[Andrew2]

Hi Al, thanks for your interest. I wasn't going to include diagrams but they are there at the bottom of the post now, so really the description is not required.
Modulation hum is a subject that comes up quite often on here these days. It seems to be caused by the RF from strong transmissions getting on the mains wiring and mixing with local 'buzz' noise also on the mains. Our local Gold 1458 KHz is a very strong signal and it suffers terribly, although rotating the radio stops it.
I didn't so much design the loop as throw it together and 'suck it and see'! I think there are about half a dozen turns in the loop and it is tuned/matched with a 1000pf compression trimmer (one of several bought surplus many moons ago) in series with a 10nf ceramic cap. The TX output goes across the 10nf and the trimmer adjusted for max field strength. A later check with my return loss bridge showed a decent match. Tuning is pretty sharp and it's easy to end up with wonky sidebands!
I'll post some piccies if I remember....