You've seen a big box of cheap ferrites at the surplus place. What kind of ferrite are they? What's their properties? This question comes up all the time, so here's some info on identification.
First, the statistical approach... Most all of the shielding ferrites you see are Fair-Rite Type 43, or the equivalent. The permeability is around 1000 for lowish RF frequencies (<1MHz), and they become mostly resistive.
(Suggested by Scott Townley) Loop a single wire through the core. Run the frequency up until R=X. The mix is identified by the following:
Mix 33 - around 10 MHz
Mix 43 - around 20-25 MHz
Mix 61 - arround 50-70 MHz
The FairRite Catalog (http://www.fair-rite.com/) has actual tables showing impedance vs frequency for a one turn loop (http://www.fair-rite.com/ds26a.htm is one such page). You'll have to measure your particular ferrite and find one with matching dimensions.
Chuck Counselman cautions that measuring the inductance at too high a frequency can perturb your measurements, because all these materials start to become quite lossy at higher frequencies, the frequency at which this occurs being dependent on the material (which is how Townley's scheme works, really).
A bit of reading of the Fair-Rite catalog will greatly repay you. The URL (as of 19 March 2003) is http://www.fair-rite.com/fr_catalog-14thed_rev3.pdf
Amidon Associates also has data http://www.amidon-inductive.com/ but it's a bit harder to find.
The info on this page was gleaned from a variety of sources, most recently, some posts on the TowerTalk antenna reflector from Chuck Counselman, Scott Townley, and Richard Karlquist, in response to just this question from Robert Thain.
radio/ferrite.htm - 19 March 2003 - Jim Lux
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