Q.What is the frequency range of signals that propagate best over the surface of the earth?

In surface-wave (ground-wave) propagation, the radio wave hugs the Earth's curved surface, diffracting around it, but loses energy to the ground as it travels.

Surface wave (ground wave) propagation: When a radio wave is radiated from a transmitting antenna close to the ground, it can travel along the surface of the Earth, guided by the boundary between the Earth (a partial conductor) and the atmosphere. This mode of propagation is called surface-wave or ground-wave propagation, and is one of the three principal modes of electromagnetic wave propagation (the others being sky wave and space wave).

The wave follows the curvature of the Earth to some extent because of diffraction — the ground effectively acts as a wave-guide. However, the conducting Earth absorbs energy from the wave as it travels (induced currents dissipate energy as heat in the ground), so the wave is progressively attenuated; the attenuation increases rapidly with frequency, which is why this mode is practical mainly at low and medium frequencies (below a few MHz, roughly up to the MF/AM broadcast band).

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