[dtvmcdonald]

I've tried several (passive) loop antenna designs for my RCA 121 superhet radio (has RF stage), both broadcast and SW.

The Broadcast one is a 15 inch square with 14 turns and a tuning capacitor;
the output is at a tap one turn from ground. This has a Q of about 100.

I tried several SW loops. The best was a 6 foot diameter single
turn tuned with a 3-50 pF capacitor. The feed was a 15 inch loop at the
opposite side from the capacitor (inconvenient)!

The broadcast one is as good as a 30 foot indoor wire. With it the
radio provides passable room volume on atmospheric static and
the rejection of CFL noise is excellent.

The SW one is nowhere near as good as the 30 foot indoor wire.
But it is good enough to put the atmospheric static above
the noise floor of the set even at 16 MHz. The static, and hence the
limit to weak signal reception, is weak enough that you need a very quiet
room to use it for DX. Actually it turns out that putting a 1.5 volt battery
in the AGC line to make it less negative give a respectable volume. An extra
audio stage would make it work perfectly.
I'm surprised that no radio maker added an extra audio stage, or added
a step-up transformer between the first audio and the output. This
latter would have been very cheap in the olden days.

Edit: I tried a new SW one using 1/4 inch copper tubing for the loops
instead of wire. It worked quite bit better, higher signal and higher Q.
The Q is so high it is very hard to tune. It went lower in
frequency with a 250 pF capacitor. I tried placing the 18 inch diameter pickup loop
at various places around the 6 foot one and found that location made little
difference.