Observed spectrum of the Coma radio halo (black data-points) and the power law (α = 1.22 ± 0.04) that best fits the spectrum at lower frequencies, ν ≤ 1.4 GHz (solid line). Empty points are the data with the SZ correction added, the SZ-decrement is calculated from Planck measurements by adopting an aperture radius =0.48   R₅₀₀. The dotted line is a synchrotron model assuming a broken power-law energy distribution of the emitting electrons, N(E) ∝ E^− δ and ∝ E^{− (δ + Δδ)}, at lower and higher energies, with δ = 2.4 and Δδ = 1.6. The dashed line is a synchrotron model assuming a power-law (δ = 2.5) with a high energy cut-off, which occurs for instance in (homogeneous) reacceleration models (see e.g. Schlickeiser et al. 1987). The red point is the flux measured in the high-sensitivity observations at 330 MHz by Brown & Rudnick (2011) within an aperture radius =0.48   R₅₀₀.