20-11-2012, 04:25 PM
Radio-Wave Propagation
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Introduction
Radio waves are one form of electromagnetic radiation
Electromagnetic radiation has a dual nature:
In some cases, it behaves as waves
In other cases, it behaves as particles (photons)
For radio frequencies the wave model is generally more appropriate
Electromagnetic waves can be generated by many means, but all them involve the movement of electrical charges
Electric and Magnetic Fields
An electromagnetic wave propagating through space consists of electric and magnetic fields, perpendicular both to each other and to the direction of travel of the wave
The relationship between electric and magnetic field intensities is analogous to the relation between voltage and current in circuits
Plane and Spherical Waves
The simplest source of electromagnetic waves would be a point in space, with waves radiating equally in all directions. This is called an isotropic radiator
A wavefront that has a surface on which all the waves are the same phase would be a sphere
Free-Space Propagation
Radio waves propagate through free space in a straight line with a velocity of the speed of light (300,000,000 m/s)
There is no loss of energy in free space, but there is attenuation due to the spreading of the waves
Transmitting Antenna Gain
In practical communication systems, it is important to know the signal strength at the receiver input
It depends on the transmitter power and the distance from the transmitter to the receiver, but also upon the transmitting and receiving antennas
Two important antenna characteristics are:
Gain for the transmitting antenna
Effective area for the receiving antenna
Antennas are said to have gain in those directions in which the most power is radiated
Reflection, Refraction, and Diffraction
These three properties are shared by light and radio waves
For both reflection and refraction, it is assumed that the surfaces involved are much larger than the wavelength; if not, diffraction will occur