1. Propagation Effects on Communication Links
Networks and Communication Department
Dr. Marwah Ahmed

2. The basic communications principles assumed a clear Line-of-Sight (LOS) and no deleterious propagation effects. However, real wireless systems are a complicated interplay between the antenna and propagation effects, especially for mobile and portable applications where there is a dynamic propagation environment.
1-May-19
Networks and Communication Department

3. 1-May-19
Networks and Communication Department

4. Atmosphere
About 99% of the total atmospheric mass is concentrated in the first 20 miles (32 km) above Earth's surface.
All weather phenomena occur within the troposphere, although turbulence may extend into the lower portion of the stratosphere.
Atmospheric regions are also named in other ways, as Ozone layer, and Ionosphere layer.
1-May-19
Networks and Communication Department

5. Ionosphere
Ionosphere: This is the region of the atmosphere that contains ions, created by the interaction of solar radiation with gas particles.
The ionosphere overlaps with the mesosphere and thermosphere, going up to an altitude of 550 km.
Its value in practical terms is that it enables the propagation of radio wave signals, which bounce off the ions and can be transmitted to distant places on the Earth.
1-May-19
Networks and Communication Department

6. 1-May-19
Networks and Communication Department

7. Ground Wave Propagation

8. Ground Wave Propagation
Follows contour of the earth
Can Propagate considerable distances
Frequencies up to 2 MHz
Example
AM radio

9. Sky Wave Propagation

10. Sky Wave Propagation
Signal reflected from ionized layer of atmosphere back down to earth
Signal can travel a number of hops, back and forth between ionosphere and earth’s surface
Reflection effect caused by refraction
Examples
Amateur radio

11. Line-of-Sight Propagation

12. Line-of-Sight Propagation
Transmitting and receiving antennas must be within line of sight
Satellite communication – signal above 30 MHz not reflected by ionosphere
Ground communication – antennas within effective line of site due to refraction
Refraction – bending of microwaves by the atmosphere
Velocity of electromagnetic wave is a function of the density of the medium
When wave changes medium, speed changes
Wave bends at the boundary between mediums

13. Attenuation
Strength of signal falls off with distance over transmission medium
Attenuation factors for unguided media:
Received signal must have sufficient strength so that circuitry in the receiver can interpret the signal
Signal must maintain a level sufficiently higher than noise to be received without error
Attenuation is greater at higher frequencies, causing distortion

14. Q & A
1-May-19
Networks and Communication Department