  Planar superconducting microwave resonators are key elements in a variety of
technical applications and also act as sensitive probes for microwave
spectroscopy of various materials of interest. Here superconducting Pb is a
suitable material as a basis for microwave stripline resonators.
  To utilize Pb stripline resonators in a variable magnetic field (e.g. in
ESR), the electrodynamics of such resonators in finite magnetic field has to be
well understood. Therefore we performed microwave transmission measurements on
superconducting Pb stripline resonators in a variable, parallel magnetic field.
We determined surface resistance, penetration depth as well as real and
imaginary parts, \sigma$_1$ and \sigma$_2$, of the complex conductivity of
superconducting Pb as a function of magnetic field. Here we find features
reminiscent of those in temperature-dependent measurements, such as a maximum
in \sigma$_1$ (coherence peak). At magnetic fields above the critical field of
this type-I superconductor we still find low-loss microwave response, which we
assign to remaining superconductivity in the form of filaments within the Pb.
Hysteresis effects are found in the quality factor of resonances once the swept
magnetic field has exceeded the critical magnetic field. This is due to normal
conducting areas that are pinned and can therefore persist in the
superconducting phase. Besides zero-field-cooling we show an alternative way to
eliminate these even at T<T$_c$. Based on our microwave data, we determine the
critical magnetic field and the critical temperature of Pb in a temperature
range between 1.6K and 6.5K and magnetic fields up to 140mT, showing good
agreement with BCS predictions. We study a Sn sample in a Pb resonator to
demonstrate the applicability of superconducting Pb stripline resonators in the
experimental study of other (super-)conducting materials in a variable magnetic
field.
