Results of precise measurements on the variation of drift cyclotron loss cone frequency and perpendicular wavenumber as a function of ion density are reported. Careful quantitative comparisons to theory are made. In particular, it was found that a local linear slab model accurately predicted the unstable mode frequencies, even under conditions where nonlocal effects should be significant—i.e., when characteristic gradient scale lengths are on the order of an unstable mode’s wavelength. Inaccurate predictions of unstable perpendicular wavelength, however, may be a consequence of nonlocal effects.