The main downside is magnetic loops is that they have very narrow bandwidth. That’s it. That’s the one. That’s also a benefit, you just gave to be more aggressive about tuning.
That and low transmit efficiency. We have a mag loop net, and despite the claims otherwise, dipole signals are almost always stronger than mag loop signals. A large amount of wishful thinking surrounds them in ham circles.
However to be fair, you need to compare the mag loop with other small antennas that are a few metres long, or dipoles that are 2m above the ground. They look a lot better when comparing to other antennas that fit into small spaces close to the ground.
An auto-tuner certainly makes them much more usable. One exception to this is the 60m band. The band is very narrow, so fixed tuning is OK, and the legal EIRP limit is much lower than most transmitters. In this case the mag loop’s low efficiency is a plus: you can get the legal EIRP with 100W transmit power and a 1m diameter loop.
Mag-loop antennas are omni-directional with respect to the plane of the loop. Tip one to the horizontal and it will radiate equally N, S, E, and W. Like so, with its characteristic nulls aimed straight up and down.
The links in the email don’t link properly..
The problem with knowing more about chemistry that I do radio, is that I saw this and thought “why on earth are you working with HF in a small space? Ventilation, people!”
The chance of another person of having even most part of those parts required lying around is probably the same as us being in a false vacuum and it collapsing as I hit ‘send’ for this comment..
Here goes nothing..