  We present detailed radio observations of the tidal disruption event (TDE)
ASASSN-19bt/AT2019ahk, obtained with the Australia Telescope Compact Array
(ATCA), the Atacama Large Millimeter/submillimeter Array (ALMA), and the
MeerKAT radio telescopes, spanning 40 to 1464 days after the onset of the
optical flare. We find that ASASSN-19bt displays unusual radio evolution
compared to other TDEs, as the peak brightness of its radio emission increases
rapidly until 457 days post-optical discovery and then plateaus. Using a
generalized approach to standard equipartition techniques, we estimate the
energy and corresponding physical parameters for two possible emission
geometries: a non-relativistic spherical outflow and a relativistic outflow
observed from an arbitrary viewing angle. We find that the non-relativistic
solution implies a continuous energy rise in the outflow from $E\sim10^{46}$
erg to $E\sim10^{49}$ erg with $\beta \approx 0.05$, while the off-axis
relativistic jet solution instead suggests $E\approx10^{52}$ erg with
$\Gamma\sim10$ erg at late times in the maximally off-axis case. We find that
neither model provides a holistic explanation for the origin and evolution of
the radio emission, emphasizing the need for more complex models. ASASSN-19bt
joins the population of TDEs that display unusual radio emission at late times.
Conducting long-term radio observations of these TDEs, especially during the
later phases, will be crucial for understanding how these types of radio
emission in TDEs are produced.
