Module 10—Introduction to Wave Propagation, Transmission Lines, and Antennas
Density of Layer
i - ix
, 1-1 to 1-10
1-11 to 1-20
, 1-21 to 1-30
1-31 to 1-40
1-41 to 1-47
2-1 to 2-10
, 2-11 to 2-20
2-21 to 2-30
, 2-31 to 2-40
2-40 to 2-47
, 3-1 to 3-10
3-11 to 3-20
3-21 to 3-30
3-31 to 3-40
, 3-41 to 3-50
3-51 to 3-58
, 4-1 to 4-10
4-11 to 4-20
, 4-21 to 4-30
4-31 to 4-40
, 4-41 to 4-50
4-51 to 4-60
Figure 2-15 illustrates the relationship between radio waves and ionization density. Each ionized layer has a
central region of relatively dense ionization, which tapers off in intensity both above and below the maximum
region. As a radio wave enters a region of INCREASING ionization, the increase in velocity of the upper part of
the wave causes it to be bent back TOWARD the Earth. While the wave is in the highly dense center portion of the
layer, however, refraction occurs more slowly because the density of ionization is almost uniform. As the wave
enters into the upper part of the layer of DECREASING ionization, the velocity of the upper part of the wave
decreases, and the wave is bent AWAY from the Earth.
Figure 2-15.—Effects of ionospheric density on radio waves.
If a wave strikes a thin, very highly ionized layer, the wave may be bent back so rapidly that it will
appear to have been reflected instead of refracted back to Earth. To reflect a radio wave, the highly ionized
layer must be approximately no thicker than one wavelength of the radio wave. Since the ionized layers are often
several miles thick, ionospheric reflection is more likely to occur at long wavelengths (low frequencies).
For any given time, each ionospheric layer has a maximum frequency at which
radio waves can be transmitted vertically and refracted back to Earth. This frequency is known as the CRITICAL
FREQUENCY. It is a term that you will hear frequently in any discussion of radio wave propagation. Radio waves
transmitted at frequencies higher than the critical frequency of a given layer will pass through the layer and be
lost in space; but if these same waves enter an upper layer with a higher critical frequency, they will be
refracted back to Earth. Radio waves of frequencies lower than the critical frequency will also be refracted back
to Earth unless they are absorbed or have been refracted from a
lower layer. The lower the frequency of a radio wave, the more rapidly the wave is refracted by a
given degree of ionization. Figure 2-16 shows three separate waves of different frequencies entering an
ionospheric layer at the same angle. Notice that the 5-megahertz wave is refracted quite sharply. The 20-megahertz
wave is refracted less sharply and returned to Earth at a greater distance. The 100-megahertz wave is obviously
greater than the critical frequency for that ionized layer and, therefore, is not refracted but is passed into
Figure 2-16.—Frequency versus refraction and distance.
Angle of Incidence
The rate at which a wave of a given frequency is refracted
by an ionized layer depends on the angle at which the wave enters the layer. Figure 2-17 shows three radio waves
of the same frequency entering a layer at different angles. The angle at which wave A strikes the layer is too
nearly vertical for the wave to be refracted to Earth. As the wave enters the layer, it is bent slightly but
passes through the layer and is lost. When the wave is reduced to an angle that is less than vertical (wave B), it
strikes the layer and is refracted back to Earth. The angle made by wave B is called the CRITICAL ANGLE for that
particular frequency. Any wave that leaves the antenna at an angle greater than the critical angle will penetrate
the ionospheric layer for that frequency and then be lost in space. Wave C strikes the ionosphere at the smallest
angle at which the wave can be refracted and still return to Earth. At any smaller angle, the wave will be
refracted but will not return to Earth.
Figure 2-17.—Different incident angles of radio waves.
As the frequency of the radio wave is increased, the critical angle must be reduced for refraction to
occur. This is illustrated in figure 2-18. The 2-megahertz wave strikes the layer at the critical angle for that
frequency and is refracted back to Earth. Although the 5-megahertz wave (broken line) strikes the ionosphere at a
lesser angle, it nevertheless penetrates the layer and is lost. As the angle is lowered from the vertical,
however, a critical angle for the 5-megahertz wave is reached, and the wave is then refracted to Earth.
Figure 2-18.—Effects of frequency on the critical angle.
Q20. What factor determines whether a radio wave is reflected or refracted by the ionosphere?
Q21. There is a maximum frequency at which vertically transmitted radio waves can be refracted back to Earth.
What is this maximum frequency called?
Q22. What three main factors determine the amount of refraction
in the ionosphere?
Skip Distance/Skip Zone
In figure 2-19, note the relationship between the sky
wave skip distance, the skip zone, and the ground wave coverage. The SKIP DISTANCE is the distance from the
transmitter to the point where the sky wave is first returned to Earth. The size of the skip distance depends on
the frequency of the wave, the angle of incidence, and the degree of ionization present.
Figure 2-19.—Relationship between skip zone, skip distance, and ground wave.
The SKIP ZONE is a zone of silence between the point where the ground wave becomes too weak for reception
and the point where the sky wave is first returned to Earth. The size of the skip zone depends on the extent of
the ground wave coverage and the skip distance. When the ground wave coverage is great enough or the skip distance
is short enough that no zone of silence occurs, there is no skip zone.
Occasionally, the first sky wave
will return to Earth within the range of the ground wave. If the sky wave and ground wave are nearly of equal
intensity, the sky wave alternately reinforces and cancels the ground wave, causing severe fading. This is caused
by the phase difference between the two waves, a result of the longer path traveled by the sky wave.
The path that a refracted wave follows to the receiver depends on the angle at
which the wave strikes the ionosphere. You should remember, however, that the rf energy radiated by a transmitting
antenna spreads out with distance. The energy therefore strikes the ionosphere at many different angles rather
than a single angle.
After the rf energy of a given frequency enters an ionospheric region, the paths that this energy might follow are
many. It may reach the receiving antenna via two or more paths through a single layer. It
may also, reach the receiving antenna over a path involving more than one layer, by multiple hops
between the ionosphere and Earth, or by any combination of these paths.
Figure 2-20 shows how radio waves
may reach a receiver via several paths through one layer. The various angles at which rf energy strikes the layer
are represented by dark lines and designated as rays 1 through 6.
Figure 2-20.—Ray paths for a fixed frequency with varying angles of incidence.
When the angle is relatively low with respect to the horizon (ray 1), there is only slight penetration
of the layer and the propagation path is long. When the angle of incidence is increased (rays 2 and 3), the rays
penetrate deeper into the layer but the range of these rays decreases. When a certain angle is reached (ray 3),
the penetration of the layer and rate of refraction are such that the ray is first returned to Earth at a minimal
distance from the transmitter. Notice, however, that ray 3 still manages to reach the receiving site on its second
refraction (called a hop) from the ionospheric layer.
As the angle is increased still more (rays 4 and 5),
the rf energy penetrates the central area of maximum ionization of the layer. These rays are refracted rather
slowly and are eventually returned to Earth at great distances. As the angle approaches vertical incidence (ray
6), the ray is not returned at all, but passes on through the layer. ABSORPTION IN THE IONOSPHERE
Many factors affect a radio wave in its path between the transmitting and receiving sites. The factor that
has the greatest adverse effect on radio waves is ABSORPTION. Absorption results in the loss of energy of a radio
wave and has a pronounced effect on both the strength of received signals and the ability to communicate over long
You learned earlier in the section on ground waves that surface waves suffer most of their
absorption losses because of ground-induced voltage. Sky waves, on the other hand, suffer most of their absorption
losses because of conditions in the ionosphere. Note that some absorption of sky waves may also occur at lower
atmospheric levels because of the presence of water and water vapor. However, this becomes important only at
frequencies above 10,000 megahertz.
Most ionospheric absorption occurs in the lower regions of the ionosphere where ionization density is
greatest. As a radio wave passes into the ionosphere, it loses some of its energy to the free electrons and ions.
If these high-energy free electrons and ions do not collide with gas molecules of low energy, most of the energy
lost by the radio wave is reconverted into electromagnetic energy, and the wave continues to be propagated with
little change in intensity. However, if the high-energy free electrons and ions do collide with other particles,
much of this energy is lost, resulting in absorption of the energy from the wave. Since absorption of energy
depends on collision of the particles, the greater the density of the ionized layer, the greater the probability
of collisions; therefore, the greater the absorption. The highly dense D and E layers provide the greatest
absorption of radio waves.
Because the amount of absorption of the sky wave depends on the density of the ionosphere, which varies with
seasonal and daily conditions, it is impossible to express a fixed relationship between distance and signal
strength for ionospheric propagation. Under certain conditions, the absorption of energy is so great that
communicating over any distance beyond the line of sight is difficult. FADING
most troublesome and frustrating problem in receiving radio signals is variations in signal strength, most
commonly known as FADING. There are several conditions that can produce fading. When a radio wave is refracted by
the ionosphere or reflected from the Earth's surface, random changes in the polarization of the wave may occur.
Vertically and horizontally mounted receiving antennas are designed to receive vertically and horizontally
polarized waves, respectively. Therefore, changes in polarization cause changes in the received signal level
because of the inability of the antenna to receive polarization changes.
Fading also results from absorption of the rf energy in the ionosphere. Absorption fading occurs for a longer
period than other types of fading, since absorption takes place slowly.
Usually, however, fading on
ionospheric circuits is mainly a result of multipath propagation. Multipath Fading
MULTIPATH is simply a term used to describe the multiple paths a radio wave may follow between transmitter and
receiver. Such propagation paths include the ground wave, ionospheric refraction, reradiation by the ionospheric
layers, reflection from the Earth's surface or from more than one ionospheric layer, etc. Figure 2-21 shows a few
of the paths that a signal can travel between two sites in a typical circuit. One path, XYZ, is the basic ground
wave. Another path, XEA, refracts the wave at the E layer and passes it on to the receiver at A. Still another
path, XFZFA, results from a greater angle of incidence and two refractions from the F layer. At point Z, the
received signal is a combination of the ground wave and the sky wave. These two signals having traveled different
paths arrive at point Z at different times. Thus, the arriving waves may or may not be in phase with each other.
Radio waves that are received in phase reinforce each other and produce a stronger signal at the receiving site.
Conversely, those that are received out of phase produce a weak or fading signal. Small alternations in the
transmission path may change the phase relationship of the two signals, causing periodic fading. This condition
occurs at point A. At this point, the double-hop F layer signal may be in or out of phase with the signal arriving
from the E layer.
Figure 2-21.—Multipath transmission.
Multipath fading may be minimized by practices called SPACE DIVERSITY and FREQUENCY DIVERSITY. In space
diversity, two or more receiving antennas are spaced some distance apart. Fading does not occur simultaneously at
both antennas; therefore, enough output is almost always available from one of the antennas to provide a useful
signal. In frequency diversity, two transmitters and two receivers are used, each pair tuned to a different
frequency, with the same information being transmitted simultaneously over both frequencies. One of the two
receivers will almost always provide a useful signal. Selective Fading
resulting from multipath propagation is variable with frequency since each frequency arrives at the receiving
point via a different radio path. When a wide band of frequencies is transmitted simultaneously, each frequency
will vary in the amount of fading. This variation is called SELECTIVE FADING. When selective fading occurs, all
frequencies of the transmitted signal do not retain their original phases and relative amplitudes. This fading
causes severe distortion of the signal and limits the total signal transmitted.
Q23. What is the skip
zone of a radio wave?
Q24. Where does the greatest amount of ionospheric absorption occur in the ionosphere?
Q25. What is
meant by the term "multipath"?
Q26. When a wide band of frequencies is transmitted simultaneously, each
frequency will vary in the amount of fading. What is this variable fading called?
All radio waves propagated over ionospheric paths undergo energy
losses before arriving at the receiving site. As we discussed earlier, absorption in the ionosphere and lower
atmospheric levels account for a large part of these energy losses. There are two other types of losses that also
significantly affect the ionospheric propagation of radio waves. These losses are known as ground reflection loss
and free space loss. The combined effects of absorption
, and free space loss
account for most of the energy losses of
radio transmissions propagated by the ionosphere.
Ground Reflection Loss
When propagation is accomplished via multihop
refraction, rf energy is lost each time the radio wave is reflected from the Earth's surface. The amount of energy
lost depends on the frequency of the wave, the angle of incidence, ground irregularities, and the electrical
conductivity of the point of reflection. Free space Loss
Normally, the major loss
of energy is because of the spreading out of the wavefront as it travels away from the transmitter. As the
distance increases, the area of the wavefront spreads out, much like the beam of a flashlight. This means the
amount of energy contained within any unit of area on the wavefront will decrease as distance increases. By the
time the energy arrives at the receiving antenna, the wavefront is so spread out that the receiving antenna
extends into only a very small fraction of the wavefront. This is illustrated in figure 2-22.
Figure 2-22.—Free space loss principle.
ELECTROMAGNETIC INTERFERENCE (EMI)
The transmission losses just discussed are
not the only factors that interfere with communications. An additional factor that can interfere with radio
communications is the presence of ELECTROMAGNETIC INTERFERENCE (EMI). This interference can result in annoying or
impossible operating conditions. Sources of emi are both man-made and natural. Man-Made
Man-made interference may come from several sources. Some of these sources, such as oscillators, communications
transmitters, and radio transmitters, may be specifically designed to generate radio frequency energy. Some
electrical devices also generate radio frequency energy, although they are not specifically designed for this
purpose. Examples are ignition systems, generators, motors, switches, relays, and voltage regulators. The
intensity of man-made interference may vary throughout the day and drop off to a low level at night when many of
these sources are not being used. Man-made interference may be a critical limiting factor at radio receiving sites
located near industrial areas.
Natural interference refers to the static that you often
hear when listening to a radio. This interference is generated by natural phenomena, such as thunderstorms,
snowstorms, cosmic sources, and the sun. The energy released by these sources is transmitted to the receiving site
in roughly the same manner as radio waves. As a result, when ionospheric conditions are favorable for the long
distance propagation of radio waves, they are likewise favorable for the propagation of natural interference.
Natural interference is very erratic, particularly in the HF band, but generally will decrease as the operating
frequency is increased and wider bandwidths are used. There is little natural interference above 30 megahertz.
Control of EMI
Electromagnetic interference can be reduced or eliminated by using
various suppression techniques. The amount of emi that is produced by a radio transmitter can be controlled by
cutting transmitting antennas to the correct frequency, limiting bandwidth, and using electronic filtering
networks and metallic shielding.
Radiated emi during transmission can be controlled by the physical separation of the transmitting and receiving
antennas, the use of directional antennas, and limiting antenna bandwidth.
Q27. What are the two main
sources of emi with which radio waves must compete?
Q28. Thunderstorms, snowstorms, cosmic sources, the
sun, etc., are a few examples of emi sources. What type of emi comes from these sources?
Q29. Motors, switches, voltage regulators, generators, etc., are a few examples of emi sources. What type of emi
comes from these sources?
Q30. What are three ways of controlling the amount of transmitter-generated
Q31. What are three ways of controlling radiated emi during transmission?
VARIATIONS IN THE IONOSPHERE
Because the existence of the ionosphere is
directly related to radiations emitted from the sun, the movement of the Earth about the sun or changes in the
sun's activity will result in variations in the ionosphere. These variations are of two general types: (1) those
which are more or less regular and occur in cycles and, therefore, can be predicted in advance with reasonable
accuracy, and (2) those which are irregular as a result of abnormal behavior of the sun and, therefore, cannot be
predicted in advance. Both regular and irregular variations have important effects on radio wave propagation.
The regular variations that affect the extent of ionization in the ionosphere can be divided into four main
classes: daily, seasonal, 11-year, and 27-day variations. DAILY
.—Daily variations in the
ionosphere are a result of the 24-hour rotation of the Earth about its axis. Daily variations of the different
layers (fig. 2-14) are summarized as follows:
· The D layer reflects VLF waves; is important for long
range VLF communications; refracts lf and mf waves for short range communications; absorbs HF waves; has little
effect on vhf and above; and disappears at night.
· In the E layer, ionization depends on the angle of the sun. The E layer refracts HF waves during
the day up to 20 megahertz to distances of about 1200 miles. Ionization is greatly reduced at night.
· Structure and density of the F region depend on the time of day and the angle of the sun. This region consists
of one layer during the night and splits into two layers during daylight hours.
· Ionization density of
the F1 layer depends on the angle of the sun. Its main effect is to absorb HF waves passing through to the F2
· The F2 layer is the most important layer for long distance HF communications. It is a very
variable layer and its height and density change with time of day, season, and sunspot activity.
.—Seasonal variations are the result of the Earth revolving around the sun; the relative position
of the sun moves from one hemisphere to the other with changes in seasons. Seasonal variations of the D, E, and
F1 layers correspond to the highest angle of the sun; thus the ionization density of these layers is greatest
during the summer. The F2 layer, however, does not follow this pattern; its ionization is greatest in winter and
least in summer, the reverse of what might be expected. As a result, operating frequencies for F2 layer
propagation are higher in the winter than in the summer. ELEVEN-YEAR SUN SPOT CYCLE
of the most notable phenomena on the surface of the sun is the appearance and disappearance of dark, irregularly
shaped areas known as SUNSPOTS. The exact nature of sunspots is not known, but scientists believe they are caused
by violent eruptions on the sun and are characterized by unusually strong magnetic fields. These sunspots are
responsible for variations in the ionization level of the ionosphere. Sunspots can, of course, occur unexpectedly,
and the life span of individual sunspots is variable; however, a regular cycle of sunspot activity has also been
observed. This cycle has both a minimum and maximum level of sunspot activity that occur approximately every 11
During periods of maximum sunspot activity, the ionization density of all layers increases. Because
of this, absorption in the D layer increases and the critical frequencies for the E, F1, and F2 layers are higher.
At these times, higher operating frequencies must be used for long distance communications. 27-DAY
.—The number of sunspots in existence at any one time is continually subject to change as
some disappear and new ones emerge. As the sun rotates on its own axis, these sunspots are visible at 27-day
intervals, the approximate period required for the sun to make one complete rotation.
The 27-day sunspot
cycle causes variations in the ionization density of the layers on a day-to-day basis. The fluctuations in the F2
layer are greater than for any other layer. For this reason, precise predictions on a day-to-day basis of the
critical frequency of the F2 layer are not possible. In calculating frequencies for long-distance communications,
allowances for the fluctuations of the F2 layer must be made. Irregular Variations
Irregular variations in ionospheric conditions also have an important effect on radio wave propagation. Because
these variations are irregular and unpredictable, they can drastically affect communications capabilities without
The more common irregular variations are sporadic E, sudden ionospheric disturbances, and
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