  In the case of an initially conical jet, we study the relation between jet
collimation by the external pressure and large-scale morphology. We first
consider the important length-scales in the problem, and then carry out
axisymmetric hydrodynamic simulations that include, for certain parameters, all
these length-scales. We find three important scales related to the collimation
region: (i) where the sideways ram-pressure equals the external pressure, (ii)
where the jet density equals the ambient density, and (iii) where the forward
ram-pressure falls below the ambient pressure. These scales are set by the
external Mach-number and opening angle of the jet. We demonstrate that the
relative magnitudes of these scales determine the collimation, Mach-number,
density and morphology of the large scale jet. Based on analysis of the shock
structure, we reproduce successfully the morphology of Fanaroff-Riley (FR)
class I and II radio sources. Within the framework of the model, an FR I radio
source must have a large intrinsic opening angle. Entrainment of ambient gas
might also be important. We also show that all FR I sources with radio lobes or
similar features must have had an earlier FR II phase.
