  We describe near-field imaging of sample sheet resistance via frequency
shifts in a resonant coaxial scanning microwave microscope. The frequency
shifts are related to local sample properties, such as surface resistance and
dielectric constant. We use a feedback circuit to track a given resonant
frequency, allowing measurements with a sensitivity to frequency shifts as
small as one parts in 50000 for a 30 ms sampling time. The frequency shifts can
be converted to sheet resistance based on a simple model of the system.
