Has anybody successfully got one of these to work.

I am having a bitch of a time making a loop that can tune.

Well I finally got it to tune.

13 foot piece of 3/8 inch copper tubing.

The issue was the coax. The 25 foot piece of RG8x would not produce am SWR less than 2.0

A 50 foot piece produced an SWR of 1.2.-1.5

Either it needed the extra capacitance or the 25 foot piece that I kept closing my truck door on got a door closed on it too many times. The MFJ-259C says the cable loss is only 0.2 db, but maybe it is pinched and causing SWR / capacitance problems. Maybe it isn't 50 ohms anymore.

I don't know.

It is still a very inefficient antenna. S7 same signal that my 40 foot wire Inverted V get an S9+20 db.

So far no contacts. Maybe if the band was better.

I was hoping this would be a hotel room antenna for 20m but it does not look like it will work very well.

I have read about people claiming DX with it in their apartment.

Maybe I am doing something wrong.

Physically tiny antennas + long wavelengths = dummy loads. You can't push or pull a basketball through a pinhole. You can't break the laws of physics. People do make extraordinary contacts with them, but only very occasionally. Just like people do when running QRP. Even a blind squirrel finds a nut every once in a while.

The Army supposedly uses these antennas and developed them in the 1960s.

Plus I got an RF burn from it at 5 watts.

So I am skeptical that it is just a dummy load.

Is a Magnetic loop / Army Loop / Short Transmitting Loop not an established antenna?

Has the ARRL been duped by publishing Magnetic Loop antennas in their ARRL Antenna Handbook?

Not exactly. The physics says an antenna can have two of the following three characteristics:

* Broadband
* Efficient
* Small Size

Mag loops can be very small, and efficient, but they have a very narrow bandwidth. If you make a broadband electrically small loop, then you've made yourself a dummy load.

There are a LOT of gotchas with electrically small loops, but from a physics standpoint, they don't violate any rules, and the Navy still uses the today.

http://aa5tb.com/loop.html

To answer your tuner question, I don't know anything about it. I'm building my own tuner so I can push 100W. This will create a 5kV potential across the capacitor, so I need a vacuum variable.

The Army did not invent the magnetic loop. But they did look at them in the 60's. And abandoned them because they do not perform well.

First, I'm surprised you got burned at only 5W. Second, even if you did get burned that proves nothing except that RF is flowing along the conductors of the antenna.

What matters is how much RF is actually radiated as an electromagnetic field. The problem with antennas (or apertures, as engineers like to say) that are so small that they only physically comprise a small fraction of a wavelength is that they simply do not radiate E fields well. It's a physical limitation of electromagnetics.

Established as well known designs, certainly. Established as poor performers, also certainly.

Not at all. People have been desperate for the holy grail of electrically small antennas for decades, and therefore there is a lot of interest in them. Many people try them. Most people abandon their efforts after a while. Naturally they are included in the antenna handbook along with a vast array of other antennas (ha, a pun!)

All antennas are trade-offs. If you want something that is electrically small (for a dipole that is generally considered to mean 1/10 wavelength or smaller) then you have to trade-off radiation performance and efficiency to get it. Nobody has solved that physics problem yet.

The Buddipole is another excellent case in point. People are intrigued by its small size and modular nature. But at the end of the day, if you are in the field and set up a Buddipole vs. throwing a big long wire in a tree or using one of those 20ft collapsible fishing poles to hold up a great big wire, the wire will win every time. Assuming neither setup has any gross errors in it, of course (improper baluns, etc.) because matching network losses are the biggest killers for any non-resonant antenna.

Guys, the antennas like "Buddi-Miracle-Pole", tactical "super antennas" or mag-loops do work but they are relatively inefficient. You'll read stories about people working DX with small antennas and 5 Watts and may get an impression that the antennas are far superior to anything else. In fact, the only reason they work well is - PROPAGATION. When the bands are open, it does not take much of an antenna to make a DX contact. It's a whole different "ball game" when the bands are in poor shape. The small portable or balcony antennas are a trade-off or a compromise that fits into a small space but provides poor efficiency. It's impossible to make a small, shortened antenna efficient. You can make it more efficient than a similar model but it won't compete with full size antennas or directional antennas. It's simple physics.
Also most small, shortened, "high Q" antennas are very sensitive to nearby conductive objects. Keep that in mind.

All HF bands have been in poor shape in the last couple of days. Don't get discouraged. We will have some nice openings on 20 and 40 meters in the next few years. Forget about the upper bands until 5-7 years from now. There will be some sporadic openings but nothing consistent.

I've used my Buddi-Stick on a balcony in a city. It was marginally better than a dummy load but I still managed several CW and Digital contacts with it. It's better than no contacts at all and I could not install any other antenna.
A few times I got a hotel room on the top floor and managed to throw a fishing line into a nearby tree. The trees were within throwing distance and far enough to accomodate my End-Fedz 10/20/40 and fed antenna. It worked A LOT better than any other portable contaption. I actually made lots of contacts with it and only 5 Watts from an FT-817d. Most contacts were of course done with CW. I would not call the end-fed an efficient antenna either but it's a good compromise of size, weight, ease of deployment and performance.

BTW, Semi-portable mag-loop antennas are very popular in Europe for apartment dwellers. Some claim good results with them but again, it's a matter of propagation conditions.

A point I try to convey somewhat frequently, is that much of traditional Amateur radio is exploring what is possible but improbable. Inefficient antennas are in that category, along with minuscule power, oddball propagation modes, and very long range contacts to unusual places for example.

The corollary is that just because something is possible, does not mean that it will be reliable.

I understand everything is a compromise.

I am building this for QRP out of a hotel room in the middle of a city with windows that do not open. It needs to fit in a carry on bag on an airliner.  It is a major compromise, but short of putting a hole in the ceiling or breaking the window, this is my only option, other that forgetting about HAM radio in a hotel and spending my time in the bar, which may very well be more fun and a better use of my time.

Anyway, I did get an RF burn at 5 watts which tells my I have some decent voltage on the loop. It felt like a bee sting, and stayed like a bee sting till the next day. It still feels a bit off.

I am testing it in my house since I am making it for use in a hotel room. I can hear lots with it when it is tuned and it is high Q because if I change freqs I have to retune it which frankly is a pain in the ass and takes several minutes. It is 20% of of wavelength, not 10%, in order to make it more efficient.

I am making it QRP because the radio also has to fit in my bag with my cloths and stuff and needs to be small, as does the capacitor box.

This may be an exercise in futility, but the new coax made a big difference. I don't know if the extra capacitance from the coax helped with the impedance match or if the 25 ft coax is bad. The 50 foot coax reads twice the capacitance of the 25 foot coax. The 25 foot coax reads a 0.2 db loss. Does that mean the coax is good? Since I did close my truck door on it a lot, how can I tell if it is bad?

I am about to try to see if I can make a contact on it, but like yesterday the band is single digit % propagation.

I made 1 contact with it. K8NY

S6 into West Virginia from MA on 20 watts. He was using a high gain beam. He switched to his G5RV and heard nothing.

He was coming in S9 with 1200 watts and a big yagi.

So it works, very very inefficiently, but I knew that going into it and the band conditions on 20m suck right now.

At least it is not a complete dummy load.
It is a coil-able 3/8 inch copper tube, expands to a 4.2 foot diameter circle
Fits in my suit case
Sets up in a hotel room, by a window
Runs on batter power.

I can also tune it to 40m but have not made a contact on 40 yet.

For some reason it is low Q on 20m. I tune it to low SWR in the middle of the band and it is good for the whole band.

It is very high Q on 20 meters. Very difficult to tune and a very small bandwidth.

Will I get a lot of contacts or DX? Nope, not now. Maybe when the 20m band gets a lot better.

But I learned a lot about STLs.

Still don't know why it is low Q on 20m

Because mag loops run such high currents, the losses are super bad unless pretty much perfect
construction is used. MFJ actually does have some proper mag loops with a welded loop element.

They are still very lossy on TX (8-10dB) but are respectable RX antennas. As you'd expect,
touchy as hell with tuning.

They have their place but I think there's better solutions. A loaded short vertical is going to be
about as small and perform similarly, but not need as much attention to tuning.

If you know of a loaded vertical that can fit in a 22 inch suitcase please let me know, I could not find anything that would pack that small.

what are you using for the loop element? Keep in mind that with a small diameter loop, the radiation resistance is extremely low, so the actual resistance of the radiating element becomes important.

Including connections, dielectric loss, all that stuff.

Buddiestick is what you seek, my friend.

the loop is a single peice of 13 foot 3/8 inch copper refrigerant tubing. I flattened the ends, drilled a hole and put it on the connector on the MFJ-9232 with stainless steel wing nuts

The way I understand it the less the resistance of the loop element the more efficient it will because the that is where the current will flow.

Low resistance in the transmitting element is important to avoid significant loss, as the radiation resistance is so low. The copper should be pretty good unless it's corroded or something. There are those who have silver plated loops.

Something else that comes to mind is to be cognizant of the signal pattern of a loop... broadside to the loop is a null. As an example, if you're putting a loop flat against a window it will not work very well. Max signal is off the sides 90 degrees from the feed IIRC. Some orient the loop horizontally as well.

I get about 2 S points difference  from pointing the hole in the loop to pointing the metal of the loop at a signal reception so it appears to have roughly a 12 db 'front to side ratio' for lack of a better loop term.

I am going to sandpaper the tube at the connection point to take off what little oxidation there is and see if it helps. I did that on one of the connections, but then trimmed the loop and didn't do it on the freshly cut end.

Horizontal might work well on the 10th floor of a hotel. I won't know till I try that.

But I have been reading a bunch and many people say having the antenna in the room surrounded by concrete, wires, electronics etc is a waste of time. So this may all be for not.

I've used the MFJ-1699S to good effect a couple of times on travel. It's not as good as a wire, but it's not terrible, either. See here.

There are two parts. a loading coil with taps and a stub vertical that unscrews. In two pieces, it will fit inside a suitcase (although you may have to put it in diagonally in a smaller suitcase).

I am still playing with the loop.

I used a 7 foot piece of the same 3/8 inch refrig tubing.

The loop is 2.2 feet in diameter and with it on my back deck surround on 3 sides by my house made a voice contact 905 miles away in IL with an S7 report on 20 watts with propagation at 14%

So it works. The 2.2 foot diameter loop is much easier to tune and much smaller. Since it has 1/2 the copper I suspect the resistance is 1/2 which should make a difference.

If propagation ever gets really good this setup will be even more fun.