  The Nan\c{c}ay Radioheliograph (NRH) routinely produces snapshot images of
the full sun at frequencies between 150 and 450 MHz, with typical resolution 3
arcmin and time cadence 0.2 s. Combining visibilities from the NRH and from the
Giant Meterwave Radio Telescope (GMRT) allows us to produce images of the sun
at 236 or 327 MHz, with a large FOV, high resolution and time cadence. We seek
to investigate the structure of noise storms (the most common non-thermal solar
radio emission). We focus on the relation of position and altitude of noise
storms with the observing frequency and on the lower limit of their sizes. We
present results for noise storms on four days. The results consist of an
extended halo and of one or several compact cores with relative intensity
changing over a few seconds. We found that core sizes can be almost stable over
one hour, with a minimum in the range 31-35 arcsec (less than previously
reported) and can be stable over one hour. The heliocentric distances of noise
storms are $\sim 1.20$ and 1.35 $R_{\odot}$ at 432 and 150 MHz, respectively.
Regions where storms originate are thus much denser than the ambient corona and
their vertical extent is found to be less than expected from hydrostatic
equilibrium. The smallest observed sizes impose upper limits on broadening
effects due to scattering on density inhomogeneities in the low and medium
corona and constrain the level of density turbulence in the solar corona. It is
possible that scatter broadening has been overestimated in the past, and that
the observed sizes cannot only be attributed to scattering. The vertical
structure of the noise storms is difficult to reconcile with the classical
columnar model.
