  Context: The study of radio emission from core-collapse supernovae (SNe)
probes the interaction of the ejecta with the circumstellar medium (CSM) and
reveals details of the mass-loss history of the progenitor. Aims: We report
observations of the type IIP supernova SN\,2016X during the plateau phase, at
ages between 21 and 75 days, obtained with the Karl G. Jansky Very Large Array
(VLA) radio observatory. Methods: We modelled the radio spectra as
self-absorbed synchrotron emission, and we characterised the shockwave and the
mass-loss rate of the progenitor. We also combined our results with previously
reported X-ray observations to verify the energy equipartition assumption.
Results: The properties of the shockwave are comparable to other type IIP
supernovae. The shockwave expands according to a self-similar law $R \propto
t^m$ with $m=0.76 \pm 0.08$, which is notably different from a constant
expansion. The corresponding shock velocities are approximately 10700 - 8000 km
s$^{-1}$ during the time of our observations. The constant mass-loss rate of
the progenitor is $\dot{M}=$ (7.8 $\pm$ 0.9)\,$\times 10^{-7} \alpha^{-8/19}
(\epsilon_B/0.1)^{-1} M_{\odot}$ yr$^{-1}$, for an assumed wind velocity of 10
km s$^{-1}$. We observe spectral steepening in the optically thin regime at the
earlier epochs, and we demonstrate that it is caused by electron cooling via
the inverse Compton effect. We show that the shockwave is characterised by a
moderate deviation from energy equipartition by a factor of $\epsilon_e /
\epsilon_B \approx 28$, being the second type IIP supernova to show such a
feature.
