  We performed MHD simulations of very light bipolar jets with density
contrasts down to 10^-4 in axisymmetry, which were injected into a medium of
constant density and evolved up to 200 kpc (200 r_j) full length. These jets
show weak and roundish bow shocks as well as broad cocoons and thermalize their
kinetic energy very efficiently. We argue that very light jets are necessary to
match low-frequency radio observations of radio lobes as well as the bow shocks
seen in X-rays. Due to the slow propagation, the backflows and their turbulent
interaction in the midplane are important for a realistic global appearance.
