  Microwave-field distribution, dissipation, and surface impedance are
theoretically investigated for superconductors with laminar grain boundaries
(GBs). In the present theory we adopt the two-fluid model for intragrain
transport current in the grains, and the Josephson-junction model for
intergrain tunneling current across GBs. Results show that the surface
resistance $R_s$ nonmonotonically depends on the critical current density
$J_{cj}$ at GB junctions, and $R_s$ for superconductors with GBs can be smaller
than the surface resistance $R_{s0}$ for ideal homogeneous superconductors
without GBs.
