I have a Grundig YachtBoy SW/MW receiver that I'd like to connect to an
external long-wire antenna, but I'd like to have the antenna on the
roof, while I'd be listening in my office in the basement. I'd like to
make a couple of simple baluns to match it to 75 ohm coax. What's the
impedance that the receiver is expecting? It's just a phono plug
input... Grundig's web site is, ah, not very technical...

Forget the balun and other matching devices. While trying to cover a wide
frequency range it is a waste of time. There will be more than enough
signal for receiving without any matching devices.