  An analytical model is presented for a rectangular lattice of isotropic
scatterers with electric and magnetic resonances. Each isotropic scatterer is
formed by putting appropriately 6 $\Omega$-shaped perfectly conducting
particles on the faces of a cubic unit cell. A self-consistent dispersion
equation is derived and then used to calculate correctly the effective
permittivity and permeability in the frequency band where the lattice can be
homogenized. The frequency range in which both the effective permittivity and
permeability are negative corresponds to the mini-band of backward waves within
the resonant band of the individual isotropic scatterer.
