Problems of Radio Communication in Hills Areas

Sri Kamalesh Deka, B.E.(Elec.)

(The author was then Dy. S. P.(Communication). This article was first published in Assam Police Training College on 15th of August. 1970)

Radio communication within recent decades, has become a popular way of conveying information between places that may be separated by thousands of miles. Basic difference between Radio-Communication and Telecommunication is that in Telecommunication signals are sent with the help of some electrical pulses flowing through some conductors. So it requires some wires. But in Radio-Communication, signals are sent with the help of electromagnetic waves produced in the space (i.e. in atmosphere). These waves travel at the velocity of light and consist of electric and magnetic fields at right angles to each other. No material medium is necessary for the propagation of these waves. So we often call it wireless communication.

In hilly areas we mainly face the following problems:

Problem of Attenuation.
Problem of distortion.
Problem of erection of wireless station.

Attenuation of a radio wave means diminishing of its amplitude (i.e. strength). In hill this is more because waves when they collide against the rocks of trees their energy contents are absorbed by these substances and become unable to move to distant place.

Signals become distorted for various reasons.

Harmonics produced by lightening
Signal of other station
Defective parts of the transmitter or receiver.

Erection of Station in Hill Areas

To install on station in hills the carrying of equipment poses one problem. Valves of receiver of transmitter of a wireless set are the most delicate parts which may get damaged due to any jerkings or rough handling.

Nowadays transistors have come in replacing valves. They are light and compact. Minor jerkings cannot cause considerable harm in transistors as they do not contain glass.

The custodians of wireless station have to face many other problems in such regions. There is the problem of scarcity of water. If ration is not supplied regularly they have to go without food for days. In rainy seasons the condition becomes much worse. The hilly tracts become slippery and muddy. Leeches and other insects are there to annoy everybody. In some areas intense cold prevails throughout the year.

As the electromagnetic waves travel through space, the following factors predominantly affects the radio communication.

Atmospheric condition
The nature of the objects on earth surface between the transmitting and receiving centres
Distance of transmission
Density of signals from other stations
Power of the transmitter and capacity of the receiver
Methods of transmission or propagation
Nature of antenna
Frequency of the carrier wave
Methods of detection

In hill areas 1st, 2nd, 3rd and 6th factor are more influential than in plains which will be clear in course of subsequent discussion.

We take the following mathematical relation first:

Wave length X frequency = Velocity of the wave.

i.e. µ x f = v (For a particular wave) = 3 x 108 m/s (for radio wave).

This above relation enables us to know the frequency of a station if we know its wave length i.e., in what meter the station is broadcasting. This relation again tells us the fact that a wave of higher frequency has a lower wave length. The range of frequencies which are audible to the ear (known as audiofrequency) lies from 20 kilocycles/second. Higher frequencies are considered radio frequencies. Now to compare we can note that if frequency is 30 KC/S wave length is 10⁴ metres but if frequency is 3 x 10⁵ KC/S i.e. 300 MC/S wave length only one metre. This is one of the reasons why audio signal is modulated with another radio frequency wave generated in the transmitter known as carrier wave. The resultant wave is a radio frequency wave having comparatively short wave length for which antenna and other short circuit condition can be designed to achieve good radiation of waves. As this article does not provide enough scope to go into details of all aspects of the the communication system only the propagation aspect will be discussed elaborately.

Propagation of radio waves:

There are three modes of propagation of the radio waves:

Ground wave propagation.
Space wave propagation
Skywave propagation

In ground wave propagation one leg of the wave front traces the ground. The signal strength and the range depends on the conductivity of the earth. The lesser the conductivity, the more will be the attenuation for a given frequency. Marshy land and sea have higher conductivity than dry soil and rock. The attenuation increases with frequency. But in a hilly region this is not suited because of the corrugated earth surface full of rocks and dry soil which are not good conductors for it.

In space wave propagation the receiving antenna receives the signal from a direct ray and a ray reflected from earth. Whether the two will reinforce or oppose each other depend on the phase difference and the path difference. This mode is important for ultrahigh frequencies as used in television. Here both transmitting and receiving antennas are placed at hill tops or without obstruction. This range is upto radio horizon and is increased by atmospheric refraction.

Skywave propagation is used extensively for short waves for long distance radio communication utilises ionised layers in the middle atmosphere. These ionised layers are mainly produced by the ultraviolet rays of the sun. At night recombination of the ions take place and ion density gets reduced. The radio-waves (depending on the frequency) can get reflected from these layers but the maximum frequency for which reflection may take place for a given angle of departure is known as the maximum usable frequency (M. U. F.) and is important in choosing frequency greater than the critical frequency there is a maximum angle of departure and have a minimum distance from the transmitter to the shortest distance of the skywave return to earth. Now the radio-wave of that frequency cover a certain small distance from the transmitter due to ground wave propagation. It follows that there is a distance lying between the farthest point of ground wave coverage and nearest point of sky wave return where no radio waves of this frequency from the transmitter will be received. This distance is known as the skip distance. It increases as the frequency is increased.

From what is discussed above it becomes clear how the hilly areas terrains largely affect propagation of radio waves. The change of atmospheric condition influences the propagation. But most important atmospheric factor is lightning. Lightening is defined as an electric discharge from cloud to cloud or cloud to earth. During this electrical discharge electromagnetic waves of all frequency (known as harmonics) are generated in space. They do not spare any radio signal from its influence and distort the signals. This fact can be seen in our own sets. Whenever we switch on any switch where a radio is on, we see some transient disturbances. This is because while switching on some spark for a very small period of time as in lightening.

Above discussion will only be useful in showing the line of thinking about problems of radio communication. Only one or two aspects are being shown here. Finally, it will be interesting to refer here to our home sets. Suppose we have tuned the set to one station. We may see that simultaneously signals of other stations are being audible to us, even if we are trying to avoid this. This only tells us the fact that there is no enough space in space for good radio communication.

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