Steve: Your results sound pretty reasonable. If you will look up a recent copy of the ARRL Antenna Handbook you will find curves showing how the inductance of a ferrite-cored loop varies with the length to diameter ratio. For very long, skinny coils full of high-permeability material you get quite an inductance change, while for short, fat coils the change is much less because a large part of the flux path is through air. Ed Phillips