  A structure formed by combined lattices of infinitely long wires and
split-ring resonators is studied. A dispersion equation is derived and then
used to calculate the effective permittivity and permeability in the frequency
band where the lattice can be homogenized. The backward wave region in which
both the effective permittivity and permeability are negative is analyzed. Some
open and controversial questions are discussed. It is shown that previous
experimental results confirming the existence of backward waves in such a
structure can be in deed explained in terms of negative material parameters.
However, these parameters are not quasi-static and thus the known analytical
formulas for the effective material parameters of this structure, which have
been widely used and discussed in the literature, were not correct, and it was
the reason of some objections to the authors of that experiment.
