Monday, May 11, 2009

Ground wave propagation

Abstract:- I avail this opportunity to convey the entire knowledge of ground wave propagation through this paper.
This paper gives the information about the ground wave propagation, line of sight, various facts effecting ground wave propagation.

Introduction to the problem:- In communication system, there are various methods or techniques for propagation. We use all the methods for communication to one place to another place. Ground wave propagation is one of the most popular technique for transfer the signals of low and medium frequency range of radio waves from one place to another place.

Radio waves
The word radio means radiation of electromagnetic waves conveying information from one end and receiving such information at other end. Within this meaning such applications telegraphy, telephony, television and a host of navigational age are classified as radio.
All radio waves are electromagnetic waves which travel with speed of light. 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.

The Electromagnetic Spectrum
Frequency Range Band Designation
30-3000 Hz ELF
3-30 kHz VLF
30-300 kHz LF
300-3000 kHz MF
3-30 MHz HF
30-300 MHz VHF
300-3000 MHz UHF
3-30 GHz SHF
30-300 GHz EHF

Propagation of Waves
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.
Ground Wave
Radio waves in the VLF band propagate in a ground, or surface wave. The wave is connected at one end to the surface of the earth and to the ionosphere at the other. The ionosphere is the region above the troposphere (where the air is), from about 50 to 250 miles above the earth. It is a collection of ions, which are atoms that have some of their electrons stripped off leaving two or more electrically charged objects. The sun's rays cause the ions to form which slowly recombine. The propagation of radio waves in the presence of ions is drastically different than in air, which is why the ionosphere plays an important role in most modes of propagation. Ground waves travel between two limits, the earth and the ionosphere, which acts like a duct. Under normal conditions, the temperature of air gradually decreases with increase in height above ground. When there is a stable high pressure system, a mass of warm air may over run cold air, causing a temperature inversion. Radio waves trapped below the mass can travel great distance with little loss. The area between the earth and the warm air mass is known as duct. In troposphere under normal conditions the air pressure water vapor pressure and temperature reduces with the increase of height above earth. As a result of this the refractive index also reduces with the increase of height. In standard atmosphere the modified refractive index M increases linearly with the increase of height. Under abnormal meteorological conditions particularly under water, variation of dielectric constant of the troposphere with height departs considerably from the standard condition. Thus under certain special conditions the dielectric constant may not decrease at all with height or may even increase with height resulting in the radio wave following the straight line path or curving away from the earth respectively. This results in reduction of the distance to the horizon. Under another type of abnormal conditions the variation of refractive index with height may be much higher than the normal in the region close to the earth. Such abnormal conditions may result when dry air flows from land out over cooler water. Evaporation of moisture from water into the lower layers of air cools the air. Thus the lower layer is cool and rich in moisture while the upper layer is warm and contains less moisture. Thus as the height increases there results increased moisture lapse rate and a temperature inversion that is a rapid increase in temperature instead of reduction. Thus there results rapid decrease in dielectric constant as the wave travels upwards. Typical result of such an abnormal condition is it cause the path of the rays traveling close to the surface of the earth to be bent as much or even more than the curvature of the earth while the rays at greater height are bent less. When the frequency is sufficiently high the region where the variation of refractive index is usually great actually traps energy and causes it to travel along the earth’s surface much as though in a waveguide. This special refraction the electromagnetic wave is called super refraction and the propagation utilizing this super refraction is called duct propagation. The duct propagation may increase the range of space wave communication two or three times the normal line of sight range. Duct propagation becomes possible only at frequencies above one thousand mega hertz. The duct is the region between the upper minimum of the M curve and either the ground or the point where the vertical projection from the upper minimum curve intersects the m curve. When the curve has a negative slope the curvature of the ray is concave downward on a plane earth diagram and the true curvature of the rays is greater than the curvature of the earth. Hence rays which enter the duct with sufficiently small angles are bent until they become horizontal and then are turned downwards.

Since the duct curves with the earth, the ground wave will follow. Therefore very long range propagation is possible using ground waves. Ground wave use for radio communication signal propagation on the long and medium wave bands for local radio communication.

Ground wave propagation
Ground wave propagation is particularly important on the LF and MF portion of the radio spectrum. Ground wave propagation is used to provide relatively local radio communication coverage especially by radio broadcast stations that required to cover particular locality.
Ground wave signal is made up of number of constituent. If the antennas are in the line of sight then there will be a direct wave as well as wave signal.
As the name suggests direct signal is that travels directly between the two antennas and is not effected by the locality. There will also be a reflected signal as the transmission will be reflected by a number of objects including the earth’s surface and any hills, or large buildings that may be present.
In addition to this there is a surface wave. This tends to follow the curvature of the earth and enables coverage to be achieved beyond the horizon. It is the sum of all these components that is known as ground wave.
Beyond the horizon the direct and reflected waves are blocked by the curvature of earth, and the signal is purely made up from the diffracted surface wave. It is for this reason that surface wave is commonly called the ground wave propagation.
The radio signals are spreads out from the transmit along the surface of earth. Instead of just traveling in a straight line the radio signals tend to flow the curvature of the earth. This is because currents are induced in the surface of earth and this action slows down the wave front in this region, causing the wave front of communications signal to tilt downward towards the earth. With the wave front tilted in this direction it is able to curve around the earth and be received well beyond the horizon.

The wave induces currents in the ground over which it passes and thus losses some energy by absorption. This is made up by energy diffracted downward from the upper portion of the wave front.
There is another way in which the surface wave is attenuated. Because of diffraction the wave front gradually tilts over as shown in above figure. As the wave propagates over the earth it tilts more and more, and the increasing tilt causes greater short circuiting of the electric field component of the wave and hence field strength reduction. Eventually, at some distance from the antenna as partly determined by the type of surface over which the ground wave propagates the wave lies down and dies. Thus in the VLF band insufficient range of transmission is cured by increasing the transmitting
Field strength at a distance
Radiation from an antenna by means of the ground wave gives rise to field strength at a distance. If the distance between two antennas is very long the reduction of field strength due to ground and atmospheric absorption reduces the value of voltage received.
Line of Sight
In the VHF band and up, the propagation tends to straighten out into line-of-sight (LOS)
waves. However the frequency is still low enough for some significant effects.
1. Ionospheric scatter. The signal is reflected by the E-region and scattered in all directions. Some energy makes it back to the earth's surface. This seems to be most effective in the range of 600-1000 miles.

Figure 16
1. Tropospheric scatter. Again, the wave is scattered, but this time, by the air itself. This can be visualized like light scattering from fog. This is a strong function of the weather but can produce good performance at ranges under 400 miles.

Figure 17
1. Tropospheric ducting. The wave travels slower in cold dense air than in warm air. Whenever inversion conditions exist, the wave is naturally bent back to the ground. When the refraction matches the curvature of the earth, long ranges can be achieved. This ducting occurs to some extend always and improves the range over true the line-of-sight by about 10 %.
1. Diffraction. When the wave is block by a large object, like a mountain, is can diffract around the object and give coverage where no line-of-sight exists.
Beyond VHF, all the propagation is line-of-sight. Communications are limited by
the visual horizon. The line-of-sight range can be found from the height of the
transmitting and receiving antennas as the addition of four third as the optical horizon.
Effect of frequency
As the wave front of the ground wave surface travels along the earth’s surface it is attenuated. The degree of attenuation is dependent upon a variety of factors. Frequency of the radio signal is one of the major determining factors as losses rise with increasing frequency. As a result it makes this form of propagation impracticable above the bottom end of the HF portion of the spectrum (3 MHz). Typically a signal at 3.0 MHz will suffer an attenuation that may be in the range of 20 to 60 db more than one at 0.5 MHz dependent upon a verity of factors in the signal path including the distance. In view of this it can be seen why high power HF radio broadcast May only audible for a few miles from the transmitting site via the ground wave.
Effect of Ground
The surface wave also very dependent upon the nature of ground over which the signal travels. Ground conductivity, terrain roughness and the dielectric constant all effect the signal attenuation. In addition to this the ground penetration varies, becoming greater at lower frequencies and this means that it is not just the surface conductivity that is of interest. At the higher frequencies this is not of greater importance, but at low frequencies penetration means that ground strata down to 100 meters may have an effect.
Despite all these variables, it is found that terrain with good conductivity gives the best result. Thus soil type and moisture content are of importance. Salty sea water is best, and rich agriculture or marshy land is also good. Dry sandy terrain and city centers are by far the worst. This means sea paths are optimum, although even these are subject to variations through due to roughness of the sea, resulting on path lose being slightly dependent upon the whether it should also be noted that in view of fact that signal penetration has an effect, the water table may have an effect dependent upon the frequency in use.

Effect of polaristion
The type of antenna has a major effect. Vertical polarization is subject to considerably less attenuation than horizontally polarized signals. In some cases the difference can amount to several tens of decibels. It is for the reason that medium wave broadcast stations use vertical antennas, even if they have to be made physically short by adding inductive loading. Ships making use of the mf marine bands often used inverted L antennas as these are able to radiate a significant proportion of the signal that is vertically polarized.




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