  The properties of the covariant quantum fields on de Sitter spacetimes are
investigated focusing on the isometry generators and Casimir operators in order
to establish the equivalence among the covariant representations and the
unitary irreducible ones of the de Sitter isometry group. For the Dirac quantum
field it is shown that the spinor covariant representation, transforming the
Dirac field under de Sitter isometries, is equivalent with a direct sum of two
unitary irreducible representations of the $Sp(2,2)$ group, transforming alike
the particle and antiparticle field operators in momentum representation. Their
basis generators and Casimir operators are written down finding that the
covariant representations are equivalent with unitary irreducible ones from the
principal series whose canonical weights are determined by the fermion mass and
spin.
