Why is communication using line of sight mode

Why is communication using line of sight mode limited to frequencies above 40 MHz?

At frequencies above 40 MHz, communication is limited by line of sight mode. At these frequencies, the sizes of transmitting and receiving antennas are relatively smaller and can be placed at heights of many wavelengths above the ground. Frequencies less than 30 MHz can be reflected from ionosphere and can be relayed over large distances using sky wave propagation.

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