  The persistently bright ultra-compact neutron star low-mass X-ray binary 4U
1820$-$30 displays a $\sim$170 d accretion cycle, evolving between phases of
high and low X-ray modes, where the 3 -- 10 keV X-ray flux changes by a factor
of up to $\approx 8$. The source is generally in a soft X-ray spectral state,
but may transition to a harder state in the low X-ray mode. Here, we present
new and archival radio observations of 4U 1820$-$30 during its high and low
X-ray modes. For radio observations taken within a low mode, we observed a flat
radio spectrum consistent with 4U 1820$-$30 launching a compact radio jet.
However, during the high X-ray modes the compact jet was quenched and the radio
spectrum was steep, consistent with optically-thin synchrotron emission. The
jet emission appeared to transition at an X-ray luminosity of $L_{\rm X (3-10
keV)} \sim 3.5 \times 10^{37} (D/\rm{7.6 kpc})^{2}$ erg s$^{-1}$. We also find
that the low-state radio spectrum appeared consistent regardless of X-ray
hardness, implying a connection between jet quenching and mass accretion rate
in 4U 1820$-$30, possibly related to the properties of the inner accretion disk
or boundary layer.
