  We conducted radio-interferometric observations of the well known binary
cataclysmic system AM Herculis. This particular system is formed by the
magnetic white dwarf (primary) and the red dwarf (secondary), and is the
prototype of so-called polars. Our observations were conducted with the
European VLBI Network (EVN) in the e-EVN mode at 5 GHz. We obtained six
astrometric measurements spanning one year, which make it possible to update
the annual parallax for this system with the best precision to date
($\pi=11.29\pm0.08$ mas), equivalent to the distance of $88.6\pm0.6$ pc. The
system was observed mostly in the quiescent phase (visual magnitude
$m_v\sim15.3$), when the radio emission was at the level of about $300~\mu$Jy.
Our analysis suggests that the radio flux of AM Herculis is modulated with the
orbital motion. Such specific properties of the radiation can be explained
using the emission mechanism similar to the scenario proposed for V471 Tauri
and, in general, for RS CVn type stars. In this scenario the radio emission
arises near the surface of the red dwarf, where the global magnetic field
strength may reach a few kG. We argue that the quiescent radio emission
distinguishes AM Herculis together with AR Ursae Majoris (the second known
persistent radio polar) from other polars, as the systems with a magnetized
secondary star.
