The modulated signal is transmitted by the antenna. It is received at the other end. 2kHz to 400 GHz Frequencies are transmitted with wireless communication.
While the signal travels from the transmitter to the receiver, the strength of the electromagnetic wave keeps decreasing. Three modes by which EM waves are transmitted. They are:
- Ground wave propagation or surface wave propagation (nearly 2 kHz to 2 Mhz)
- Skywave propagation or ionospheric propagation nearly 3 MHz to 30 MHz.
- Space wave propagation nearly 30 MHz to 400 GHz.
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” and the waves used are called surface waves or ground waves.
Both the transmitting and receiving antennas are to the earth. The efficiency of the radiation of the signal is decided by the size of the antenna. The signal gets attenuated during transmission over a long distance.
Causes for Attenuation are:
Increasing Distance – The attenuation of the signal directly depends on the transmitter’s power, frequency of the transmitter, and nature of the earth’s surface(Hill, plateau, Water bodies, etc).
Absorption of energy by the Earth – When the transmitted Electromagnetic wave comes in contact with the earth, it causes changes in the earth and form a current.
Due to this, the earth acts as a capacitor that leads to the attenuation of the wave.
Tilting of the wave – As the wave advances, the wavefront starts slowly tilting as per the curvature of the earth.
This increase in the tilt decreases the electric field strength of the wave (Tamil Nadu Text Book Corporation #).
Finally, at some distance, the surface wave dissolves out due to energy loss.
The frequency of the ground wave is essentially less than 2Mhz because the high-frequency waves experience more loss of energy in the earth’s atmosphere.
Ground wave propagation is mostly used in local broadcasting, radio navigation for the ship to ship, ship to shore communication, and mobile communication.
“The mode of propagation in which the electromagnetic waves radiated from an antenna, directed upwards at large angles gets reflected by the ionosphere back to earth is called skywave propagation or ionospheric propagation”. (Tamil Nadu Text Book Corporation #)
The corresponding waves are called sky waves. For Skywave propagation, the frequency range must be 3 to 30 MHz.
Electromagnetic waves of frequency greater than 30 Mhz can easily penetrate the ionosphere and do not suffer from reflection. Skywave propagation is used in short wave(SW) radio services.
Medium and high frequencies are used for long-distance radio communication. As the skywave propagation uses a high frequency that undergoes multiple internal reflections between the earth and ionosphere it can be used for extremely long-distance communication.
A single reflection makes the radio waves travel a distance of approximately 4000 km. The Ionosphere serves as a reflecting surface. The ionosphere is at a distance of approximately 50 km and spreads up to 400km above the earth’s surface.
The air molecules in the ionosphere get ionized due to UV rays, cosmic rays, alpha rays, beta rays, etc.
This ionized molecule helps in reflecting the radio waves back to the earth’s surface. This phenomenon of bending the radio waves back to earth is internal reflection.
This is the reason EM waves are transmitted at a critical angle to ensure the waves get total reflection and reach the ground without escaping into space.
“The shortest distance between the transmitter and the point of reception of the skywave along the surface is called the skip distance”. (Tamil Nadu Text Book Corporation #)
“When the angle of emission increases, the reception of ground waves decreases”. (Tamil Nadu Text Book Corporation #)
At one point there will be no reception due to ground waves. “There is a zone in between where there is no reception of electromagnetic waves neither ground nor sky called as skip zone or skip area”. (Tamil Nadu Text Book Corporation #)
“The higher the frequency higher is the skip distance and for a frequency, less than the critical frequency skip distance is zero”. (Tamil Nadu Text Book Corporation #)
Space wave Propagation
The Process of sending and receiving a signal via space is called space wave communication. The electromagnetic waves of very high frequencies above 30 MHz are called space waves.
Travel in a straight line and used for line of sight communication (LOS). Space wave propagation used television broadcast, satellite communication and RADAR are based on space wave propagation.
The mode of communication using the satellite above the earth is called Satellite communication.
The message signal from the Earth station is transmitted to the satellite by an uplink (frequency band 6 GHz) amplified by a transponder and then retransmitted to another earth station via a downlink frequency band 4 GHz.
Application of Satellite Communication
- Weather Satellites
- Communication Satellites
- Navigational satellite