Explain Propagation Modes.
In the earth environment, electromagnetic waves propagate in ways that depend own properties but also on those of the environment itself.
The various methods of propagation depend largely on frequency.
Wireless transmissions propagate in three modes. They are −
1. Ground Wave
2. Sky Wave
3. Line of Sight (LOS)
1. Ground Wave Propagation
Radio waves in the VLF band propagate in a ground, or surface wave. The wave is connected at one end to the surface of the earth and to the ionosphere at the other.
The ionosphere is the region above the troposphere (where the air is), from about 50 to 250 miles above the earth.
It is a collection of ions, which are atoms that have some of their electrons stripped off leaving two or more electrically charged objects. The sun's rays cause the ions to form which slowly modified.
Fig. Ground Wave Propagation
The propagation of radio waves in the presence of ions is drastically different than in air, which is why the ionosphere plays an important role in most modes of propagation.
Ground waves travel between two limits, the earth and the ionosphere, which acts like a channel. Since the channel curves with the earth, the ground wave will follow. Therefore very long range propagation is possible using ground waves.
Example of ground wave communication is AM radio.
2. Sky Waves Propagation
Radio waves in the LF and MF ranges may also propagate as ground waves, but suffer significant losses, or are attenuated, particularly at higher frequencies. But as the ground wave mode fades out, a new mode develops: the sky wave.
Sky waves are reflections from the ionosphere. While the wave is in the ionosphere, it is strongly bent, or refracted, ultimately back to the ground.
From a long distance away this appears as a reflection. Long ranges are possible in this mode also, up to hundreds of miles.
Sky waves in this frequency band are usually only possible at night, when the concentration of ions is not too great since the ionosphere also tends to attenuate the signal.
Fig. Sky Wave Propagation
However, at night, there are just enough ions to reflect the wave but not reduce its power too much.
Example: Used in amateur radio, CB radio, international broadcast such as BBC.
3. Line-of-sight propagation
Fig. Line-of-sight propagation
Transmitting and receiving antennas must be within line of sight.
1. Satellite communication – signal above 30 MHz not reflected by ionosphere
2. Ground communication – antennas within effective line of site due to refraction
Refraction – bending of microwaves by the atmosphere.
1. Velocity of electromagnetic wave is a function of the density of the medium
2. When wave changes medium, speed changes
3. Wave bends at the boundary between mediums
Optical and Radio line of sight
𝒅 = 𝟑. 𝟓𝟕√h' and 𝒅 = 𝟑. 𝟓𝟕√𝑲𝒉'
Where:
d = distance between antenna and horizon (km)
h = antenna height (m)
K = adjustment factor to account for refraction, rule of thumb K = 4/3
Maximum distance between two antennas for LOS propagation:
= 𝟑. (4𝑲𝒉𝟏' + 4𝑲𝒉𝟐')
Where:
h1 = height of antenna one
h2 = height of antenna two
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