Chapter Fifteen:Radio-Wave Propagation :Chapter Fifteen:Radio-Wave Propagation


Introduction :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


Electromagnetic Spectrum :Electromagnetic Spectrum


Electromagnetic Waves :Electromagnetic Waves Electromagnetic transmissions move in space as Transverse waves Waves are characterized by frequency and wavelength:


Electric and Magnetic Fields :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 This relationship is expressed by:


Power Density :Power Density Power density in space is the amount of power that flows through each square meter of a surface perpendicular to the direction of travel


Plane and Spherical Waves :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


Circular Polarization :Circular Polarization The polarization of a plane wave is simply the direction of its electric field vector The wave can rotate in either direction - it is called right-handed if it rotates clockwise


Free-Space Propagation :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


Attenuation of Free Space :Attenuation of Free Space An isotropic radiator would produce spherical waves The power density of an isotropic radiator is simply be the total power divided by the surface area of the sphere, according to the square-law:


Transmitting Antenna Gain :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


Receiving Antenna Gain :Receiving Antenna Gain A receiving antenna absorbs some of the energy from radio waves that pass it A larger antenna receives more power than a smaller antenna (in relation to surface area) Receiving antennas are considered to have gain just as transmitting antennas do The power extracted from a receiving antenna is a function of its physical size and its gain


Path Loss :Path Loss Free-space attenuation is the ratio of received power to transmitted power The decibel gain between transmitter and receiver is negative (loss) and the loss found this way is called free-space loss or path loss


Reflection, Refraction, and Diffraction :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


Reflection :Reflection Reflection of waves from a smooth surface (specular reflection) results in the angle of reflection being equal to the angle of incidence


Other Types of Reflection :Other Types of Reflection Corner reflector Parabolic reflector Diffuse Reflection


Refraction :Refraction A transition from one medium to another results in the bending of radio waves, just as it does with light Snell’s Law governs the behavior of electromagnetic waves being refracted:


Diffraction :Diffraction As a result of diffraction, electromagnetic waves can appear to “go around corners” Diffraction is more apparent when the object has sharp edges, that is when the dimensions are small in comparison to the wavelength


Ground-Wave Propagation :Ground-Wave Propagation Most of the time, radio waves are not quite in free space Terrestrial propagation modes include: Line-of-sight propagation Space-wave propagation Ground waves Sky waves


Ionospheric Propagation :Ionospheric Propagation Long-range communication in the high-frequency band is possible because of refraction in a region of the upper atmosphere called the ionosphere The ionosphere is divided into three regions known as the D, E, and F layers Ionization is different at different heights above the earth and is affected by time of day and solar activity


Line-of-Sight Propagation :Line-of-Sight Propagation Signals in the VHF and higher range are not usually returned to earth by the ionosphere Most terrestrial communication at these frequencies uses direct radiation from the transmitter to the receiver This type of propagation is referred to as space-wave, line-of-sight, or tropospheric propagation


Propagation in a Mobile/Portable Environment :Propagation in a Mobile/Portable Environment Multipath propagation creates interference for communication systems Mobile environments are often so cluttered that the square-law attenuation of free space does not apply (for example, in a city with many buildings)


Repeaters and Cellular Systems :Repeaters and Cellular Systems Because mobile systems have relatively small antenna heights, systems must be in place to improve signal strength and reception capabilities Mobile units make use of repeaters that are full-duplex and use resonant cavities called a duplexer Cellular systems do not use the horizon as the limit of coverage Antennas may still be mounted high, but the range is deliberately limited by using as low a transmitter power as is possible


Control of Fading in Mobile Systems :Control of Fading in Mobile Systems Fading is a problem with mobile systems and increasing power and typical frequency diversity are not workable solutions to this problem Spread-spectrum systems can correct fading through alternative frequency diversity systems such as CDMA Using a rake receiver, a CDMA system can receive several data streams at once


Other Propagation Modes :Other Propagation Modes Tropospheric Scatter - makes use of the scattering of radio waves in the troposphere to propagate signals in the 250 MHz –5 GHz range


Ducting :Ducting Under certain conditions, especially over water, a superrefractive layer can form in the troposphere and return signals to earth The signals can then propagate over long distances by alternately reflecting from the earth and refracting from the superrefractive layer A related condition involves a thin tropospheric layer with a high refractive index, so that a duct forms


Examples of Ducting :Examples of Ducting


Meteor-Trail Propagation :Meteor-Trail Propagation Meteors are constantly entering the earth’s atmosphere and being destroyed The meteors that enter the atmosphere leave behind an ionized trail that can be used for communication. It is not suitable for voice communication