Monopole signal for the considered combined Comptonization and diffuse FF distortion models after subtracting the current CMB spectrum in the blackbody approximation at the effective temperature T₀. Solid and three dots-dashed lines refer to positive and negative values, respectively. Free-free distortions are evaluated for a reionization model compatible with Planck results and for an extreme model involving a population of ionized halos. Comptonization distortions are evaluated for u = 2 × 10⁻⁶ and u = 10⁻⁷, representative of imprints by astrophysical or minimal reionization models. For comparison with previous works that focussed on the CMB, they are also displayed, other than in terms of antenna temperature (left panel), which is typically used at radio frequencies, in terms of equivalent thermodynamic temperature (right panel); the differences appear above ≃40 GHz. See also the legend and the text.