 |
| Importance of Radio Waves in Modern Communication |
|
| Radio waves are electromagnetic waves of wavelength 10-3m and higher. Their frequency range is from a few kHz to nearly a few hundred MHz. The propagation of radio waves through the atmosphere is relevant in all modern forms of communication: radio, television, microwaves etc. The earliest form of radio communication used electromagnetic waves of the type experimented upon by Hertz and Marconi-waves having wavelength of 10 m or more (frequencies less than 30 MHz). The waves in this range are termed as AM (for amplitude modulated) band in modern technology. The below figure shows different frequency bands. In the below figure, different frequencies are expressed in MHz. |
| |
 |
| |
| Frequency bands in radio wave communication |
| |
| With reference to the frequency range, the radio waves are classified as under: |
| |
| Radio frequency bands |
| |
 |
| |
|
| |
| The amplitude-modulated band (AM band) contains the radio waves having frequency of less than 30 MHz or having wavelength of 10 m or more. The lower atmosphere is more or less transparent to radio waves. However, the ionosphere reflects back the radio waves. Thus, a signal emitted by an antenna from a certain place can be received at another place on the surface of Earth in the following two ways: |
| |
| Ground wave propagation Or Surface wave propagation |
| |
| The AM radio waves which propagate from one point to another following the surface of Earth are known as ground waves or surface waves. This type of propagation is called ground wave propagation or surface wave propagation. |
| |
| In ground wave transmission, the intensity of the signal falls with distance due to its absorption by the ground. So, ground wave propagation cannot take place up to very large distance. If the energy of the signal is increased by increasing the frequency of the carrier wave, then there is corresponding increase in absorption of signal by the ground. This sets an upper limit to the frequency at which ground wave propagation can be done. It has been practically observed that ground wave propagation is useful up to 1500 kHz (or wavelength nearly 200 m). This part of AM band is called medium wave band. |
| |
| Ground waves progress along the surface of the Earth and must be vertically polarized to prevent short-circuiting the electric component. A wave induces currents in the ground over which it passes and thus loses some energy by absorption. This is made up by energy diffracted down- ward from the upper portions of the wavefront. |
| |
| There is another way in which the surface wave is attenuated. Because of diffraction, the wavefront gradually tilts over, as shown in the below figure. As the wave propagates over the Earth, it tilts over more and more. The increasing tilts cause greater short-circuiting of the electric field components of the wave. Eventually, at some distance (in wavelengths) from the antenna, as partly determined by the type of surface over which the ground wave propagates, the wave "lies down and dies". It is important to realise this, since it shows that the maximum range of such a transmitter depends on its frequency as its power. Thus, in the VLF band, insufficient range of transmission can be cured by increasing the transmitting power. This remedy will not work near the top of the MF range. This is because propagation is now definitely limited by tilt. |
| |
 |
| |
| Ground-wave propagation |
| |
| VLF Propagation |
| |
| When propagation is over a good conductor like seawater, particularly at frequencies below about 100 kHz, surface absorption is small. Also, the attenuation due to the atmosphere is small. Thus, the angle of tilt is the main determining factor in the long distance propagation such waves. The degree of tilt depends on the distance from the antenna in wavelengths. This explains the early disappearance of the surface waves in HF prorogation. Conversely because of the large wavelengths of VLF signals, waves in this range are able to travel long distances before disappearing (right around the globe if sufficient power is transmitted). |
| |
| At distance up to 1000km, the ground wave is remarkably steady. This shows little diurnal, seasonal or annual variation. Farther out, the effects of the E layers contribution to propagation are felt. Both short- and long-term signal strength variations take place, the latter including the 11-year solar cycle. The strength of low-frequency signals changes only very gradually. So, rapid fading does not occur. Transmission at these wavelengths proves a very reliable means of communication over long distances. |
| |
|
| |
| Sky waves are the AM radio waves, which are received after being reflected from the ionosphere. The propagation of radio wave signals from one point to another via reflection from ionosphere is known as sky wave propagation. |
| |
| The sky wave propagation is an important consequence of the total internal reflection of radio waves. As we go higher in the ionosphere, there is an increase in the free electron density. Consequently there is a decrease of refractive index. Thus, as a radio wave travels up in the ionosphere, it finds itself traveling from denser to rarer medium. It continuously bends away from its path till it suffers total internal reflection to reach back the Earth. |
| |
| The sky waves are of great importance for very long distance radio communication. The sky waves are in the frequency range of 2MHz to 30 MHz. [Due to their high frequency, the sky waves are called short waves]. This region in AM band is called short wave band. |
| |
| Note that the communication in AM band below 200 m wavelength is via the wave only. |
| |
| Effect of ionosphere on sky wave propagation |
| |
| Even before Sir Edward Appleton's pioneering work in 1925, it had been suspected that ionization of the upper parts of the Earth's atmosphere played a part in tile propagation of radio waves, particularly at high frequencies. Experimental work by Appleton showed that the atmosphere receives sufficient energy from the Sun for its molecules to split into positive and negative ions. Thus, they remain ionised for long periods of time. He also showed that there were several layers of ion is at ion at differing heights, which (under certain conditions) reflected back to Earth the high-frequency waves that would otherwise have escaped into space. The various layers, or strata, of the ionosphere have specific effects on the propagation of radio waves, and must now be studied in detail. |
| |
| The ionosphere is the upper portion of the atmosphere. It absorbs large quantities of radiant energy from the Sun, becoming heated and ionised. The most important ionizing agents are ultraviolet and a, b and g radiation from the Sun, as well as cosmic rays and meteors. The overall result is a range of four main layers, D, E, F1 and F2, in ascending order. The last two combine at night to form one single layer. |
| |
| The D layer is the lowest, existing at an average height of 70 km, with an average thickness of 10 km. The degree of its ionisation depends on the altitude of the Sun above the horizon, and thus it disappears at night. It is the least important layer from the point of view of HF propagation. It reflects some VLF and LF waves and absorbs MF and HF waves to a certain extent. |
| |
| The E layer is next in height, existing at about 100 km, with a thickness of perhaps 25 km. Like the D layer, it disappears at night. The reason for these disappearances is the recombination of the ions into molecules. This is due to the absence of the Sun (at night), when radiation is no longer received. The main effects of the E layer are to aid MF surface-wave propagation a little and to reflect some HF waves in daytime. |
| |
| The E8 layer is a thin layer of very high ionisation density, sometimes making an appearance with the E layer. It is also called the sporadic E layer; when it does occur, it often persists during the night also. On the whole, it does not have an important part in long-distance propagation. But it sometimes permits unexpectedly good reception. Its causes are not well understood. |
| |
| The F1 layer exists at a height of 180 km in daytime and combines with the F2 layer at nights. Its daytime thickness is about 20 km. Although some HF waves are reflected from it, most pass through to be reflected from the F2 layer. Thus, the main effect of the F1 layer is to provide more absorption for HF waves. |
| |
| Note that the absorption effect of this and any other layer is doubled, because HF waves are absorbed on the way up and also on the way down. |
| |
| The F2 layer is by far the most important reflecting medium for high-frequency radio waves. Its approximate thickness can be up to 200 km, and its height ranges from 250 km to 400 km in daytime. At night, it falls to a height of about 300 km, where it combines with the F1 layer. Its height and ionisation density vary tremendously. It is most noticeable that the F layer persists at night, unlike the others. |
| |
| This is due to the following reasons: |
| |
| (i) Since this is the topmost layer therefore it is also the most highly ionised. There is some chance for the ionization to remain at night, to some extent at least. |
| |
| (ii) Although ionisation density is high in this layer, the actual air density is not. This low actual density gives the molecules a large mean free path. So, there is low molecular collision rate. This explains as to why the ionization does not disappear as soon as the sun sets. It must be mentioned that the reasons for better HF reception at night are the combination of the F1 and F2 layers into one F layer, and the virtual disappearance of the other two layers (which were causing noticeable absorption during the day). |
| |
| Reasons for better HF reception at night |
| |
| (i) The combination of the F1 and F2 layers into one F layer |
| |
| (ii) The virtual disappearance of the other two layers, which were causing noticeable absorption during the day. |
| |
| Reflection mechanism |
| |
| Electromagnetic waves returned to Earth by one of the layers of the ionosphere appear to have been reflected. In actual fact, the mechanism involved is refraction. As the ionisation density increases for a wave approaching the given layer at an angle, so the refractive index of the layer is reduced. Alternatively, this may be interpreted as an increase in the conductivity of the layer, and therefore a reduction in its electrical density or dielectric constant. Hence the incident wave is gradually bent farther and farther away from the normal. |
| |
| If the rate of change of refractive index per unit height (measured in wavelengths) is sufficient, the refracted ray will eventually become parallel to the layer. It will then be bent downward, finally emerging from the ionised layer at an angle equal to the angle of incidence. Some absorption has taken place. But the wave has been returned by the ionosphere. |
| |
