Yes, this might be good for another thread...

Given the limited LF bandwidth you're interested in, I can't imagine
that making a transformer would be all that difficult. Not trivial,
certainly, but far from impossible. A 5:1 turns ratio will give you a
25:1 impedance ratio. You'll want to use a core material that doesn't
introduce distortion. A transformer like that also gives you a way to
keep the loop loading balanced. But--are you going to put the tuning
capacitor at the loop, or do you have in mind putting it, say, at the
receiver end of the feedline? If it's at the loop, how will you adjust
it? And just what size signals do you expect to get? One of the nice
things about LF/VLF is the predictability of signal strengths. Also,
beware of worrying a lot about feedline impedance. How long will your
feedline be, in wavelengths? If it's, say, 0.05 wavelengths at 100kHz
(and THAT's 150 meters long!), does it really make much difference that
it's quite a different impedance than the antenna? And...what IS the
impedance of the line, at that frequency? It may well be a bit
different from what you calculate for the line at 10MHz. What would
happen if you fed your one turn loop with 100 feet of "300 ohm
twinlead" or "450 ohm ladder line" and just tuned it at the receiver
with a capacitor across that line? Small transmission line wire size
would ding the Q some, but would that be an issue? I'm just
speculating here, and maybe someone with direct experience with that
sort of feed will offer suggestions.

One thing to keep in mind here is that the LOOP construction will
almost certainly be the most challenging and expensive part, for a big
loop. And--you may really not NEED THAT big a loop! More signal also
means more atmospheric noise, and you won't improve signal:noise ratio
just by getting more of both signal and noise. Anyway, once you have a
well-constructed loop, it's relatively easy to play around with
different feed systems--preamp and remote tuning, straight feedline,
whatever.

Cheers,
Tom