  A well known property of the gamma-ray sources detected by COS-B in the
1970s, by the Compton Gamma-ray Observatory in the 1990s and recently by the
Fermi observations is the presence of radio counterparts, in particular for
those associated to extragalactic objects. This observational evidence is the
basis of the radio-gamma-ray connection established for the class of active
galactic nuclei known as blazars. In particular, the main spectral property of
the radio counterparts associated with gamma-ray blazars is that they show a
flat spectrum in the GHz frequency range. Our recent analysis dedicated to
search blazar-like candidates as potential counterparts for the unidentified
gamma-ray sources (UGSs) allowed us to extend the radio-gamma-ray connection in
the MHz regime. We also showed that below 1 GHz blazars maintain flat radio
spectra. Thus on the basis of these new results, we assembled a low-frequency
radio catalog of flat spectrum sources built by combining the radio
observations of the Westerbork Northern Sky Survey (WENSS) and of the
Westerbork in the southern hemisphere (WISH) catalog with those of the NRAO
Very Large Array Sky survey (NVSS). This could be used in the future to search
for new, unknown blazar-like counterparts of the gamma-ray sources. First we
found NVSS counterparts of WSRT radio sources and then we selected flat
spectrum radio sources according to a new spectral criterion specifically
defined for radio observations performed below 1 GHz. We also described the
main properties of the catalog listing 28358 radio sources and their logN-logS
distributions. Finally a comparison with with the Green Bank 6-cm radio source
catalog has been performed to investigate the spectral shape of the
low-frequency flat spectrum radio sources at higher frequencies.
