Propagation of radio waves
PROPAGATION OF RADIO WAVES :
TROPOSPHERIC MODES OF PROPAGATION :
- The propagation through the troposphere consists of :
- Tropospheric scattering , troposperic ducting and tropospheric delay .
Tropospheric scattering :
At high frequencies and a height of about 6 miles the frequency back to the ground is scattered and this allows the communication over the horizon in between 500 miles . The White Alice Communications System was developed using this tropospheric scattering .
Troposperic Ducting :
The UHF and VHF signals can travel through hundreds of kilometres due to the changes in moisture in atmosphere and temperature and it can travel even more farther or beyong the normal radio horizon for ducting mode . These random occurring propagation mode are mostly created by the tropospheric weather conditions . An example is the early morning enhancements of troposphere in which signals from distances can be received for a couple of hours still the sun’s warming effect is undone .
Tropospheric delay :
The tropospheric delay is a source of error in the technique of radio ranging .
STRUCTURE OF TROPOSPHERE :
The lowest portion of the earths atmosphere is the troposphere . The structure of troposphere cosists of its composition , pressure and temperature . The composition in this troposphere is mostly uniform . The surface is the source of water vapour by evaporation and even along with the decrease in height , the temperature decreases . Near the surface and with decrease in height the water vapour portion is great . Similarly at sea level the atmospheric pressure is maximum and this decreases with increase in altitude . The height after the change in pressure can be shown by :
Where z stands for altitude and R is gas constant .Similarly the temperature also decreases with the increase in altitude . This decrease in temperature rate can be represented by -dT/dz .
The reason of such temperature difference is because at the ground the absorption of sun’s energy takes place and radiation takes place at the top of the atmosphere leading to cooling of the earth .
STRATOSPHERE AND IONOSPHERE :
The stratosphere lies just above the troposphere and belo the mesosphere . It is always an opposite to troposphere in earths surface . It is mostly situated between 10 to 13 km and 50 km above the earths surface . In this layer with the increase in aircraft flight altitude the temperature also increases . The UV light passes through the mid stratosphere layer and O and O2 combines to form maximum natural ozone . This layer is mostly useful in air. Similarly the ionosphere lies in upper atmosphere . The ionosphere plays an important role in atmospheric electricity and also forms the inside layer of magnetosphere . It also helps in propagation of radio waves to long distance places on earth .
GROUND WAVE PROPAGATION :
This type of propagation is used for very short distances during daytime . A number of constituents is used to build a ground wave radio signal . There will be a direct wave and a reflected signal if the antennas are placed in a a single line and due to this it is not harmed by locality . The transmission also gets reflected due to a number of objects like hills , buildings etc . To achieve the complete coverage the radio communication stations depend on the ground wave propagation . There will also be a surface wave in addition to this ground wave propagation . The surface wave follows the earths curvature and hence it can also be achieved beyond the horizon .
INFLUENCE OF EARTHS MAGNETIC FIELD ON RADIOWAVE :
The mode condition and various other conditions of the waves in a mode are described by evaluation of the residues of a series of poles of a contour integral. The curvature of earth is allowed for the method of `modified refractive index’, thus to avoid the use of cumbersome spherical wave function. This also allows the replacement of a point source by a line source with repeated simplification of the mathematical calculation .
LUF :
It stands for lowest usable high frequency ( LUF) . It is a frequency in high frequency band in which the required signal to noise ratio is sufficiently provided by field intensity that is received for a given time . The LUF has a direct impact by the amount of energy that is accepted by lower regions .
MUF :
It is the maximum ratio frequency . It is used for reflection from the ionosphere between two points at a given time . It is not dependent on transmitter power . Mostly MUF is a number which is predicted . It is the maximum frequency due to which in 90 % days communication is possible . To calculate the communication on a given day , it may suceed or may not suceed in MUF .
CRITICAL FREQUENCY :
It is alo defined as the limiting frequency in which the reflection of the wave component takes place and above which it enters an ionospheric layer .
With the time of delay the critical frequency changes along with the conditions of atmosphere . It is always the result of limitation of electron . In short it is the maximum level of magnitude above which the wave enters the ionosphere and below which the reflection of wave takes place from the ionosphere .
The electron values are the main factors on which its value depends upon .
LOS :
It stands for line of sight propagation . It is a form of electromagnetic radiation or a acoustic propagation of a wave . It consists of transmission of light emission in a straight line . The atmosphere may absorb , reflect , refract or defract the waves and such waves cannot travel behind obstacles . It travels as a ground wave at less frequency and follows the curvature of earth because of defraction in the layers of atmosphere . Hence it helps in receiving AM signals after the antenna transmission falls below horizon . Quasi curved paths is a result of multiple reflection or defraction . Radio horizon is defined as the farthest possible point of propagation .
Fiels strength in free space belongs to the branch of telecommunications . Under ideal conditions a half wave dipole is caused by field strength . We use empirical formulas to calculate the field strength .
QUESTIONS AND ANSWERS :
QUE > WHAT ARE THE TROPOSPHERIC MODES OF PROPAGATION ? EXPLAIN THEM ?
ANS > The propagation through the troposphere consists of 3 TYPES :
Tropospheric scattering , troposperic ducting and tropospheric delay .
Tropospheric scattering :
At high frequencies and a height of about 6 miles the frequency back to the ground is scattered and this allows the communication over the horizon in between 500 miles . The White Alice Communications System was developed using this tropospheric scattering .
Troposperic Ducting :
The UHF and VHF signals can visit through hundreds of kilometres due to the changes in moisture in atmosphere and temperature and it can travel even more farther or beyong the normal radio horizon for ducting mode . Let us take an example is the early morning enhancements of troposphere in which signals from distances can be received for a couple of hours still the sun’s warming effect is undone .
Tropospheric delay :
The tropospheric delay is a source of error in the process or technique of radio ranging .
QUE > WHAT IS GROUND WAVE PROPAGATION ?
ANS > The ground wave propagation is used for very short distances in daytime . A number of constituents used to build a ground wave radio signal . A direct wave and a reflected signal are present. If the antennas are placed in a a single line and due to this it is not harmed by locality . The transmission also gets reflected due to a number of objects like hills , buildings etc . To achieve the complete coverage the radio communication stations depend on the ground wave propagation . There will also be a surface wave in addition to this ground wave propagation . The surface wave follows the earths curvature and hence it can also be achieved beyond the horizon.
GATE Syllabus-
1.Gate Syllabus for Electronics and Communication 2014
2.Gate Syllabus for Physics 2014
3.Gate Syllabus for Engineering Science 2014
IES Syllabus-
1. IES Syllabus for Electronics and Telecomm
2. IES Syllabus for General Ability
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All wireless communication depends on the radio waves, so to know about the propagation and diversion of the radio waves are must for an engineer.
I hope this article clearly explains the concept very well
this article gives much knowledge to electronics students..
gives information on radio waves