If you are building radios etc. I recommend a 
couple pieces of test equipment:
Elecraft XG3 S9 
generator and more.
L/C 
Meter IIB for measuring inductance and capacitance.
Reference material
Ferrite beads and toroidal cores good information
Ferrite & Iron Powder Toroid Info another good reference
Toroid 
inductance chart
Data books
PL-259 coaxial cable 
assembly
FSK/CW interface for my IC-7600 and other radios
 
Toroid_Info_chart
Toroids
Amidon Site