  The synchrotron radio morphology of bilateral supernova remnants depends on
the mechanisms of particle acceleration and on the viewing geometry. However,
unlike X-ray and $\gamma$-ray morphologies, the radio emission does not depend
on the cut-off region of the parent electron population, making it a simpler
and more straightforward tool to investigate the physics of cosmic ray
production in SNRs. Our aim is to derive from the radio morphology tight
constraints on the direction of the local magnetic field and its gradient, and
on the obliquity dependence of the electron injection efficiency. We perform a
set of 3D MHD simulations describing the expansion of a spherical SNR through a
magnetized medium with a non-uniform ISMF. From the simulations, we derive
non-thermal radio maps and compare them with observations of the SN1006
remnant. We find that the radio morphology of SN1006 at 1 GHz is best-fitted by
a model with quasi-parallel injection efficiency, a magnetic field aspect angle
of $38^\circ\pm 4^\circ$ with the line of sight, and a gradient of the field
strength toward the galactic plane, higher then the expected variations of the
large scale field of the Galaxy. We conclude that the radio limbs of SN1006 are
polar caps, not lying in the plane of sky}. The study of the synchrotron radio
emission of SNRs is of crucial importance to derive information on the galactic
magnetic field in the vicinity of the remnants, and more hints on the actual
injection efficiency scenario.
