  In this paper, we report mean-field and effective-field renormalization group
calculations on the isotropic Heisenberg antiferromagnetic model under a
longitudinal magnetic field. As is already known, these methods, denoted by
MFRG and EFRG, are based on the comparison of two clusters of different sizes,
each of them trying to mimic certain Bravais lattice. Our attention has been on
the obtantion of the critical frontier in the plane of temperature versus
magnetic field, for the simple cubic and the body-centered cubic lattices. We
used clusters with $N=1,2,4$ spins so as to implement MFRG-12, EFRG-12 and
EFRG-24 numerical equations. Consequently, the resulting frontier lines show
that EFRG approach overcomes the MFRG problems when clusters of larger sizes
are considered.
