Thermal fraction at λ6 cm versus the spectral index β of observed Galactic emission. The spectral index of thermal free-free emission is fixed to be −2.1, while the spectral index of non-thermal synchrotron emission is assumed to be −3.0 (black lines), −2.8 (red lines), and −2.6 (green lines). The curves calculated between λ6 cm and λ11 cm are shown by a solid line and λ6 cm and λ21 cm by a dashed line.