Hi Tom,


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.

OK, I just ran some numbers for a 6000 ohm to 100 ohm toroid
transformer at 190 Khz. I didn't have to deal with the secondary at
all, because the primary has to be so large...I never made it past the
primary! Perhaps I made an error in the calculations? The transformer
has to present about 6000 ohms of inductive reactance, which is 16.6
millihenrys. Even on a large high mu core, I'd have to wind 400
turns!!!!!!!!!!! With that many turns, the losses would be big, and
would still have to wind a secondary (although it would much much
smaller). Did I make a mistake in the calculations?

I don't mind going to the antenna to tune it-lowfer signals don't
change frequency much.


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.


Was hoping to use twisted wire which can be homebrewed or cat 5...it's
cheap and available. I think the twisted wire runs 80 to 90 ohms
impedance. I considered that I might just tolerate the mismatch since
the run was so short....but it's such a big difference, I am not sure
the input filter in the receiver will react as expected.

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.

Are there any other ways to convert the impedance without big losses
or without resorting to an active preamp? I'd like to avoid the preamp
if possible, especially since I expect a good output voltage from the
big and relatively high Q loop.

T