  In a parameter study extending to jet densities of $10^{-5}$ times the
ambient one, I have recently shown that light large scale jets start their
lives in a spherical bow shock phase. This allows an easy description of the
sideways bow shock propagation in that phase. Here, I present new, bipolar,
simulations of very light jets in 2.5D and 3D, reaching the observationally
relevant scale of $>200$ jet radii. Deviations from the early bow shock
propagation law are expected because of various effects. The net effect is,
however, shown to remain small. I calculate the X-ray appearance of the shocked
cluster gas and compare it to Cygnus A and 3C 317. Rings, bright spots and
enhancements inside the radio cocoon may be explained.
