Hi Capt. Oveur,
A SDF is a Simplified Directional Facility - basically as far as I know it, similar to an ILS but a non-precision aid (i.e. less accuracy in the readouts or setup in relation to the runway).
I find
http://users.pandora.be/flyingineurope/aaa.htm a great resource for helping to remember some of those TLAs!
Chrs, CS