  Binary systems that harbor a non-accreting pulsar are efficient non-thermal
emitters, from radio to gamma rays. This broadband emission is thought to come
from the region where the companion star and pulsar winds collide. A
paradigmatic example of this source type is PSR B1259$-$63. Whereas the
high-energy radiation probes the shock structure at the binary scale, the radio
emission is produced well outside the system, in regions where the shocked
stellar and pulsar winds are likely mixing due to hydrodynamical instabilities.
Understanding the evolution of the shocked flow depends strongly on a proper
characterization of the low-energy radiation. We have performed numerical
calculations of the radio emission produced in a high-mass binary hosting a
young pulsar. Adopting a prescription for the shocked flow in the system and
the non-thermal particle injection, we have generated synthetic radio emission
maps that can be compared with observations. Preliminary results suggest that
the emitting bulk flow should be rather slow, with a pulsar wind magnetization
at the shock higher than in isolated pulsars.
