Propagation as defined by Webster Dictionary as "the phenomenon of radio frequency energy traveling through the earth's atmosphere, as well as through the empty space above the atmosphere".
Once radio-frequency energy has been emitted from an antenna it will travel away from the antenna, where it is generated from, at the speed of light. Once it is sent the wave exists and propagates independently of the system that produces it. This means that changes, which occur at the transmitter (antenna), will have no effect on the energy that has already been radiated. The front of the wave is round or spherical in nature. The direction in which the wave will travel is dependent on the characteristics of the antenna from which the wave is generated.
For the purpose of analyzing and predicting propagation, it will take place in two manners or modes: ground wave propagation mode and sky wave propagation mode. These two modes are very different and distinct, as described below.
Ground - Wave Propagation Mode:
Ground - Wave propagation mode refers to the propagation effects, which are most directly determined by the earth's presence. Ground - Wave propagation include several different components (See Figure 1). The direct wave component will include the radiation that travels through the atmosphere on a direct line from the sending antenna to the receiving antenna. The direct wave case is sometimes referred to as line of sight propagation since it is limited by the curvatures of the earth's surface. (See Figure 2). However in actual fact radio waves travelling near the earth's surface bend and follow the surface to a small degree. This bending is refraction caused by the atmosphere. As a result of the refraction, the "radio horizon" is slightly beyond the visual horizon. Direct - wave propagation is especially important when the frequency of radiation is above approximately 30 MHz. At such frequencies the direct wave is the major component of ground-wave propagation. It is also important to remember to know that line of sight transmission means that signals which are propagated mainly as direct waves do not go around or through objects or any other obstructions of large size.
Therefore, a ground wave includes these components; surface wave, direct wave, tropospheric wave, and ground reflected waves. A surface wave will travel near the earth's surface and is significantly affected by that surface. This wave is works best with vertical polarization and relatively low frequencies, approximately 2MHz. Absorption by the earth's surface limits the range of the wave. Direct wave propagation is utilized for transmissions of frequencies higher than those for which the surface wave is used. Polarization may be vertical or horizontal - both are used. Vertical polarization is used commonly for two-way radio communication; horizontal polarization is used for commercial broadcast FM and television transmissions. The range of direct wave propagation is limited to line of sight, which, because an effect called atmospheric refraction, is approximately 15% greater than visual line of sight. The tropospheric wave is a direct wave that has received significant refraction by the troposphere, the lower atmosphere. Refraction occurs when weather conditions, such as temperature inversion, produce layers of differing refractive indexes in the atmosphere. The boundaries of such layers produce refraction. Tropospheric wave propagation's achieve remarkable ranges at times, but may also be unreliable. Ground reflected wave might enhance a direct wave but often cause undesirable effects such as television ghost images.
Sky - Wave Propagation Mode:
The second major type of wave propagation includes what is called a sky wave. The sky wave is the product of radiation whose direction initially is above the horizontal, toward the sky. It is a relatively complex event; there are several factors that determine the ultimate nature of this mode of propagation (See Figure 3). This diagram depicts the several different paths of transmission associated with sky waves. The most important factor that effects the propagation of sky waves is the ionosphere.
The meaning of ionosphere means literally means the sphere of ions. It is a sphere of ionized gases that encompass the earth. An ion is an atom or molecule (group of atoms) which has a net electrical charge. The sphere of ions has four particular layers; they are termed the D, E, F1 and F2 layers. The layers are different heights above the earth's surface, the order of the heights is the same as that shown in (See Figure 4), with the D layer nearest to the earth's surface.
The molecules of the gases at the upper ends of the earth's atmosphere are ionized by radiation from the sun. Ionization is maintained for long periods because the low density of the gases in the upper atmosphere reduces significantly, this will produce what is called de-ionization, the recombination of electron and atom. Most of the ionizing energy is produce from the sun, there are also other sources of ionizing energy, and these are a direct result of cosmic energies.
The amount of ionization, is a directly depends on the intensity of the ionizing energy. Since the majority of that energy is ultraviolet radiation from the sun, therefore the effect on radio wave propagation is practically totally by the character of the sun's radiation striking the earth. Therefore because of the variable nature of the sun's energy reaching the earth, the intensity of ionization and the number and altitude of the layers of the ionosphere undergo constant changes, these changes will follow a repetitive pattern. There is a cycle of changing radiation of a much greater period of time, which is termed, the sun spot cycle. Sunspots, which make themselves apparent as dark spots on the sun's surface. These are tremendous storms, or localized disturbances, these storms consists of explosions of gases which flare out from the suns surface to distances of half a million miles. These explosions increase the distance between the sun's surface to that of the earth's, therefore increasing the ionization energy of the sun. This occurrence therefore increases the intensity of ionization in the atmosphere, which will intern effect any kind of electrical propagation, these sunspots occur once every eleven years.
An ionized gas is a relatively good conductor; the layers of the ionosphere affect the propagation of radio waves, because they are large conducting surfaces. Depending on particular basis, the ionosphere may absorb, reflect or dispense electromagnetic radiation. The overall effect of the ionosphere on the transmission depends on several factors, including: height of layer, frequency of wave, angle between direction of propagation and surface of layer, intensity of ionization of layer, and density of layers.
For long distance communication, the most useful effect of the ionosphere is the reflection of a wave. When a wave is reflected, the angle of the reflection is equal to the angle of incidence. A reflected sky wave is capable of traveling a much further around the than a surface wave or direct wave (See Figure 5).
Whether or not a wave is reflected from an ionospheric layer depends on its frequency and the ionization density of the layer. This density determines the distance between the ionized molecules in a gas form. In a high ionization density gas, the distance between the ionized molecules is relatively shorter than in a low ionization density gas. A high-density gas will be able to turn back (reflect) a shorter wave than a low-density gas. A low-density gas will allow a short wave to pass through (between the molecules, as it were) and turn back only long waves.
For a given ionization density of layer, there is a highest frequency that will be reflected; waves of higher frequency will simply pass on through. This frequency is called the critical frequency. Since the ionospheric layers nearest the sun - the E and F (F1 and F2) layers have the greatest ionization, their critical frequencies are greater (they reflect higher frequencies than does the D layer). However, as described above, the ionization of the various layers are changing constantly, sometimes predictable, but most times unpredictably. Therefore, ever layer will have a potentially different critical frequency, and also these critical frequencies will change dependent on the different ionization levels present.
The concept of critical frequencies, as described, applies to radiation that is beamed directly at the ionosphere from an antenna that is based on earth, the direction of travel makes a 90 angle with respect to the earth's surface. Reflection of waves whose frequencies are higher than the critical frequency occurs when the angle of travel is smaller (See Figure 5). However, as the angle between the direction of travel and the earth's surface increases (approaches 90), an angle is reached at which the angle of the radiation is not bent sufficiently to permit to the earth. The radiation is effectively lost in, or absorbed, by the ionospheric layer (See Figure 6). This angle is called the critical angle.
The critical angle also depends on the ionization density of the layer and the frequency of the wave. The critical angle is smaller for higher frequencies: it is smaller for lower ionization densities. For a given frequency, the critical angle is larger i.e.: the E layer than for than D layer. When the direction of travel that a waves makes with the earth's surface is greater than the critical angle, the wave simply passes through a given layer. It may or may not be reflected by a higher layer, however it depends on whether the critical angle is exceeded for such a layer.
Before looking at the overall effect of the ionosphere on the radio wave transmission let us examine specific details concerning the ionospheric layers. The D layers have the least amount of ionization density; this is because it is further from the sun. The higher layers and between it and the sun, intercepting and diminishing the energy available for the ionization at the lower level. The D layer is present only during daylight hours and has its greatest significance at midday when the sun's effect is the greatest. Its ionization is not great enough to bend the wave directions expect for the lower frequencies. It will only effect waves whose frequency levels are 500 kHz. It has a negative effect on the propagation of sky waves because it will absorb the energy from them, therefore attenuates their field strength, as they pass through. The D layer extends for 30 to 55 miles above the earth's surface.
The band of ionization extending above 55 to 90 miles above the earth's is designated to the E layer. Like the D layer the conditions of this layer will be effected by various reasons mentioned above. It however, will reflect waves of greater frequencies - 20 MHz.
The effective range of a wave reflected from an ionospheric layer is determined by the basic geometry involved: the layer is at some definite distance from the earth's surface; the direction of travel most take into effect the limitation imposed by the critical angle concept. The travel path of the waves forms two sides of a triangle (See Figure 7). The effective communication distance - the distance from the transmitter to the receiver, measured along side the earth's surface - forms the third side of the triangle. The length of this third side is determined by the lengths of the other two sides and the angles between them. Since the wave is not turned around sharply at the surface of the layer but travels along side an arc, the effective height of the point of reflection is slightly greater then the height of the lower surface of the layer.
To obtain greater transmission distances it would be necessary to decrease the angle the direction of travel makes with respect to the earth' surface (See Figure 7). Since this may to be so effective since the wave travels a much larger distance through the ionized layer and therefore is attenuated.
The most ionized layer is the F layer. It is also, therefore, the most useful for long distance transmission of higher radio frequencies. The F layer exists from about 90 miles above the earth's surface to an altitude of approximately 250 miles. During times of maximum ionization in the cycles of variations the F layer is seen as splitting into two layers F1 and F2. The F1 layer exceeding from about 90 to about 150 miles above the earth, and the F2 layer, which exists between 150 to 250 miles above the earth. The F1 layer is lowest during the daylight hours; the two layers appear to merge into one at night and the lower surface is somewhat higher than during the day.
Beyond the ground wave range, a distance of 100 miles or so from the transmitting antenna, receivers would be in what is called the skip zone. This is a region in which the transmitted signal is highly attenuated, basically to weak to be received satisfactorily. At some considerable distance from the transmitter, an area were the reception is very good as a result of reflection of the transmitted wave from an ionospheric layer. The distance from the transmitting antenna to such reception points is called the skip distance. Skip distance and the transmission range due to skip propagation, varies with the conditions that alter the characteristics of the ionosphere.
Conditions that determine the skip distance for a particular transmission can change very quickly. The result is a sudden attenuation of the received signal, a phenomenon called fading. Signals will fade in and out.
Unfortunately, fading does not only effect the amplitude of a received signal. Because a modulated RF signal represents a band of frequencies, fading may cause distortion of the signal. This is an effect known as selective fading. Different frequencies are reflected by different amounts. Thus different frequencies, even in a given transmission, may be attenuated differently, producing selective fading and the resulting distortion
All of the propagation effects, which are dependent on the reflection of radio waves from the ionosphere, are limited to those frequencies that can be reflected. As a result, the term maximum usable frequency is used often is discussions involving ionospheric propagation. The maximum usable frequency (MUF) is the highest frequency at which signals will be reflected by the F2 layer, the layer of greatest ionization.
As can be seen there are many factors to consider when rebounding signals off the ionosphere (i.e. utilizing transmission paths made possible by reflections from the ionosphere). Also to the mostly predictable cycled variations of characteristics of the ionosphere, there are numerous sudden, unpredictable variations. All of these factors make designing and working with RF equipment very interesting.
I have covered the terms and principles require I have offered descriptions for all the terms listed on the handout assignment. I hope, from reading my report, a greater knowledge has been gained about propagation.