30-01-2013, 11:14 AM
Propagation of Waves
[attachment=48855]
The process of communication involves the transmission of information from one
location to another. As we have seen, modulation is used to encode the information onto
a carrier wave, and may involve analog or digital methods. It is only the characteristics
of the carrier wave which determine how the signal will propagate over any significant
distance. This chapter describes the different ways that electromagnetic waves propagate.
Basics
An electromagnetic wave is created by a local disturbance in the electric and
magnetic fields. From its origin, the wave will propagate outwards in all directions.
If the medium in which it is propagating (air for example) is the same everywhere,
the wave will spread out uniformly in all directions.
Refraction
When the wave enters the new medium, the speed of propagation will change.
In order to match the incident and transmitted wave at the boundary, the transmitted
wave will change its direction of propagation. For example, if the new medium has
a higher index of refraction, which means the speed of propagation is lower, the
wavelength will become shorter (frequency must stay the same because of the boundary
conditions). For the transmitted wave to match the incident wave at the boundary,
the direction of propagation of the transmitted wave must be closer to perpendicular.
The relationship between the angles and indices of refraction is given by Snell's Law:
Interference
All electromagnetic waves can be superimposed upon each other without limit.
The electric and magnetic fields simply add at each point. If two waves with the
same frequency are combined there will a be a constant interference pattern caused
by their superposition. Interference can either be constructive, meaning the
strength increases as result, or destructive where the strength is reduced.
The amount of interference depends of the phase difference at a particular point.
It can be shown that constructive interference occurs for phase differences of 0-1200,
and 240-3600. Thus destructive interference occurs from 120-2400. For two identical
waves, no phase shift results in total constructive interference, where the strength
is maximum and 1800 phase shift will create total destructive interference (no signal
at all).
[attachment=48855]
The process of communication involves the transmission of information from one
location to another. As we have seen, modulation is used to encode the information onto
a carrier wave, and may involve analog or digital methods. It is only the characteristics
of the carrier wave which determine how the signal will propagate over any significant
distance. This chapter describes the different ways that electromagnetic waves propagate.
Basics
An electromagnetic wave is created by a local disturbance in the electric and
magnetic fields. From its origin, the wave will propagate outwards in all directions.
If the medium in which it is propagating (air for example) is the same everywhere,
the wave will spread out uniformly in all directions.
Refraction
When the wave enters the new medium, the speed of propagation will change.
In order to match the incident and transmitted wave at the boundary, the transmitted
wave will change its direction of propagation. For example, if the new medium has
a higher index of refraction, which means the speed of propagation is lower, the
wavelength will become shorter (frequency must stay the same because of the boundary
conditions). For the transmitted wave to match the incident wave at the boundary,
the direction of propagation of the transmitted wave must be closer to perpendicular.
The relationship between the angles and indices of refraction is given by Snell's Law:
Interference
All electromagnetic waves can be superimposed upon each other without limit.
The electric and magnetic fields simply add at each point. If two waves with the
same frequency are combined there will a be a constant interference pattern caused
by their superposition. Interference can either be constructive, meaning the
strength increases as result, or destructive where the strength is reduced.
The amount of interference depends of the phase difference at a particular point.
It can be shown that constructive interference occurs for phase differences of 0-1200,
and 240-3600. Thus destructive interference occurs from 120-2400. For two identical
waves, no phase shift results in total constructive interference, where the strength
is maximum and 1800 phase shift will create total destructive interference (no signal
at all).