  We present new measurements of the diffuse radio emission from the Coma
cluster of galaxies at 1.4 GHz using the Effelsberg 100-m-telescope. The halo
source Coma C has an extent down to noise of about 80 arcmin corresponding to 3
Mpc (H_0 = 50 km/s/Mpc) in Coma. The radio map reveals clear similarities with
images of the extended X-ray halo of the Coma cluster. After subtracting the
contributions from point sources we obtained an integrated diffuse flux density
of S = 640 +- 35 mJy from Coma C. We derive relations between the various
observationally determined spectral indices and the spectral index of the
synchrotron emissivity, which allow one to achieve a rough estimate concerning
the consistency of the presently available data at different frequencies and to
place constraints on the emissivity index distribution. In the halo's core
region, the inferred emissivity index between 0.3 GHz and 1.4 GHz appears to be
in the range 0.4 - 0.75 implying that there must be some very effective
mechanism for particle acceleration operating in the intracluster medium.
