  We demonstrate the operation of superconducting coplanar microwave resonators
in a very large frequency range up to 50 GHz. The resonators are fabricated
from niobium thin films on sapphire substrates and optimized for these high
frequencies by small chip sizes. We study numerous harmonics of the resonators
at temperatures between 1.5 K and 6 K, and we determine quality factors of up
to 25000 at 1.5 K. As an example for spectroscopy applications of such
resonators we detect the superconducting transition of a bulk tin sample at
multiple probing frequencies.
