  We have conducted a study at radio wavelengths of the spectral behaviour of
the supernova remnant (SNR) CTB 80. Based on an homogenised data set of
integrated flux densities, we calculated for the whole SNR a radio index -0.36
+/- 0.02. The shape of the global spectrum suggests absorption by ionized gas
in the interstellar medium (ISM) along the line of sight. Spatial spectral
variations across the SNR are investigated based on high-angular resolution
data at 240, 324, 610, and 1380 MHz using different techniques. The three
extended arms associated with this SNR, show a clear indication of spectral
steepening when moving outwards from the central nebula, with variations of up
to - 0.9. However, while the spectral steepening is smooth along the eastern
arm, the northern and soutwestern arms include locally flatter structures,
which in all cases coincide with radio, IR, and optical emission enhancements.
We interpret this spectral property as the result of the combination of two
different particle populations: aging relativistic electrons injected by PSR
B1951+32 and particles accelerated at the sites where the SNR shock front
encounters interstellar gas inhomogeneities. Concerning the central nebula, the
angular resolution of the available database does not permit a detailed
spectral study of the core region, i.e. the 45'' region around PSR B1951+32,
where we can only confirm an average spectral index alpha=0.0. The surrounding
8' plateau nebula has an <alpha> ~ -0.25, with a peak of alpha ~ -0.29
coincident with a secondary maximun located at the termination of a twisted
filament that trails to the east, behind the pulsar.
