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In contrast, at very low operating frequencies (<30 MHz) where the
wavelength is very large, radio signals can propagate extremely long distances (in fact
around the earth) owing to the various layers within the ionosphere acting as giant wave
guides reflecting the signal to and fro between the layers. Unfortunately the properties
of the ionospheric layers change with time of day, season, temperature, and so on,
and the propagation characteristics can be very unpredictable and data
transmission rates are often limited to a few kbps. Above 30 MHz, ionospheric reflection
begins to pack up and propagation is principally by line-of-sight path. In order to
communicate over long distances, high antenna towers are required to combat the curvature
of the earth.
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