  We present an analytical model for the evolution of extended active galactic
nuclei (AGNs) throughout their full lifecycle, including the initial jet
expansion, lobe formation, and eventual remnant phases. A particular focus of
our contribution is on the early jet expansion phase, which is traditionally
not well captured in analytical models. We implement this model within the
Radio AGN in Semi-Analytic Environments (RAiSE) framework, and find that the
predicted radio source dynamics are in good agreement with hydrodynamic
simulations of both low-powered Fanaroff-Riley Type-I and high-powered Type-II
radio lobes. We construct synthetic synchrotron surface brightness images by
complementing the original RAiSE model with the magnetic field and
shock-acceleration histories of a set of Lagrangian tracer particles taken from
an existing hydrodynamic simulation. We show that a single set of particles is
sufficient for an accurate description of the dynamics and observable features
of Fanaroff-Riley Type-II radio lobes with very different jet parameters and
ambient density profile normalisations. Our new model predicts that the lobes
of young (< 10 Myr) sources will be both longer and brighter than expected at
the same age from existing analytical models which lack a jet-dominated
expansion phase; this finding has important implications for interpretation of
radio galaxy observations. The RAiSE code, written in Python, is publicly
available on GitHub and PyPI.
