If you live (or are staying) in a really confined space and can't even accommodate a temporary fishing pole mast and thin wire then a magnetic loop is probably your next best choice for HF.
Only reaching up to your chest, such a loop can fit onto an apartment balcony and attract little more attention than a bicycle or washing line.
Building an efficient loop
An efficient loop needs to be made of a fat, low resistance conductor. Not only that but the connections must be excellent. If they're not you will get loss. With QRP that loss may
deliver poor results that you might simply write off as a natural consequence of using an electrically small. Whereas with more than about 10 or 20 watts you will get arcing and sparking
and the antenna won't be usable. Reducing the power cuts out these symptoms but then your signal is doubly weak - firstly due to the low power and secondly due to the inefficient antenna.
The most critical connections in a magnetic loop are at the ends - where the pipe loop connects to the capacitor that brings it to resonance. Some builders use short lengths of coaxial cable
shield but even so there can be losses as its resistance is higher than the loop's solid copper.
A critical choice when building magnetic loops is the variable capacitor. The 'Rolls Royce' for loop builders is the vacuum variable capacitor and associated control mechanism.
If you don't have the budget for this the
next step down is a wide spaced transmitting variable capacitor. Cheaper and more obtainable are receiving type variable capacitors as used in tube (valve) radios. Two gang is
good as you can connenct the ends of the loop to the two stators and increase efficiency compared to if you had one side connected to the frame (which is connected to the rotor). However you
do lose half your capacitance which can make the loop harder to bring to resonance much below 7 MHz. Also the narrow capacitor plate spacing (compared to transmitting variable capacitors) makes
arcing likely if you're running more than QRP power levels. Still if you value multiband coverage and operate QRP then such a loop can still give good contacts. Home station QRP magnetic loop for 160 to 15m is a simple example.
A single band alternative
What if, mindful of the losses in magnetic loops, you wish to run more than QRP power levels to be comfortably heard by others? This puts us back into vacuum variable capacitor
land and the attendant cost and construction complications.
Making your loop tune a single band simplifies things a lot. You don't need much of a capacitance range and construction becomes much easier. The band restriction might not even be such a big deal -
many, especially SSB operators, would rather having a good capability on one band than mediocre capability on several.
The video below describes a single band loop where I use RG213
coax (which has about 1pF per cm capacitance) to bring the loop to resonance at the top end of the 7 MHz band. In parallel with this is a home made variable capacitor, using pieces of printed circuit
board, to bring this to resonance on your desired frequency. The whole thing is supported on a timber cross.
As you heard from the on-air tests the loop isn't as good as a full sized antenna but still gives worthwhile results. Stations worked were up to 2500km away. I've run 80 to 100 watts into
it with good results but you wouldn't want to feed it with much more otherwise arcing will start.
Construction effort and cost are low so making loops for other bands isn't out of the question. A pair of loops for say 7 and 14 MHz should give a good range of local and DX contacts
for the space constrained ham.
Precautions
Since we are now dealing with higher power and higher voltages precautions when using the loop need to be taken more seriously than at lower transmit powers. Risks include EMR exposure and shock.
Mitigate these by having a good separation between the loop and inhabited rooms (eg having the loop outside rather than inside) and ensuring that others can't touch the loop when transmitting.
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