  The electronic properties of superconducting Sn films ($T_c \approx$ 3.8 K)
change significantly when reducing the film thickness down to a few nm, in
particular close to the percolation threshold. The low-energy electrodynamics
of such Sn samples can be probed via microwave spectroscopy, e.g. with
superconducting stripline resonators.
  Here we study Sn thin films, deposited via thermal evaporation -ranging in
thickness between 38 nm and 842 nm- which encompasses the percolation
transition. We use superconducting Pb stripline resonators to probe the
microwave response of these Sn films in a frequency range between 4 GHz and 20
GHz at temperatures from 7.2 K down to 1.5 K. The measured quality factor of
the resonators decreases with rising temperature due to enhanced losses. As a
function of the sample thickness we observe three regimes with significantly
different properties: samples below percolation, i.e. ensembles of disconnected
superconducting islands, exhibit dielectric properties with negligible losses,
demonstrating that macroscopic current paths are required for appreciable
dynamical conductivity of Sn at GHz frequencies. Thick Sn films, as the other
limit, lead to low-loss resonances both above and below $T_c$ of Sn, as
expected for bulk conductors. But in an intermediate thickness regime, just
above percolation and with labyrinth-like morphology of the Sn, we observe a
quite different behavior: the superconducting state has a microwave response
similar to the thicker, completely covering films with low microwave losses;
but the metallic state of these Sn films is so lossy that resonator operation
is suppressed completely.
