Hi Daniele,

Could it be possible to tune a ferrite loop over a frequency range as wide as 100-500KHz? Or at least over the EU NDB band (say 300-500KHz)?

Any practical suggestion (and/or references on the web) about how to build it?

I think it is no problem. A narrow band but adjustable antenna is a good choice. Just use a large variable cap, e.g. 2x500 pF. It may be helpful to use switched caps and a smaller variable cap like i did. If you have a high Q antenna, you can then easily fine tune the system.
So maybe 5...100 pF +100 pF+180 pf+270 pF+560 pF or so, depending on your L.

About the ferrites, there are several sellers on eBay who offer a russian MU400HH type. Would it be suitable for LF?

Yes, very suitable. I have exactly these types in my LF ferrite antenna and got a Q of 480. Q will change over the frequency. So maybe it is even helpful to realise a switched L, for the higher frequencies.

73, Stefan