  The observed excess radio background has remained a puzzle for over a decade.
A recent new physics solution involves dark matter that decays into dark
photons in the presence of a thermal dark photon background. The produced
non-thermal dark photon spectrum then converts into standard photons around the
reionization era, yielding an approximate power-law radio excess with
brightness temperature $T(\nu)\simeq \nu^{-2.5}$ over a wide range of
frequencies, $\nu$. This simple power-law model comes intriguingly close to the
current data, even if several ingredients are required to make it work. In this
paper, we investigate some of the details of this model, showcasing the
importance of individual effects. In particular, significant deviation from a
power law are present at $\nu\lesssim 100\,{\rm MHz}$ and $\nu\gtrsim 1\,{\rm
GHz}$. These effects result in improving the fit to data compared to a
power-law spectrum, and may become testable in future observations. We also
highlight independent signatures that can be tested with future CMB spectral
distortion experiments such as {\it PIXIE}.
