Radio spectral index as a function of inclination angle. Panel a: integrated radio spectral index between 144 and 1400 MHz as a function of cos(i). Panel b: spectral curvature Δα = α_low − α_high as a function of cos(i). The high-frequency radio spectral index α_high is defined between 1400 and 5000 MHz. Both spectral index measurements show at most only a weak correlation with the inclination angle.