Context : Determining ages of the outer and the inner lobes of so-called double-double radio galaxies ( DDRGs ) is crucial for understanding the active cycles of galactic nuclei , the phases of interruption of the jet flow , and physical conditions in the surrounding galactic and intergalactic medium governing the jets ’ propagation .
A recognition and understanding of these conditions during the restarted jet activity is of special interest .

Aims : We determine the ages and other physical characteristics of the outer and the inner lobes of the DDRG J1548 - 3216 , as well as the properties of the surrounding environment during the original and the restarted phase of the jets ’ activity .

Methods : Using the new low-frequency and high-frequency radio images of this galaxy , we determined the shape of the spectrum along its lobes and performed the classical spectral-ageing analysis .
On the other hand , we applied the analytical model of the jet ’ s dynamics , which allowed us to derive the physical conditions for the source ’ s evolution during the original jet propagation through the unperturbed IGM , as well as those when the restarted new jet propagates inside the outer cocoon formed by the old jet material that passed through the jet terminal shock .

Results : The dynamical age estimate of the outer and the inner lobes is 132 \pm 28 Myr and \sim 9 \pm 4 Myr , respectively .
The synchrotron age in the outer lobes systematically rises from \sim 25 Myr in the vicinity of the lobes ’ edges to about 65–75 Myr in the centre of the old cocoon .
These ages imply an average expansion speed along the jets ’ axis : ( 0.012 \pm 0.003 ) c in the outer lobes and ( 0.058 \pm 0.025 ) c in the inner lobes , but the latter speed would be \sim 0.25 c when they were of age less than 1 Myr .
We find that the jet power during the restarted activity is about ten-fold fainter than that of the original jet .
Similar disproportion is found for the internal pressures and the magnetic field strengths in the old cocoon and those in the inner lobes .
This disproportion can be effectively reduced by assuming the same equations of state for the emitting particles and the magnetic fields within the old and the new lobes .
However , we think that our assumption of the non-relativistic equation of state for the old cocoon and the relativistic one for the new lobes is more justified .

Conclusions :