Some of the most important Physics Topics include energy, motion, and force.
What is ‘Radio Horizon’? What are the Different Mediums for Wave Propagation?
When electromagnetic carrier waves ranges from Very Low Frequency (VLF) to microwave (as stated in table 1) propagate through atmosphere, mainly three types of waves are produced-
- ground wave
- space wave
- sky wave.
Suitable wave for transmission and its corresponding range of frequency of carrier wave are to he selected first and their technological set-ups are to be arranged accordingly.
Ground wave: This wave propagates from one point to another, along the surface of earth following earth’s curvature. For the propagation of this wave, the earth surface at one side and the lowest layer of ionosphere (which is nearly 80 km above the earth surface) at the other—both act as reflectors. The wave progresses through successive reflections which it suffers from these two surfaces [Fig.].
Another special feature of the wave is polarisation of the wave.
The transmitting antenna is placed vertically, so the wave gets polarised at vertical plane at the start i.e., its electrical field has no horizontal component and initial wavefront is a vertical wavefront [Fig.].
Now, as the wave progresses along the earth surface, the wave-front gradually gets tilted from the vertical line, which means that the magnitude of horizontal component of its electrical field starts increasing. The horizontal component of the transmitted energy gets absorbed on the earth surface. As the tilte of the wave- front towards horizontal plane increases, the rate of absorption of energy Continues to increase. Therefore ground wave is not suited for long range communication,
Ground wave plays a very important role in transmission through carrier wave of Very Low Frequency (VLF) and Low Frequency (LF). In case of higher frequencies, the energy carried by ground wave gets absorbed very fast. This is why FM radio or television signal can be transmitted only up to a short distance.
Space Wave: This type of wave can propagate through the atmosphere in a straight line almost without any deviation. Due to curvature of earth surface, we can communicate through rectilinear space wave upto a distance of a certain limit. It is called radio horizon of the corresponding antenna.
In Fig., the space wave can propagate upto a maximum point O from the transmitting antenna. The distance PO of the earth surface is the radio horizon of the antenna.
Let R = radius of the earth,
H = height of transmitting antenna A,
From Fig. we get,
Similarly in Fig., the radio horizon of receiving antenna B,
QO = \(\sqrt{2 h R}\)
Clearly, when both transmitting and receiving antennas are used at the same time, the magnitude of radio horizon is,
PQ = PO + QO = \(\sqrt{2 H R}\) + \(\sqrt{2 h R}\)
For example, if a space wave, transmitted from a transmitting antenna, 300 m high, is to be received by a receiving antenna 30 m high, the radio horizon will be
= \(\sqrt{2 \times(6400 \times 1000)}(\sqrt{300}+\sqrt{30})\)
≈ 81.6 × 103m = 81.6 km
[The radius of earth has been taken as 6400 km ]
If carrier wave of High Frequency (HF) or of even higher frequency is transmitted as space wave, then a little energy is absorbed. So, inspite of the limitations of radio horizon, space waves are most suitable for very long distance transmission. In this process, the application of one or more intermediate antennas is most prevalent. Each Intermediate antenna receives transmitted signal and transmits the signal to the next antenna. In this way, the radio horizon can be enlarged to a great extent.
As a high quality communication technique, the use of microwave communication Is increasing day by day. The frequencies of these microwaves are very high, 300 MHz to 300 GHz. These waves are transmitted mainly as space waves. For long distance communication, instead of using a series of antennas, artificial earth satellites are employed. Space waves transmitted from earth surface after getting reflected from the artificial satellites, reach distant places on earth surface. This arrangement is called satellite communication.
Sky wave: Carrier waves of Medium Frequency (MF) or Lower High Frequency (LHF) are mostly transmitted as sky wave. In sky wave propagation, the ionosphere extending from a height of 80 km to about 400 km above earth surface plays a very important role. The ionisation of air in the ionosphere is mainly due to solar radiation in day time and cosmic rays of much less intensity during night. Hence the rate of ionisation is much greater during day time than at night. In ionosphere, electrons and ions formed due to ionisation remain mixed with neutral molecules and atoms. The rate of ionisation is measured by the number of free electrons in unit volume, i.e., by electron number density. Here two factors are important—
i) Since the solar radiation or cosmic radiation falls on the upper surface of ionosphere first, the rate of ionisation is higher in the upper surface. Depending on the difference of rate of ionisation, ionosphere is divided into three layers— upper layer, middle layer and lower layer. Rate of ionisation is maximum at upper layer. The middle and the lower layer almost do not exist at night.
ii) With the change of ionisation rate in air, the velocity of electromagnetic waves through it also changes. As a result, ionosphere acts as a refract-ing medium. Higher the ionisation rate in a layer, higher is the velocity of electromag-netic waves through it.
That is, the refractive index of this layer decreases. In the layers of ionosphere, shown in Fig., the rate of ionisation of a higher layer A is maximum and its refractive index is minimum. It can be proved that for an incident electromagnetic wave of frequency f, the refractive index of a layer of ionosphere,
μ = (1 – \(\frac{81 n}{f^2}\))1/2 …… (3)
where, n = electron number density of that layer of the medium.
Whenever a sky wave propagates through ionosphere, to higher layer, it advances from a higher refractive index layer to lower refractive index layer. When the slope of incident sky wave with respect to vertical axis becomes very small, it is refracted from highest ionisation layer i.e., from the lowest refractive index layer and escapes the ionosphere (ray ‘a’ of Fig.).
On the other hand, if any wave with a higher slope is incident from below onto the ionosphere, ¡t suffers total internal reflection from one or the other layer and is reflected back to the earth surface. In other words, it is due to reflection in ionosphere that the sky wave is transmitted from one point of earth surface to another.
As the values of f are relatively low for the waves of Medium Frequency (MF) the refractive indices of the layers of ionosphere, by equation (3), are also low. As a result, from even the lower layer of the medium, the sky wave gets reflected. That is why sky waves of such frequencies travel relatively shorter distances on earth surface.
On the other hand, sky waves of High Frequency (HF) (i.e., short wave) are reflected from the upper layers of ionosphere. Hence, the reflected waves can reach distant places on earth surface. In this case, these waves are rather absorbed to some amount in medium and lower layers, hence the intensity of the wave is reduced. Again at night, as the middle layer and lower layer almost do not exist, the amount of absorption is negligibly small. Hence, the signal carried by sky wave is more intense and clear at night.
Sky wave plays the mam role in radio transmission. Normally, medium wave transmission for short distances and short wave transmission for longer distances are made through sky waves.
Other media for the propagation of carrier waves:
It has already been mentioned that apart from air medium, two other media are used for communication. These are:
i) Coaxial cable: For short distance communication, this medium is most effective. As for example, in a telephone communication system or a computer network limited to a small area, the coaxial cable is widely used.
ii) Optical fibre: Unimaginable success has been achieved in world wide telephone communication system by using optical fibre. Through optical fibre, it is possible to transmit electromagnetic waves in the range of almost infinite frequency, very effectively.
[The detailed discussion regarding these media is not included in the present syllabus.]