The discussion on ferrites and how to use them (November and December 2011 Tech Tips) seems to have hit home with many of fellow broadcasters out there. In fact, my friend Clay Freinwald from Seattle wrote me:
"I trust I am like most and have a collection of these ferrites left over from previous projects or things I have scrapped. For me, they end up in a drawer. The real issue here is what is the core material and how will it perform at AM or FM frequencies? Then again even if the material has a label (not likely) how do you know what that means? Sure it's easy to go out and buy something that's pre-packaged (and labeled), but what about the collection in the drawer in your shop?" Clay forwarded a link regarding a method of identifying the ferrite mix.
You'll need an impedance bridge (and signal generator) for that method.
Another good method I found with a little search explains how to tell using either a grid dip oscillator or a plain LCR meter. The link I originally found is no longer active, but search for the appropriate terms and you should be able to find the information.
Another useful resource regarding ferrites is this data sheet from CWS Bytemark.
And finally, by way of the Pubtech message list: Magnetics is a source of the large ferrite "donuts" that go around transmission lines.
Irwin is transmission systems supervisor for Clear Channel NYC and chief engineer of WKTU, New York. Contact him at email@example.com.