I wouldn't argue with real-life experience, but here's my take based on
a wobbly grasp of theory:
Since it's a passive design, the amount of signal captured is limited
by its very short 2m length, which is considerably under 1/10th
wavelength for many SWL frequencies in heaviest use right now as the
solar cycle has waned. So although it is potentially *very* quiet, I'd
predict its performance would rely quite heavily on having a quiet,
low-noise receiver.
So for a good tabletop, it might do well. For a noisier radio, you
might as well forget it.
Performance would also depend on being mounted relatively far from
local electrical noise sources -- so you wouldn't want to mount it
close to a house with PCs and electronic gizmos running while you SWL.
That's because, even though a passive antenna won't add (more than a
tiny bit of thermal) noise, if it picks up local noise it's going right
down the coax into your receiver.
A few years ago I did a ton of experiments with homebrew antennas with
9:1 transformers in different configurations, and came to the
conclusion that all of those options were inferior to a Wellbrook loop
(which is an active design, btw.) Even with a Drake R8B, which is no
slouch of a radio.
-- Ross