  I present low-frequency observations of the lobes of the microquasar GRS
1758-258, which, together with earlier radio observations, provide the best
constraints yet on the lobe spectra, and show that they are consistent with the
electron energy indices expected from standard shock acceleration models. There
is no statistically significant evidence for synchrotron ageing along the
length of the lobes, which is consistent with expectation if the magnetic field
strength is close to the minimum-energy value. If the environment of the lobes
has typical ISM properties, the lobes will dissipate on timescales of thousands
of years or less, long before synchrotron ageing can become significant.
