  Microwave spectroscopy is a powerful experimental tool to reveal information
on the intrinsic properties of superconductors. Superconducting stripline
resonators, where the material under study constitutes one of the ground
planes, offer a high sensitivity to investigate superconducting bulk samples.
In order to improve this measurement technique, we have studied stripline
resonators made of niobium, and we compare the results to lead stripline
resonators. With this technique we are able to determine the temperature
dependence of the complex conductivity of niobium and the energy gap
$\Delta(0)=2.1$ meV. Finally we show measurements at the superconducting
transition of a tantalum bulk sample using niobium stripline resonators.
