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14. General Discussion
The atmosphere of the earth is not uniform. Moisture content, density, and temperature changes in the atmosphere occur erratic ally at various times. Moreover, the composition of the atmosphere is affected by geographical location. This lack of uniformity influences the passage of electromagnetic waves through the atmosphere in many ways. A knowledge of the composition of the earth's atmosphere is essential to an understanding of the effects of atmosphere on the uniformity of wave propagation. As will be explained in more detail in the remaining sections of this Section, the atmosphere can be regarded as being formed from three separate adjacent regions, each of which exerts different influences on the passage of an electromagnetic wave. These regions are known respectively as the troposphere, the stratosphere, and the ionosphere.
An understanding of the effects of these regions is further complicated by variations in the frequency of the transmitted waves; low frequency waves behave differently from high frequency waves. For ease in identification, radio frequencies are normally classified in ranges as shown in Table 1.
15. Wave Types
There are a number of paths by which a radio wave may reach a receiving antenna. Waves traveling these paths are generally given specific names. By far the most important ones are the ground wave, the sky wave, and the space wave.
The term ground wave is generally applied to waves that travel from the transmitting to the receiving antenna with the bottom of the wave front touching the ground, as illustrated in Fig. 11. The wave is always vertically polarized, because any horizontal component existing in the wave is essentially shorted by the earth.
As the ground wave progresses outward from the transmitting antenna, it sets up a minute electrical current within the earth directly beneath the wave. The energy for this earth current is always supplied by the wave above; the two are inseparable. In addition to creating this current, the wave will continue to supply energy in order to lessen inherent power losses within the earth.
These power losses are determined by the frequency of operation and the conductive properties of the ground. Clearly, then, the ground wave will suffer less attenuation over salt water than over dry, sandy soil. The power losses, however, are not purely resistive, but significantly reactive. For this reason the frequency of the wave is the primary determinant in ground wave propagation.
As the frequency is increased, earth losses increase, so that for frequencies much above one megacycle the ground wave is virtually useless, except for local coverage. As a matter of fact, at television frequencies, the attenuation becomes so great that the ground wave is essentially useless at distances as short as one mile from the transmitter.
------------ Table 1.
Frequency:
below 30 khz
30 khz to 300 khz
300 khz to 3 mhz
3 mhz to 30 mhz
30 mhz to 300 mhz
300 mhz to 3000 mhz
3000 mhz to 30,000 mhz
30,000 mhz to 300,000 mhz
Range:
very low frequency (vlf)
low frequency (l-f)
medium frequency (m-f)
high frequency (h-f)
very high frequency (vhf)
ultra high frequency (uhf)
super high frequency (shf)
extremely high frequency (ehf)
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The energy that reaches the receiving antenna by virtue of reflection from or refraction by an ionized layer of air encircling the earth is called the sky wave. Most of the long distance communication carried on below a frequency of 30 mhz is accomplished by means of this sky wave. (Sec Fig. 12.)
16. The Ionosphere
Fig. 11. Propagation of ground waves.
The upper regions of the earth's atmosphere absorb a large amount of radiant energy from the sun, which produces significant ionization of the air molecules. It is this region that reflects or refracts the sky wave back to earth. Because of the variation in atmospheric properties at different altitudes, the ionosphere tends to form in layers, the properties of each layer being dependent upon the specific type of radiation reaching it from the sun and the atmospheric characteristics of a given altitude.
The mechanism by which radiation from the sun produces ionization cannot be adequately described by classical theory. In terms of modern quantum mechanics, photons of different energies (but all traveling at the speed of light) strike these molecules and produce a number of effects, depending upon specific photon energies. (Gas molecules normally exist in their lowest rotational and vibrational states.) Photons of definite kinetic energy excite the molecules to higher states of rotation and vibration, and are absorbed in the process. Other photons of different kinetic energy ionize individual atoms, and are also absorbed in the process. It is this latter phenomenon in which we are most interested.
An ionized atom, by definition, is one that has lost or gained one or more orbital electrons. Since the negative electrons and positive protons cancel each other in a neutral, un-ionized atom, the removal or gain of an electron leaves the remaining atom with a net positive or negative charge.
Fig. 12. Sky wave propagation.
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Planck (18.~8-1948) introduced the idea that light was made up of elemental particles called photons or quanta; this explains numerous phenomena that could not then be justified by the classical wave theory of light. Planck stated that the energy (in ergs) contained in a photon of light was proportional to its frequency, with a constant of proportionality known as Planck's constant.
E = hf
Where:
E Energy of photon in ergs.
h = Planck's constant = 6.56 X 10^-27 erg sec.
f = Frequency of light in cycles per second.
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Ionization by virtue of photon collision can occur in any atmospheric density, but only in the rarefied atmosphere existing above 50 miles from the earth can the ionization, once produced, exist for any appreciable time. This is possible because of the near vacuum that exists at these altitudes. Free electron and ion density are so low that the chances of their collision (and consequent re combination into neutral atoms) are relatively small. This is not, however, a hard and fast rule, since the ionosphere is over one hundred miles thick. The ionization of its lower levels is keyed directly to the amount of sunlight, whereas the air density of the upper extremities is sufficiently low to allow residual ionization to last throughout the night, when the sun's radiation is absent.
If a sky wave enters the ionosphere, the electric field of the wave exerts a physical force upon the free electrons and ions existing in this region. The amount of force may be calculated by means of equation 1. The electric field of any radio wave varies sinusoidally, and therefore the force exerted will tend to make both electrons and ions oscillate sinusoidally, in unison with the applied field. Since an ion has more than 1800 times the mass of an electron, ionic movement may be neglected; only the electrons are appreciably affected by the electric field. The distance over which an unimpeded electron moves varies inversely as the frequency ol the wave; i.e., at a low frequency there is more time, and thus the electron moves a greater distance before the electric field reverses.
As mentioned previously, any accelerating charge radiates electromagnetic waves. Thus these electrons, which absorb energy when set in motion, reradiate it by virtue of that same motion. Because of a phase difference between the oscillating electron current and the original electric field, the direction of re-radiated energy is not the same as that of the incident wave. The direction is de pendent upon the electron density; it always takes the path toward lower electron density. The effect is analogous to having the incident wave bent as it travels through a portion of the ionosphere.
For this reason the bending of the wave is best described with optical analogies.
An electric field in the ionosphere forces the free electrons to oscillate sinusoidally. Without regard to their reradiation, these moving particles constitute an electron (or actual conductive) current -- similar to the type that exists in a wire -- whose magnitude is proportional to the instantaneous velocity of these electrons.
The motion of the free electrons lags the applied field in phase by 90 degrees, in accordance with the laws of simple harmonic motion. This means that the electron current also lags the applied field by 90 degrees.
It should now be clear that the current flowing through a given portion of space in the ionosphere is made up of two components: the previously described space current, which leads the electric field by 90 degrees, and the electron current, which lags the electric field by 90 degrees. The two currents tend to subtract, resulting in a net reduction in the space current. The net space current flowing between any two points in a medium is directly proportional to the dielectric constant of that medium, hence any reduction in the space current is equivalent to a reduction in the dielectric constant below what it would have been had the electron current not been present. Since the dielectric constant for air is essentially I, it follows that it is less than l in the ionosphere, where there is an electron current. The extent of the reduction is principally dependent upon the electron density.
The optical analogy may now be introduced, because the refractive index of any portion of the ionosphere is equal to the square root of its dielectric constant; that is, n=yk
Where: n = refractive index
Where: k = dielectric constant (5)
Moreover, the phase velocity of any wave is inversely proportional to the refractive index. That is, phase velocity = c/n where c is the velocity of light in a vacuum. Putting equation 5 in reciprocal form we have:
1 1 "=v"°k (6)
Multiplying both sides of the equation by c, the velocity of light in a vacuum:
C C n y7c (7)
But c/n is equal to the phase velocity, hence phase velocity = _c_
yk (8)
The most significant part of the above is that the wave's phase velocity will be greater as the dielectric constant is reduced. In fact, since the dielectric constant is less than I, the phase velocity will actually exceed the speed of light.
Imagine a wave front entering the ionosphere at an angle J. Since the top part of the wave front will enter the ionosphere first, its velocity will be greater than that of the lower part of the wave front, which has not yet entered. Under the above conditions one of two things must happen: either the wave front breaks up, or it bends in the direction shown. The latter happens, and this may be justified by using classical Huygens constructions. As the wave front continues in its penetration of the ionosphere, the top part of the wave front will always be in contact with a region of higher electron density, and will thus have a greater velocity, causing a continual bending (or refraction) of the wave. The exact direction the wave takes is governed by Snell's law.
(See Fig. 13~ n = s!n<f>1 ( 9)
sm4>,
Where: 4>1
The angle that the direction of propagation makes with the normal to a unit volume of space in the ionosphere containing a particular electron density (labeled B). The wave front enters this unit volume of space from a different unit volume of space, containing a different electron density (labeled A).
ef>r =
The angle that the wave front makes with the normal as it proceeds through this unit volume (B) of space in the ionosphere.
n =
The relative index of refraction between the two unit volumes being considered (A and B). This is shown in Fig. 12. Note that, since the ionosphere varies in electron density (becoming progressively greater as the middle is approached) , the wave continues to suffer a refraction as it progresses in the medium. If the electron density is sufficient, or the ionosphere thick enough, the wave is returned to earth as shown.
17. Layers or Regions of the Ionosphere
The term ionosphere was first applied broadly to an ionized region above the earth. Upon investigation, however, it was found that the ionosphere was split into a number of layers or regions, corresponding to altitudes where ionic density was at a maximum.
Layers are formed because at different altitudes there are different atmospheric properties, and differences in the sun's radiation.
As an example, the highest regions are produced principally by ultraviolet radiation. On the other hand, cosmic rays, with their greater penetrating power, are responsible for some of the lower region ionization. During sun-spot disturbances, when cosmic ray radiation is at a maximum, the lowest region is often intensely ionized, bringing about communication failure at certain frequencies.
The regions vary in ionic density and height above the earth, depending upon whether or not the area is in daylight -- and, with some regions, depending on the time of day or night. The four principal daytime regions are called the D, E, F1, and F2, regions, as illustrated in Fig. 14.
Fig. 13. Angles between Incident and emerging wave fronts and the normal to
adjacent volumes of different electron densities In the Ionosphere.
The lowest of these regions is the D region. It is at such a low altitude (30 to 55 miles) that the molecules, once ionized, quickly recombine because of the higher air density. For the same reason, the degree of ionization at any time is governed by the amount of sunlight. Far from being a good reflector of radio waves, the D region is responsible for most of the low frequency sky wave attenuation. Much of the energy absorbed by the free electrons from an incident wave is lost in the form of heat through collision of these electrons with un-ionized air molecules.
The collisions may wholly or partially impede the re-radiation of the wave as it passes through the region, depending on the number of molecules present, the amount of ionization of the medium, the frequency of the incident sky wave, and the distance the wave travels through the medium. During the middle of the day, when the D region ionization is at a maximum, the effect is particularly evident.
Fig. 14. Variation of Ionization (electron density) for the Ionospheric layers
under daytime and nighttime conditions.
D region absorption varies inversely as the frequency squared.
It becomes an increasing deterrent to daytime communication as the frequency dips below 6 or 7 mhz -- and absorption reaches a maximum at about mhz. Below this frequency, D region penetration decreases, and so does the absorption.
The E region is the lowest region that is effective in returning the sky wave to earth, and thus affording long distance communication. In many respects, however, it resembles the D region. Its height (about 65 miles) is sufficient to insure that a relatively low amount of electron collision with air molecules will occur, except for periods of high ionization. Its height, however, is not sufficient to allow much residual ionization. Therefore, at noon, when the ionization is most intense, E region absorption becomes pronounced.
Another important region for communication is the F2 region.
The F2 region is the most prominent, and the most intensely ionized region of the ionosphere. The rarefied atmosphere at its height (150 to 250 miles) is sufficient to allow a high degree of residual ionization. The ionization is most intense at about noon; then it gradually decreases, reaching a minimum just before sun rise. Its height varies with sun-spot activity, season, and even time of day.
The F2 region often splits into two distinct regions during the day. The new region is termed the F1 region, while the original main region retains the F2 designation. The main effect that the F1 region has on sky wave communication is to introduce some additional absorption. Thus, during the night the over-all ionization of the atmosphere is lower than during the day. The F1 layer rejoins the F2 layer, making a single F2 layer; the E layer almost completely disappears.
The extent to which a given wave is refracted in the ionosphere is a function of the refractive index. The refractive index is a function of the number of free electrons per cubic centimeter and the frequency.
Where: n
N f (10)
the index of refraction.
the number of free electrons per cubic centimeter.
the frequency of the sky wave in kilocycles.
From examination of the above formula, it can be seen that n must always -- for all real values -- be less than 1 (or zero). The ratio 81 N /f2 is the factor that determines whether the wave will be returned to earth because of refraction in a region, or whether the value will be attenuated when passing through the region. If the ratio is less than 1, n is real and the wave is refracted without significant attenuation. If the wave's angle (unless angle from normal is specified, the angle is measured with respect to the earth or the tangent to the ionosphere) of penetration in the region is low enough, and the region thick enough, the wave will be re turned to earth. If the ratio is greater than one, n is imaginary and the wave is attenuated. The attenuation would be linear with distance of penetration if the layer were homogeneous. Because of the concentration of free electrons at the center, the attenuation is more exponential in nature. Attenuation under these circum stances is roughly analogous to a waveguide that is operating below the cutoff frequency -- the phase velocity approaches infinity and the group velocity approaches zero.
18. Angle of Incidence
If a wave enters a region perpendicularly, there obviously can be no refraction. What does happen is that the wave will penetrate the layer until N increases to the point at which the refractive index is zero (or the ratio of 81N/f2 is 1). If there is sufficient electron density within the region, or if the frequency is low enough, this value of refractive index can be achieved - and the wave is reflected and returned to earth. Of the two variables (electron density and frequency) the only one we have any control over is the frequency, and thus the highest frequency that can be returned at an incident angle of 90 degrees is called the critical frequency. If the frequency is increased above this critical value, the wave can only be returned to earth if the incident angle with the tangent to the region is less than 90 degrees.
19. Critical Angle
Fig. 15. The effect of the angle of radiation on sky wave transmission.
If a wave above the critical frequency is to be returned to earth it must be radiated at a sufficiently low angle with respect to the earth's surface to insure refraction. The highest angle of radiation that permits the radio waves of a given frequency to be returned to earth by a region is called the critical angle for that region. Examples of this are shown in Fig. 15. Because radio waves travel in an approximately straight line from the transmitter to the ionosphere, the angle of incidence to the ionosphere is 90 degrees minus the radiation angle from the transmitting antenna to the ground.
Clearly, as the frequency is increased, increased attention must be paid to the angle of radiation if penetration of the ionosphere, and subsequent loss of the signal, is to be avoided. Each region has a critical frequency and a critical angle for frequencies above the critical value. Often one region will be penetrated (because the sky wave may exceed either or both values) and still the signal may be refracted from a higher region, if it has sufficient electron density. This is shown in Fig. 15, where ray I is easily refracted by the E region because it enters below the E region critical angle. Ray 3 penetrates the E region but is returned to earth by the F2 region because it is below the F2 region critical angle. Ray 4 also penetrates the E region. It enters the F2 region at its critical angle and is returned to earth. Ray 5 penetrates both regions and is lost in space. This diagram applies 'for one frequency only. It a lower frequency is used, higher critical angles for both regions are present; conversely, if the frequency is increased, both regions have lower critical angles. If the frequency is increased sufficiently, there comes a time when, even if a wave is emitted from the transmitter parallel to the earth, it will exceed the critical angle for any region. This condition is reached at about 30 mhz. Above it, the sky wave cannot be used for reliable communication.
20. Skip
If the critical frequency of the ionosphere has been exceeded, and hence there exists a critical angle, there is a minimum ground distance from the transmitter at which the sky wave will be re turned to earth. Note that in Fig. 15 Ray 2 is incident to the E region at its critical angle. Note where the refracted wave reaches the ground. If the critical angle is exceeded, the signal is lost -- yet the distance between the transmitter and the point where the sky wave returns to earth is too great for communication with the ground wave. This is a "silent" zone that encircles the transmitter from the point where the ground wave fades out to the point where the sky wave is first heard. The skip distance is denoted by the minimum terrestrial distance from the transmitter to the point of sky wave return.
As the frequency is raised the skip distance increases, because of a lowering of the critical angle. For communication to a point at a certain distance from the transmitter, and at a certain time of day, and season, there exists a maximum usable frequency (abbreviated MUF) that may be used for sky wave communication.
Each ionospheric region has is own MUF for communication over a given distance from the transmitter. This information is made available in chart form for commercial services.
21. Sporadic E Reflection
In addition to the regular E region ionization there often exists within that region a thin intensely ionized region called a sporadic E "cloud." Sporadic E clouds vary in size from one to several hundred miles across. They usually drift from place to place, and appear and disappear in a most unpredictable manner, during both day and night.
A sporadic E cloud has a rather well defined boundary, and as a rule, waves are reflected from its surface, rather than refracted.
They have no critical frequency. As the frequency of a vertical wave is increased, the returning signal dissipates to a non-usable value. The clouds have no critical angle either, although again, if the sky wave is of a high enough frequency and strikes the cloud at too high an angle, little of the signal will be returned.
Because of their intense ionization, sporadic E clouds are often able to return to earth waves of much higher frequency than 30 mhz.
If the cloud is correctly situated between transmitter and receiver, it can greatly enhance normal E region communication. Great distances have been covered on frequencies up to 50 mhz and higher by using sporadic E reflection. The great disadvantage inherent in this type of communication is that it is unreliable. Clouds appear, vary in ionization intensity and location, and then disappear with an utter lack of regularity. The source of sporadic E ionization is not fully understood.
22. QUIZ
(1) Why does the composition of the atmosphere affect wave propagation?
(2) Define the three regions of the atmosphere.
(3) What is the frequency range of medium frequency waves?
(4) What is the primary determinant in ground wave propagation?
(5) Describe the four ionized layers of the atmosphere.
(6) Explain the process of ionization.
(7) Define critical frequency.
(8) How does the critical angle control wave reflection?
(9) Define "skip"; "silent zone"; "skip distance".
(10) What is sporadic E reflection?