In the earth environment,
electromagnetic waves propagate in ways that depend not only on their own
properties but also on those of the environment itself.
The waves with frequencies above HF
(High Frequency) travel in a straight line. They propagate by means of so
called waves. They travel in the troposphere, the portion of the atmosphere
closest to the ground.
Frequencies below the HF (High
Frequency) range travel around the curvature of the earth. They are called
Ground Waves or Surface Waves. All broad-casting radio signals received in day
time propagate by means of surface waves.
Waves in HF (High Frequencies)
ranges are reflected by the ionized layers of the atmosphere and are called sky
waves. Such signals are beamed into the sky and comedown again after
reflection, returning to earth.
Types of propagation can be written
as:
1.
Line of Sight Propagation (Space Waves): It requires a
path where both antennas are visible to one another i.e. both transmitting and
receiving antennas must be within line of sight. VHF (Very High Frequencies)
& UHF (Ultra High Frequencies) communication typically use this path. For
example Satellite Communication, Ground Communication.
Fig1: Line of sight propagation
1.
Ground Wave Propagation (Ground Waves): Ground waves
progresses along the surface of the earth i.e. the signal follows contour of
the earth and can propagate considerable distances. This type of wave induces
currents in the ground over which it passes and thus loses some energy by
absorption. Frequencies up to 2MHz (VLF and MW uses it). For example AM Radio.
Fig2: Ground
wave propagation
1.
Sky-Wave Propagation: In sky wave
propagation the waves reflects back to the earth from the Ionosphere of the
earth atmosphere i.e. the signal is reflected from the ionized layer of the
atmosphere and travel forward with a number of hops, back and forth. For
example SW Radio (HF uses it).
Fig3: Sky wave propagation
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