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Types of Signal Propagation (Radio Wave Propagation)

Propagation of radio waves from a radiating transmitting antenna to the receiving antenna may take place in one of the following routes:
  1. Ground-wave or surface-wave propagation (up to 2 MHz)
  2. Space-wave or Line of Sight (LOS) or tropospheric propagation (above 30 MHz)
  3. Sky-wave or Ionospheric-wave propagation (2 MHz to 30 MHz)
  4. Satellite communication (1 GHz to 50 GHz)

Some Terminologies of Ionospheric Wave Propagation

  1. Critical Frequency
In case of sky-wave propagation, there is an upper limit on frequency that will result in a wave being returned back to the earth, known as the critical frequency.


The critical frequency of a layer is that frequency for which a vertically incident wave just fails to be reflected back from the layer.
It is the highest frequency of an ionised layer that is reflected by the layer at vertical incidence.
It is given as,
Description: 28114.png
where, Nmax is the maximum ionisation density, i.e., number of electrons per unit volume.
  1. Maximum Usable Frequency (MUF)
The maximum frequency that can be reflected back for a given distance of transmission is called the Maximum Usable Frequency (MUF) for that distance.
  1. Skip Zone and Skip Distance
The skip distance is the distance from the transmitter to the point where the sky-wave is first returned to the earth.
  1. Virtual Height
The virtual height of an ionospheric layer is the equivalent altitude of a reflection that would produce the same effect as the actual refraction.
  1. Radius of Curvature
A radio-wave propagating horizontally in the earth’s atmosphere follows a path with slightly downward curvature due to refraction of the wave in the atmosphere. This refraction of the wave occurs due to the variation of dielectric constant, and hence the refractive index, with the height above the earth.
  1. Effective Radius of the Earth
In working with propagation problems, the wave paths are considered to be straight lines instead of being as they actually are due to atmospheric refractions. To compensate for the curvature, a larger value of the radius of the earth is used, which is known as the effective radius of the earth.

Duct Propagation

Electromagnetic waves with frequencies in the range of VHF, UHF and microwaves can propagate through the troposphere of the earth’s atmosphere due to repeated refraction from the troposphere. This mode of propagation is called tropospheric ducting or duct propagation.

The troposphere is the layer of the atmosphere that lies directly above the surface of the earth and extends to a height of about 10 km. Under normal conditions, the density of the troposphere is higher near the surface of the earth and becomes progressively lower at higher altitudes. However, because of turbulence and temperature and water-vapour variations in the atmosphere, there is formation of ‘ducts’ in the atmosphere.

The duct may be formed between one layer of the troposphere and the earth surface or it may be formed between two layers of the troposphere. This duct acts as a leaky waveguide and guides the electromagnetic waves between its walls.

Description: 51042.png
Duct propagation
When the duct is formed between the earth and one layer of troposphere, the wave is continuously refracted by the tropospheric layer and reflected by the earth’s surface so that the wave propagates around the curvature of the earth even beyond the line of sight.

When the duct is formed between two layers of the troposphere, the refracted wave does not travel back to the earth; it is refracted back and forth by both boundaries on each side of the duct and the wave propagates through this tropospheric duct. In order to use the tropospheric duct, both transmitting and receiving antennas must be within the duct.

As with ordinary waveguide propagation, there is a certain critical frequency below which the duct propagation will not occur. This critical frequency depends upon the thickness of the duct.


Fading is the fluctuations in signal-field strength at a receiver. It can be broadly classified into two categories:
  1. Selective fading
  2. Multiple path fading

Satellite Communication

The vagaries like fading, ionospheric disturbances and storms put some limitations on the use of sky waves for reliable communication.
Reliable communication can be made at large distances by using artificial satellites. In this system, an artificial satellite is placed in a circular orbit at an altitude of 36,000 km above the earth's surface in the equatorial plane. At this orbit, the period of revolution of the satellite around the earth is 24 hours. Hence, to an observer on the earth, the satellite appears stationary. Such satellites are called geostationary satellites and the orbit is called synchronous orbit.
Basic Elements of Satellite Communication
Satellite communications comprise two main components:
  1. The Satellite The satellite itself is also known as the space segment, and is composed of three separate units:
    • The fuel system
    • The satellite and telemetry controls
    • The transponder
The transponder includes the receiving antenna to pick up signals from the ground station, a broadband receiver, an input multiplexer, and a frequency converter which is used to reroute the received signals through a high powered amplifier for downlink.
  1. The Ground Station This is the earth segment. The job of the ground station is two-fold. In the case of an uplink, or transmitting station, terrestrial data in the form of baseband signals is passed through a baseband processor, an up-converter, a high-powered amplifier, and through a parabolic dish antenna up to an orbiting satellite. A downlink, or receiving station, works in the reverse fashion as the uplink, ultimately converting signals received through the parabolic antenna to baseband signal.

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