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I love small transmitting loops (STLs), aka magnetic loop antennas.  They are VERY misunderstood, and underestimated.  A properly constructed and sited loop can easily perform as well as a dipole, within 6dB.  And in a situation where a ham cannot install a dipole at it's proper 2 λ height, often times the loop will outperform the dipole.

I've read enough from those who have done EXTENSIVE testing, along with my own comparisons, to know right away that when someone starts yammering on about how inefficient they are, or how terrible they perform, they have either never used one themselves or the one they did use was terribly inefficient due to it's design or construction.

One complaint I hear often is that they are "figgity" or "fussy" to tune.  For those of us that use them, know what they consider fussy to tune is simply a demonstration of the high Q of the STL.  Let me clear that up...... you've probably heard this said before: all antennas are a compromise.  This is true. The fundamental limitations and trade-offs of an antenna system are: small physical size, bandwidth, gain.  Want a great antenna system?   Good, you get to pick two! 

Let’s look at a few 20 meter antennas:

1/4 wavelength vertical: compact compared to a 1/2 wavelength dipole for the same band, usually wide bandwidth that will cover the entire band, and with a great radial field will afford the user around 2-3 dBi of gain. With the 1/4 wavelength vertical, you end up with an antenna that is about 16.5' tall compared to a 1/2 wavelength dipole at about 33'. 

- SIZE: height is not usually a problem, but if you in an HOA, the visual aspect  might be a no-no. Also, the vertical will need a radial field which will require  enough ground for them to be laid out.

- BANDWIDTH: no real loss here, a vertical will usually exhibit  decent  enough bandwidth to cover most bands except for the larger  ones, like 80/75m  or 10m. 

- GAIN: around 2-3 dBi with a good ground plane system. Will have a nice low  take-off angle for hunting DX.

* The downside here is SIZE. Even though the shorter size is usually seen as a bonus, the height is what can sometimes be an issue for people depending on their location and things like an HOA

1/2 wavelength dipole: somewhat large footprint at around 33' long, wide bandwidth that will usually cover an entire band, and gain of around 2.15 dBi.  In reality, the 1/2 wavelength dipole is the antenna that all other antennas are compared to. 

- SIZE: most urban back yards can accommodate a 33’ dipole. But that not  always the case. The most often problem I hear with dipole installations is not  being able to install at the proper height (0.6 wavelengths above ground)

- BANDWIDTH: most 2λ dipoles will usually cover an entire band with some  exceptions, like 80/75m.

- GAIN: the dipole sets the standard that gain is usually measured to. However,  without the proper installed height, gain can be dramatically affected.

* The downside here again is larger physical size that make it difficult for some folks to put one up in the back yard.

Yagi beam, 3 element: has a large footprint at around 12' long x 27' wide (average size, some are bigger, some are smaller), narrower bandwidth that decreases with gain, and with gain numbers around 8 dBi and up. 

- SIZE: a beam works very well if you have the space to install one. Usually a  beam installation will require a lot of ancillary items like a tower and guying  system

- BANDWIDTH: beams tend to have a slightly narrower bandwidth than a  vertical or dipole, and the bandwidth can decrease as gain increases. 

- GAIN: isn’t this the reason we all want a beam?? Besides gain, a bean is most  excellent at nulling out unwanted noise.

* The downside here is the much larger physical size and the need for a tower or large mast, and other constraints like an HOA.

20 meter small transmitting loop: small footprint(on average the radiating element is about 3' in diameter and 3 to 6' off the ground from the bottom of the loop), narrow bandwidth, and gain of around 1-2 dBi. 

- SIZE: very small footprint, and no ground plane needed. 

- BANDWIDTH: very narrow bandwidth, usually in the 20-100 kHz range.

- GAIN: about 1.5 dBi over average ground.

* The downside here is the very narrow bandwidth that, without a proper tuning system, can make tuning the antenna difficult.

 So the STL exhibits similar gain to a dipole, as is physically small in size. The trade-off with them is the very narrow bandwidth. And this is the real issue people usually have with them, and the ability to tune it properly. Myself, like many others, have devised ways to precisely tune the capacitors without having to leave the shack. It can be a simple system like I use, or some rather ingenious system that use small computers like the Arduino to automatically tune the capacitor to whatever frequency the radio has been tuned to. I use a simple battery powered home brewed tuner for my POTA loop consisting of a geared motor, a couple automotive relays, and a couple switches. At home I use a PWM motor controller I bought off Amazon for around $10. Both are installed in small metal enclosures. Once you get the hang of it, they can be dialed in rather quickly! I start with listening for the loudest noise as I tune the capacitor, then adjust the capacitor for the lowest SWR. I’m at the point at home I can do this very quickly.

"It ain't what a man don't know that gets him in to trouble; it's what he does know for sure that just ain't so!" - Mark Twain

One of the BEST papers written on the subject of small transmitting loop antennas, The Underestimated Magnetic Loop HF Antenna,  was composed by Leigh Turner, and can be downloaded HERE.  Leigh is very knowledgeable on these remarkable antennas, and has put together a very well written and informative 33 page research paper on the matter.

Are they the greatest thing since sliced bread?  No.  Do they have a big advantage over most any other HF antenna one could install at their QTH?  No.  But, if you have to deal with restrictive local ordinances or HOA/CCRs that dictate what you can or can't install, a STL might just be the thing that keep a ham on the air and involved in the hobby.  Another nice thing about an STL is they exhibit both low and high take-off angles!

When I can be in a QSO with someone, and switch back and forth between my wire and the loop, and they can't tell the difference between the two antennas, I'm happy with that.  Even better are the times when I make the switch, and get a report that I'm an S-unit better, or just sound better on the loop..... well, that makes me happy too.  

My two most memorable QSO's on one of my loops have both been during a POTA activation at K-7490, Pawnee National Grassland.  The first was to JR7TKG in Japan on 17m in January of this year, a distance of 5820 miles.  I recorded a short video of that QSO, and Saki was 59 and 59+ the entire time.  I still listen to part of it just to smile at the conversation.  It sounded like a repeater QSO lol!  The other, and one of my best whether at home or portable, was to VK4SX in May, a distance of 7959 miles and also on 17m.   He was pretty stumped as I described my antenna to him.  But that's pretty normal when I describe the loop, as a lot of people have never heard of them, or believe all the negative things they hear.  Experience and use matters.

Bottom line is..... if you are limited on space, or have restrictions in place that do not allow you to install a traditional antenna, you can buy or build a great antenna that can be easily hidden or not readily seen that will still allow you to make all the contacts you want with very little disadvantage compared to other hams in your situation.  At my QTH, a 3' diameter STL is my main antenna for 20 to 10 meters.