Tropospheric scatter

Tropospheric scatter (also known as troposcatter) is a method of communicating with microwave radio signals over considerable distances – often up to 300 kilometres (190 mi), and further depending on terrain and climate factors. This method of propagation uses the tropospheric scatter phenomenon, where radio waves at UHF and SHF frequencies are randomly scattered as they pass through the upper layers of the troposphere. Radio signals are transmitted in a narrow beam aimed just above the horizon in the direction of the receiver station. As the signals pass through the troposphere, some of the energy is scattered back toward the Earth, allowing the receiver station to pick up the signal.

Normally, signals in the microwave frequency range travel in straight lines, and so are limited to line of sight applications, in which the receiver can be 'seen' by the transmitter. Communication distances are limited by the visual horizon to around 30–40 miles (48–64 km). Troposcatter allows microwave communication beyond the horizon.

Because the troposphere is turbulent and has a high proportion of moisture the tropospheric scatter radio signals are refracted and consequently only a tiny proportion of the radio energy is collected by the receiving antennas. Frequencies of transmission around 2 GHz are best suited for tropospheric scatter systems as at this frequency the wavelength of the signal interacts well with the moist, turbulent areas of the troposphere, improving signal to noise ratios.

Historically, high gain dish or billboard antennas were required for tropospheric scatter systems as the propagation losses are very high; only about one billion-billionth (1 x 10⁻¹²) of the transmit power is available at the receiver. Paths were established at distances over 1,000 km. They required antennas ranging from 9 meters to 36 meters and amplifiers ranging from 1 kW to 50 kW. These were analogue systems which were capable of transmitting a few voice channels.

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