Advertisements
Advertisements
Question
Explain the ground wave propagation and space wave propagation of electromagnetic waves through space.
Solution
Ground wave propagation:
If the electromagnetic waves transmitted by the transmitter glide over the surface of the earth to reach the receiver, then the propagation is called ground wave propagation. The corresponding waves are called ground waves or surface waves.
Both transmitting and receiving antennas must be close to the earth. The size of the antenna plays a major role in deciding the efficiency of the radiation of signals. During transmission, the electrical signals are attenuated over a distance.
Ground wave
Some reasons for attenuation are as follows:
1. Increasing distance: The attenuation of the signal depends on
- power of the transmitter
- frequency of the transmitter
- condition of the earth surface
2. Absorption of energy by the Earth: When the transmitted signal in the form of an EM wave is in contact with the Earth, it induces changes in the Earth and constitutes a current. Due to this, the earth behaves like a leaky capacitor which leads to the attenuation of the wave.
3. Tilting of the wave: As the wave progresses, the wavefront starts gradually tilting according to the Skywave curvature of the Earth. This increase in the tilt decreases the electric field strength of the wave. Finally, at some distance, the surface wave dies out due to energy loss.
The frequency of the ground waves is mostly less than 2 MHz as high-frequency waves undergo more absorption of energy at the earth’s atmosphere. The medium wave signals received during the daytime use surface wave propagation. It is mainly used in local broadcasting, radio navigation, ship-to-ship, ship-to-shore communication and mobile communication.
Space wave propagation:
The process of sending and receiving information signals through space is called space wave communication. The electromagnetic waves of very high frequencies above 30 MHz are called space waves. These waves travel in a straight line from the transmitter to the receiver. Hence, it is used for line of sight communication (LOS).
Space wave
For high frequencies, the transmission towers must be high enough so that the transmitted and received signals (direct waves) will not encounter the curvature of the earth and hence travel with less attenuation and loss of signal strength. Certain waves reach the receiver after getting reflected from the ground.
APPEARS IN
RELATED QUESTIONS
(i) Which mode of propagation is used by shortwave broadcast services having frequency range from a few MHz upto 30 MHz? Explain diagrammatically how long distance communication can be achieved by this mode.
(ii) Why is there an upper limit to frequency of waves used in this mode?
State the factors which limit its range of propagation
Derive an expression for the maximum line of sight distance between two antennas for space wave propagation.
Explain why ground wave communication mode cannot be used for long-distance communication using high frequencies
Light from a sodium lamp (S) passes through two polaroid sheets P1 and P2 as shown in fig. What will be the effect on the intensity of the light transmitted (i) by P1 and (ii) by P2 on the rotating polaroid P1 about the direction of propagation of light? Justify your answer in both cases.
The length of half wave dipole at 30MH2 is
The component of the ground wave attenuated by conducting earth is ______.
Three waves A, B and C of frequencies 1600 kHz, 5 MHz and 60 MHz, respectively are to be transmitted from one place to another. Which of the following is the most appropriate mode of communication ______.
The maximum frequency for reflection of sky waves from a certain layer of the ionosphere is found to be fmax = 9(Nmax)1/2, where Nmax is the maximum electron density at that layer of the ionosphere. On a certain day, it is observed that signals of frequencies higher than 5 MHz are not received by reflection from the F1 layer of the ionosphere while signals of frequencies higher than 8 MHz are not received by reflection from the F2 layer of the ionosphere. Estimate the maximum electron densities of the F1 and F2 layers on that day.
(i) The intensity of a light pulse travelling along a communication channel decreases exponentially with distance x according to the relation I = Ioe–αx, where Io is the intensity at x = 0 and α is the attenuation constant. Show that the intensity reduces by 75 per cent after a distance of `((In 4)/α)`
(ii) Attenuation of a signal can be expressed in decibel (dB) according to the relation dB = `10 log_10 (I/I_0)`. What is the attenuation in dB/km for an optical fibre in which the intensity falls by 50 per cent over a distance of 50 km?
