John,

That is it! Your picture shows exactly what I see on 73.3kHz. It is not

RTTY

but 100Hz wide FSK centred on 73.25.
You evidently have a very good antenna for this direction. I would very

much

appreciate some details please.

I'm using a square loop, 2 meters on a side, 14 turns, with the turns spaced
2.5 cm. The wire is 12 gauge, and the loop is series-tuned into a coupling
transformer. There is no ground connection on the loop side of the
transformer, and I've taken pains to keep things balanced. The transformer
presently has a 2:1 ratio, with the "2" side connected to coax that runs
back to the shack. Tuning of the loop is done with relay-switched capacitors
in increments of 50 pf (a 1-2-4-8... sequence with 8 relays). At 73 kHz,
about 6400 pf is in use. I'll need finer tuning than that if I want to
increase the turns ratio of the transformer, so I'll probably be adding a
motor-driven variable cap. The preamp has an honest 50 ohm input (grounded
base), and about 30 dB of gain.

I don't think there's anything unusual here, but it does work. My QTH is in
a residential area just outside a mid-sized city. It's not an RF-quiet spot.

I'll be experimenting with ways to track the signal strength on 73.25. My
receiver's s-meter is probably the least useful thing in the whole setup!

John Andrews, W1TAG