Terman also says the radio horizon as seen from a ground-mounted vertical
antenna is at distance of -

50 / Cuberoot(FreqMhz) miles.

This does not mean the ground wave suddenly weakens at this distance but
that Earth curvature and atmospheric refraction begin to have a significant
effect on ground-wave propagation.

At MF and LF, useful ground-wave propagation can occur at distances
considerably greater than the radio horizon. At HF the skywave can provide
stronger signals.
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Reg, G4FGQ