  Non-Planckian (NP) spectral modifications of the CMB radiation spectrum can
be produced due to the existence of a non-zero value of the plasma frequency at
the recombination epoch. We present here an analysis of NP effects on the radio
cosmological background and we derive, for the first time, predictions of their
amplitude on three different observables: the CMB spectrum, the
Sunyaev-Zel'dovich (SZ) effect in cosmic structures, and the 21-cm background
temperature brightness change. We find that NP effect can manifest in the CMB
spectrum at $\nu \simlt 400$ MHz as a drastic cut-off in the CMB intensity.
Using the available CMB data in the relevant $\nu$ range (i.e., mainly at
$\simlt 1$ GHz and in the COBE-FIRAS data frequency range), we derive upper
limits on the plasma frequency $\nu_p$ = 206, 346 and 418 MHz at 1, 2 and 3
$\sigma$ confidence level, respectively. We find that the difference between
the pure Planck spectrum and the one modified by NP effects is of the order of
mJy/arcmin$^2$ at $\nu \simlt 0.5$ GHz and it becomes smaller at higher
frequencies where it is $\sim 0.1$ mJy/arcmin$^2$ at $\nu \simgt 150$ GHz, thus
indicating that the experimental route to probe NP effects in the early
universe is to observe the radio cosmological background at very low
frequencies.(abridged)
