  The space-like electromagnetic form factor is expressed in terms of the
overlap integral of the initial and final meson wave functions written as
Lorentz covariant distributions of internal momenta. The meson constituents are
assumed to be valence quarks and an effective vacuum-like field. The momentum
of the latter represents a relativistic generalization of the potential energy
of the quark system. The calculation is fully Lorentz covariant and the form
factors of the charged mesons are normalized to unity at t=0. The numerical
results have been obtained by freezing the transverse degrees of freedom. We
found r_\pi^2=0.434 fm^2, r_{K^+}^2=0.333 fm^2, r_{K^0}^2=-0.069 fm^2 by taking
m_{u,d}=430 MeV, m_s=700 MeV.
