  We use the recent releases of sensitive VLA/LOFAR large-area surveys at 340
MHz and 54 MHz, in conjunction with the 1.4 GHz NVSS, to accurately determine
the `spectral index - flux density relation' ($\alpha$ - S) for extragalactic
radio sources selected at metre and decametre wavelengths, the latter for the
first time. This newly determined $\alpha$ - S$_{\rm 340~MHz}$ relation shows a
progressive flattening of $\alpha_{\rm median}$ towards lower flux densities,
starting from its steepest value (peak)occurring near S$_{\rm 340~MHz}$ $\sim$
1-2 Jy. This resolves the controversy extant in the literature since the 1980s.
The $\alpha$ - S$_{\rm 54~MHz}$ relation, too, shows a spectral index
flattening with decreasing flux density which, however, is significantly milder
and the relation is less sharply peaked than that found at 340 MHz. A possible
reason for the difference could be that the 54 MHz sample has a distinctly
stronger/conspicuous presence ( at $\sim$ 20% level) of very steep spectrum
sources having $\alpha_{54}^{1400} <$ -1.3, most of which are probably
associated with clusters of galaxies.
