Sky Wave Propagation
Sky Wave propagation is the mode of propagation in which electromagnetic waves emitted from an antenna and directed upward at great angles are reflected back to earth by the ionosphere. Since sky wave propagation takes place after reflection from the ionosphere, so it is called as ionospheric propagation. The Sky Waves are of practical importance at medium and high frequencies for very long-distance radio communications. In this mode of communication electromagnetic waves reach the receiving point after reflection from the ionized region in the upper atmosphere called ionosphere situated between 50km to 400km above earth surface under favorable conditions. The ionosphere acts like a reflecting surface and is able to reflect back the electromagnetic waves of frequencies between 2 to 30 MHz.
The Ionosphere Layer
Ionosphere is a region of the Earths upper atmosphere characterized by the presence of charged particles, primarily electrons and ions. It plays a crucial role in radio wave propagation, especially for sky wave communication. The ionosphere is composed of several distinct layers, each with its own unique properties and interactions with radio waves. There will be certain number of atoms presents in this layer. We could observe that the density of these of atoms will reduce as it moves away from the earth surface. These layers are essential to understanding how radio signals are reflected and refracted through the ionosphere.
1. D Layer: The D layer is the innermost layer, which is the lowest, thinnest layer. Which is located at altitudes of approximately 48 to 90 kilometers (30 to 56 miles) above the Earths surface. The electron density of this layer is maximum, i.e a large number of atoms will present here. Radio waves passing through the D layer experience significant attenuation due to absorption, making it less suitable for long-distance communication. It absorbs low-frequency radio waves during the day but disappears at night. It is most prominent during daylight hours when it is strongly ionized by solar ultraviolet (UV) radiation.
2. E Layer: The E layer is the middle layer, which is altitudes ranging from 90 km to 150 km (56 to 93 miles) above the surface of the Earth. It is less dense than the D layer, i.e comparatively lesser number of atoms will present in this layer. This layer can only reflect radio waves having frequencies lower than about 10 MHz and may contribute a bit to absorption on frequencies above. It can also produce sporadic E layers that are highly variable and unpredictable, which can reflect frequencies up to 50 MHz and higher. At night the E layer weakens because the primary source of ionization is no longer present. After sunset an increase in the height of the E layer maximum increases the range to which radio waves can travel by reflection from the layer.
3. F Layer: The F layer, which is the highest and thickest, and occurs between 150 and 500 km above the Earth’s surface. The electron density is least in this layer i.e least number of atoms will present here. It splits into two sub-layers: F1 layer, which often forms in the electron density profile during the day and F2 layer, remains by day and night, it is responsible for most skywave propagation of radio waves and long-distance high frequency radio communications due to its high electron density and strong ionization. The F2 layer has the ability to reflect radio waves over very long distances, making it essential for global communication, particularly during the nighttime. It is the most important layer for sky wave propagation, as it allows radio signals to travel beyond the horizon.
Let’s see how the ionospheric layers are produced. We know that we have a powerful source which is referred to as the Sun. When the sun shines brightly on the earths surface the sun produces certain rays called Cosmic Rays and these Cosmic Rays fall on top of the atmosphere of our earth. At the topmost F layer, the density of these Cosmic Rays would be the maximum that is a large number or a huge number of cosmic rays would be present at the F layer. When Cosmic Ray strikes a particular atom, it will eject an electron out of this atom. We know that when an electron is ejected out of this atom this atom becomes an ion. So that is this atom thus becomes a charged ion. In the F layer the number of atoms is very few and therefore these Cosmic Rays interact with these atoms and therefore a less dense ionization layer is formed over here. Now after these cosmic rays interact with E layer, a comparatively lesser number of Cosmic Rays would then fall on the next layer and therefore when these Cosmic Rays falls on these particular atoms it will again form another ionization layer over here. After ionization happens here then the cosmic rays will further fall on the bottommost D layer and then ionization happens. But when it reaches D layer only a very few numbers of Cosmic Rays would reach here and therefore only very few atoms would get ionized over this layer. Therefore, when you observe this the topmost layer will have a very less ionization density because the number of atoms is less now when you observe the middle most layer ionization density would be comparatively more now when you observe the lowermost layer since the number of Cosmic Rays are less and the ionization density would also be less. So, this does is how the various ionization layers are produced.
How Sky Wave Propagation Work:
Sky wave propagation begins with the generation of radio waves at a transmitting antenna. These radio waves are typically in the form of electromagnetic waves, and they carry information, such as voice, data, or other signals, and are sent into the atmosphere. These waves travel in all directions of atmosphere from the transmitting antenna. As the radio waves travel upward into the atmosphere, they encounter the ionosphere.
We know that radio waves are composed of electric and magnetic fields. Also, the charged particles present in the ionosphere layers and they have their own electric field. So, when radio wave is allowed to be propagated through the earth’s atmosphere then the field of the radio wave and the charged particles interact with each other. And this leads to cause reflection of the electromagnetic wave by the atmosphere. We know that when light propagates from a denser to a rarer medium with an angle equal to or greater than a critical angle then it gets reflected back towards the same medium. This is referred as Total Internal Reflection.
In a similar way when the transmitting antenna transmits the electromagnetic wave with a certain angle (equal or greater than critical angle) then due to ionization on the earth’s atmosphere it gets reflected back towards the surface of the earth. This causes the reception of the reflected signals by the receiving antenna. It is to be noted here that the field in the atmospheric layer must be sufficiently large so that it can allow reflection of the electromagnetic waves through it. This is so because it may be possible that a high-frequency wave may not be reflected by the lower region of the ionosphere. However, with upward movement, even the high-frequency wave will get reflected due to a higher degree of ionization.
So, we can say this as a low-frequency wave is reflected by the lower layer and the high-frequency wave is reflected by the upper layer. But beyond a certain permissible frequency (generally 30 MHz) the wave despite getting reflected penetrates the atmospheric region and is lost.
Hence sky wave propagation is suitable for the frequency range from 2 MHz to 30 MHz.
The range of frequencies that can be used for sky wave propagation is limited by two factors: the critical frequency and the maximum usable frequency.
Critical Frequency: The critical frequency is defined as the maximum frequency at which the total internal reflection takes place from the ionosphere. The critical frequency varies depending on atmospheric conditions, time of day, season, solar activity, and angle of incidence of the radio waves by the antenna.
Maximum Usable Frequency: The maximum usable frequency is defined as the highest frequency that can be used for sky wave propagation between two points on Earth at a given time. The maximum usable frequency depends on the critical frequency, the angle of incidence, and the distance between the transmitter and receiver.
Skip Distance: The skip distance is defined as the minimum distance between the earth’s surface and the radio signal’s transmission point. For flat earth, skip distance depends on the height at which reflection happens, maximum usable frequency and critical frequency.
Advantage
- It enables long-distance communication without relying on satellites or cables.
- The frequency range of operation is considerably high.
- In this propagation attenuation due to atmospheric conditions is less.
Disadvantage
- It is affected by various factors that can change signal quality, such as atmospheric conditions, time of day, season, solar activity, and structure of the ionosphere.
- Long-distance propagation requires large-sized antennas.
- It requires complex circuitry and equipment for modulation and demodulation, as it involves nonlinear operations.
Application
- It is commonly used in shortwave broadcasting, which allows radio signals to be transmitted over long distances.
- It is used in high frequency communications, particularly in long-distance communication for aviation, maritime, and military purposes.
- It can be used by radio astronomers to study celestial objects that emit radio waves, such as pulsars, quasars, galaxies, and stars.
- It can also help detect signals from extraterrestrial intelligence, if they exist.
Written By,
Department of ICT,
Comilla University, Cumilla.