  New radio data at 330 MHz are presented for the rich clusters Abell 665 and
Abell 2163, whose radio emission is characterized by the presence of a radio
halo. These images allowed us to derive the spectral properties of the two
clusters under study. The integrated spectra of these halos between 0.3 GHz and
1.4 GHz are moderately steep: alfa(0.3-1.4) = 1.04 and = 1.18, for A665 and
A2163, respectively. The spectral index maps, produced with an angular
resolution of the order of 1 arcmin, show features of the spectral index
(flattening and patches), which are indication of a complex shape of the
radiating electron spectrum, and are therefore in support of electron
reacceleration models. Regions of flatter spectrum are found to be related to
the recent merger activity in these clusters. This is the first strong
confirmation that the cluster merger supplies energy to the radio halo. In the
undisturbed cluster regions, the spectrum steepens with the distance from the
cluster center. This is interpreted as the result of the combination of the
magnetic field profile with the spatial distribution of the reacceleration
efficiency, thus allowing us to set constraints on the radial profile of the
cluster magnetic field.
