I'm trying to pick up and transmit a signal atop a few, maybe 20 amps DC in a tube (although it may as well be solid bar, I can't get to the inside). Frustratingly, I can't go into details. The tube is extant and non-negotiable.
So not a current transformer as such and with 40kHz maximum signal frequency.
This nano-crystalline and metallic glass stuff sounds interesting, but looks like a bit of a palaver to get hold of. Rogowski coils would be easy, but could one work as well as a ferrite toroid? I'm not sure why not, but, for example, current clamps use a coil with a core, don't they?
Not my strong point, magnetics. Then if it was, I wouldn't be asking here ;-)
Cheers