Not-to-scale sketch showing the geometry of cyclotron maser emission on a single dipolar field line that lies in the plane of the page. The higher frequency emission cone (blue) is oriented at a smaller polar angle, α, when compared to the emission cone at a lower frequency (red). Here, L is the distance to the field line for a polar angle of 0°, R is the distance from the star’s centre to the emission cone along the field line, θ is the angle between the direction of emission and the interior wall of the cone, and Δθ is the angular width of the emission cone’s wall. The effects of α and θ on the emission frequency has been exaggerated here and is not to scale for the LOFAR frequency range.