  Diffractive electroproduction of rho and phi mesons is measured at HERA with
the H1 detector in the elastic and proton dissociative channels. The data
correspond to an integrated luminosity of 51 pb^-1. About 10500 rho and 2000
phi events are analysed in the kinematic range of squared photon virtuality 2.5
< Q^2 < 60 GeV^2, photon-proton centre of mass energy 35 < W < 180 GeV and
squared four-momentum transfer to the proton |t| < 3 GeV^2. The total,
longitudinal and transverse cross sections are measured as a function of Q^2, W
and |t|. The measurements show a transition to a dominantly "hard" behaviour,
typical of high gluon densities and small q\bar{q} dipoles, for Q^2 larger than
10 to 20 GeV^2. They support flavour independence of the diffractive exchange,
expressed in terms of the scaling variable (Q^2 + M_V^2)/4, and proton vertex
factorisation. The spin density matrix elements are measured as a function of
kinematic variables. The ratio of the longitudinal to transverse cross
sections, the ratio of the helicity amplitudes and their relative phases are
extracted. Several of these measurements have not been performed before and
bring new information on the dynamics of diffraction in a QCD framework. The
measurements are discussed in the context of models using generalised parton
distributions or universal dipole cross sections.
