Data transmission through air (and not through any channel) is called unguided transmission. Data is carried over electro-magnetic radiation in the form of radio waves. Such propagation is classified by the type of wave used for propagation. There are three types of RF (radio frequency) propagation.
- Ground wave
- Ionospheric
- Line of Sight (LOS)
Ground Wave Propagation
Ground wave propagation follows the curvature of the earth. Ground waves have carrier frequencies up to 2 MHz. The AM radio is an example of ground wave propagaiton.
Ionospheric Propagation
Ionospheric propagation bounces off the Earh's ionspheric layer in the upper atmosphere. It is sometimes called double hop propagation. It operates in the frequency range of 30 - 85 MHz. Because it depends on the Earth's ionosphere, it changes with the weaher and the time of the day. The signal sent from a radio tower bounces off the ionosphere and comes back to earth to a receiving station.
Line of Sight Propagation
Line of sight propagation transmits exactly in the line of sight. The receiving station must be in the view of the transmit station. It is someimes called space wave or tropospheric propagation. It is limited by the curvature of the Eartth for ground-based stations (100 km. from horizon to horizon). Reflected waves can cause problems. Examples of line of sight propagation are FM radio, and satellite microwave.
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