The first step in using electromagnetic waves in space for radio communication was taken by James Clark Maxwell when he came up with “the theory of the electromagnetic field” in 1873. Maxwell claimed that magnetic waves were subject to reflection, refraction, and absorption, just as light is. The existence of these waves was first demonstrated by Heinrich Rudolph Hertz in some experiments carried out in 1888. His studies constituted the base for Guglielmo Marconi to conduct experiments with wireless telegraphy using Morse code.
In 1896, Marconi was successful in sending signals through a wireless telegraph to a distance of a few kilometers away. However, how would it be possible to provide intercontinental communication via radiotelegraphy and radiotelephone? In 1901, together with his assistants, G.S. Kemp and P.W. Paget, Marconi successfully transmitted and received transatlantic signals between Poldhu, Cornwall and New Foundland, Canada, using a kite aerial at Signal Hill in Cornwall, England. It was Edward Appleton who first discovered that radio waves were broadcast around the world after they are reflected back from the ionosphere, one of the highest electrified layers of the atmosphere that contains large concentrations of charged particles (ions) and free electrons. Electromagnetic waves that are sent from radio transmitters to outer space are reflected back to every corner of the Earth after hitting this gas and plasma layer that is composed of charged particles. Thus, radio and radiotelephone communication is made possible for the benefit of human beings. After that time, being able to use a law that had been ordained by the Supreme Creator, human beings were able to reach a level that enabled them to conduct transatlantic communications via radiotelegraphy. But what makes radio waves so special?
The frequency spectrum of electromagnetic waves begins from the “sub-sound frequency region” (1Hz) stretching up until cosmic rays (Figure 1). Radio communication is made using the electromagnetic waves that form part of this frequency spectrum. Radio communication systems can be classified into four groups relating to their frequency regions:
- LF/MF (Low Frequency/Medium Frequency)
- HF (High Frequency)
- VHF/UHF (Very /Ultra High Frequency)
- SHF (Super High Frequency)
Specifications of radio waves are taken into account in this classification. The main element that makes radio waves similar or different from each other is the frequency band that determines their wave length. Radio waves move at the speed of light (300 thousand km per second), much faster than sound itself, so to find the wave length of a radio wave, we divide its velocity by its frequency.
Frequencies used within the radio frequency spectrum measure between 20 KHz and 30 GHz. Theoretically, the high frequency band is between 3 and 30 MHz, while in practice it is between 1.6 and 30 MHz. The interval between 4 and 18 MHz is the most-widely used region in the spectrum.
Our Lord, Who incessantly prepares the Earth in a beautiful manner, also protects all of life with a perfect shield called the “atmosphere.” Scientists have divided the atmosphere into seven layers in order to reveal the unknown facts about it. These seven layers are different from each other in terms of temperature, pressure and humidity levels, and the natural events that occur in them. If we ascend from the Earth toward the sky, we pass through the layers of the troposphere, stratosphere, ozonosphere, mesosphere, thermosphere, ionosphere and the exosphere. All these layers cover a distance of about 3,000 km. Each of the atmospheric layers serves a vital cause. Every layer has many functions, ranging from the formation of rain clouds to the prevention of harmful beams reaching the Earth, from reflecting radio waves to inactivating meteors. One duty of the ionosphere that we are aware of today is to act as a reflector and distributor for radio waves.
The ionosphere and distribution of radio waves
Good transatlantic radio communication depends upon many factors. Depending on the frequency of the radio waves, the season of the year, the position of the Sun, the location of the broadcasting area and the time of day, the communication area may vary from 100 km to 10,000 km.
Radio waves are propagated around the Earth in two forms, either as ground waves or as sky waves (Figure 2). In high-frequency radio communication, it is important to choose the best frequency for the time and means of propagation.
Starting from 50 km above the Earth and stretching 440 km, the ionosphere is filled with a high concentration of free electrons and gases. Why is the ionosphere important for transatlantic radio communication? The electrified ions that fill the whole of the ionospheric layer that completely surrounds the Earth reflect radio waves from all directions to every part of the world. According to their frequencies and ionization, radio waves are completely absorbed in the ionosphere and they are either partly refracted and distributed to the outer space or are reflected and returned to the world. The electromagnetic waves within a range of 30 MHz can return to Earth after being reflected by the ionosphere.
It is accepted that the ionosphere is formed at different ionizing levels in different layers, known as D, E, F1, and F2 (Figure 3). The ionization level in the outer layers of the ionosphere is higher than that of the inner layers. The D layer, the innermost layer of the ionosphere, is 76-93 km above the Earth and is characterized by low ion densities and low collision frequencies of electrons and ions with neutral particles. Serving to absorb most energy below 7 MHz, this layer is ionized during the daylight hours, completely disappearing at night. It reaches full ionization level just after sunrise and is at its peak at noon time, immediately losing energy after sun-set.
The E layer is the region of the ionosphere that was discovered first. In this layer, molecular ion production is at its peak at about 110-115 km above the Earth. There are plenty of molecular gases at this height. This layer is a suitable platform from which radio operators can reflect signals to distant stations. Reaching a maximum at noon, the ionization in the E layer decreases towards the end of the day, disappearing completely at midnight. Moreover, at unpredictable intervals, ionized gas clouds accumulate in certain regions of this layer. This can be detected by the variable dense clouds of ionization that occur in the E layer due to the spatial and temporal structure in the ionizing particle precipitation. The plasma density of the E layer can be greatly changed because of these occasional formations. These formations, which are called “sporadic E layers,” are used by radio amateurs for long distance VHF (Very High Frequency) operation. Since the plasma density in layers D and E is highest at noon and present during the other hours of daylight, these layers are used in the daytime.
The next layer of ionosphere exists at about 160 and 400 km above the Earth and consists of layers that have a higher density of free electrons caused by the ionizing effect of solar radiation. Since the density of gas molecules at this height is low, ion and electron collisions occur very slowly in this layer. When solar radiation is high (during the day) this layer can be divided into two independent regions, F1 and F2. The F1 layer is present at 152 and 203 km above the surface of the Earth. During the night, the F1 layer merges with the F2 layer. The F2 layer exists at 250 and 400 km above the surface of the Earth. The majority of HF (shortwave) transmissions are propagated by the F2 layer, which is the main reflecting layer for HF communications both at day and at night. Reaching its maximum level of ionization just after midday, the layer is at its minimum just before sunrise. The F2 layer can be used for 10-20 MHz during the day and 3-8 MHz during the night. Since the F layer exists at a very high altitude, it is exposed to sunlight for longer periods of the day and it dissipates very slowly at night. In this case, the only layer of the ionosphere that can be used during the night is the F layer, which I is composed of the F1 and F2 layers.
Solar radiation, and consequently ionization, alters periodically. For instance, as the days are long during the summer months, ionization is also high at this period. During this time, radio waves are absorbed or attenuated more in layers E and D, and propagation covers only a small area. However, since the days are shorter during the autumn and winter, less solar energy reaches these ionospheric layers. Hence, low frequencies can easily pass through the weakly ionized D and E layers and reach the stronger F layer from where they can be propagated over long distances.
Another long term factor in ionization is the regular 11-year activity cycle of sun spots. Sun spots are believed to be caused by violent eruptions on the Sun and they are characterized by unusually strong magnetic fields. During periods of maximum sun spot activity, the density of ionization increases in all the layers of the ionosphere. During these periods, the D layer absorbs more and the critical frequencies of layers E, F1 and F2 are higher, therefore, for long distance communication higher operating frequencies over 30 MHz should be used. During terms of minimum sun spot activity, the E and F layers have weak ionization, so they cannot reflect the radio waves back onto the Earth. In this period, frequencies over 20 MHz are not used much. Along with this regular variation, “sudden ionospheric disturbances (SID)” also negatively affect the propagation of radio waves. SID are thought to be caused by severe solar eruptions, but the real cause of this phenomena is still not clearly known. (Figure 4)
Sudden ionospheric disturbances can disturb radio communication for hours or even days. Strong solar eruptions cause a sudden abnormal increase in the ionization density in the D layer, hence even the high frequency radio waves coming from the side of the Earth that is facing the Sun are completely absorbed by this layer and frequencies above 2 MHz are unable to penetrate it. When SID occurs, long distance propagation of HF radio waves may be completely blocked.
Ionospheric storms are another disturbing factor for radio communication. When a solar eruption occurs, it takes between 20 and 40 hours for the magnetic storm to reach the Earth. The ionospheric storms cause the F2 layer to virtually lose its ion density. At this time, when the range of frequencies used for communication is much smaller than normal, communication is only possible at lower frequencies.
Frequency and propagation routes in radio communications
The definition of the frequency to be used for radio communication is an important parameter for ensuring healthy propagation. For this, the Maximum Usable Frequency (MUF), and the Lowest Usable Frequency (LUF) are determined. Frequencies over MUF penetrate the ionosphere, shooting right through the ionosphere and going out into space, whereas frequencies below MUF are reflected. LUF is the lowest frequency that is completely absorbed in the D layer. To conduct good communication, a frequency, calculated as MUFÃ—0.85, should be used. This frequency may be lower at night and higher during the day.
Apart from the propagation frequency, the path that is chosen to transmit the radio waves from one point to the other also must be calculated accurately. The angle at which the radio waves enter the atmosphere (angle of incidence) defines the path that will be covered by the waves on their way to Earth. The angle of incidence should be small enough for the waves to be reflected back to Earth and large enough so that the waves will not penetrate the ionospheric layer. Smaller critical angles should be used for smaller frequencies and larger critical angles should be used for larger frequencies so that they will not penetrate through the ionospheric layer and be lost in space.
Consequently, apart from periods when solar eruptions are strong, radio waves that are over 30 MHz frequency are not reflected and can penetrate the atmosphere and reach outer space, hence making the communication between outer space and the Earth possible.
For transatlantic communications conducted via communication satellites, radio waves over 30 MHz are used. Artificial satellites imitate the ionospheric layer, their original source of inspiration, and act as a reflector for these waves (Figure 5). Waves coming from the Earth are reflected by these satellites if they are within their coverage area. However, these manmade satellites have very limited coverage areas. Although they are produced with the highest technology available, their cost is very high and they last only for about 25 years. Nevertheless, for radio waves lower than 30 MHz, the ionosphere, that covers the whole of our planet, acts as a natural satellite. Because of this characteristic of the ionosphere, we do not have to focus at any certain point. Moreover, there is no need for maintenance, nor any energy supplement, and the ionosphere is permanent. The atmosphere has been granted for our service for as long as Earth survives. Through searching and exploring new facts about the universe and all beings, we realize more and more that neither meaningless nor useless matter exists in the material world of creation. Therefore, we are better able to understand that the universe is packed with wonderful favors and blessings that are addressed directly to humanity.