  Within a covariant Bethe-Salpeter approach a rank-six separable
neutron-proton interaction kernel for the triplet coupled $^3S_1$-$^3D_1$
partial-wave state is constructed. Two different methods of a relativistic
generalization of initially nonrelativistic form factors parametrizing the
kernel are considered. The model parameters are determined by fitting the
elastic $^3S_1$ and $^3D_1$ phase shifts and the triplet scattering length as
well as the asymptotic $D/S$ ratio of the deuteron wave functions and the
deuteron binding energy. The $D$-state probability constraints 4-7\% are taken
into account. The deuteron magnetic moment is calculated. The half-off-shell
properties are further demonstrated by the Noyes-Kowalski functions. The first
test of the constructed kernel is performed by calculating the deuteron
electrodisintegration at three different kinematic conditions.
