Download Principles of Electronic Communication Systems

1
Principles of Electronic
Communication Systems
Third Edition
Louis E. Frenzel, Jr.
© 2008 The McGraw-Hill Companies
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Chapter 14
Antennas and Wave Propagation
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Topics Covered in Chapter 14
 14-1: Antenna Fundamentals
 14-2: Common Antenna Types
 14-3: Radio-Wave Propagation
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14-1: Antenna Fundamentals
 The interface between the transmitter and free space
and between free space and the receiver is the
antenna.
 At the transmitting end the antenna converts the
transmitter RF power into electromagnetic signals; at
the receiving end the antenna picks up the
electromagnetic signals and converts them into
signals for the receiver.
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14-1: Antenna Fundamentals
Radio Waves
 A radio signal is called an electromagnetic wave
because it is made up of both electric and magnetic
fields.
 Whenever voltage is applied to the antenna, an electric
field is set up.
 This voltage causes current to flow in the antenna,
producing a magnetic field.
 These fields are emitted from the antenna and
propagate through space at the speed of light.
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14-1: Antenna Fundamentals
Radio Waves: Magnetic Fields
 A magnetic field is an invisible force field created by a
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magnet.
An antenna is a type of electromagnet.
A magnetic field is generated around a conductor when
current flows through it.
The strength and direction of the magnetic field depend
upon the magnitude and direction of the current flow.
The SI unit for magnetic field strength is ampere-turns
per meter.
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14-1: Antenna Fundamentals
Figure 14-1: Magnetic field around a current-carrying conductor. Magnetic field strength
H in ampere-turns per meter = H = I I(2 πd).
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14-1: Antenna Fundamentals
Radio Waves: Electric Field
 An electric field is an invisible force field produced by
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the presence of a potential difference between two
conductors.
For example, an electric field is produced between the
plates of a charged capacitor.
An electric field exists between any two points across
which a potential difference exists.
The SI unit for electric field strength is volts per meter.
Permittivity is the dielectric constant of the material
between the two conductors.
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14-1: Antenna Fundamentals
Figure 14-2: Electric field across the plates of a capacitor.
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14-1: Antenna Fundamentals
Radio Waves: Magnetic and Electric Fields in a
Transmission Line
 At any given time in a two-wire transmission line, the
wires have opposite polarities.
 During one-half cycle of the ac input, one wire is
positive and the other is negative.
 During the negative half-cycle, the polarity reverses.
 The direction of the electric field between the wires
reverses once per cycle.
 The direction of current flow in one wire is always
opposite that in the other wire. Therefore, the magnetic
fields combine.
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14-1: Antenna Fundamentals
Radio Waves: Magnetic and Electric Fields in a
Transmission Line
 A transmission line is made up of a conductor or
conductors.
 Transmission lines do not radiate signals efficiently.
 The closeness of the conductors keeps the electric field
concentrated in the transmission line dielectric.
 The magnetic fields mostly cancel one another.
 The electric and magnetic fields do extend outward from
the transmission line, but the small amount of radiation
that does occur is extremely inefficient.
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14-1: Antenna Fundamentals
Figure 14-3: (a) Magnetic and electric fields around a transmission line. (b) Electric
field. (c) Magnetic fields.
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14-1: Antenna Fundamentals
Antenna Operation: The Nature of an Antenna
 If a parallel-wire transmission line is left open, the
electric and magnetic fields escape from the end of the
line and radiate into space.
 This radiation is inefficient and unsuitable for reliable
transmission or reception.
 The radiation from a transmission line can be greatly
improved by bending the transmission-line conductors
so they are at a right angle to the transmission line.
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14-1: Antenna Fundamentals
Antenna Operation: The Nature of an Antenna
 The magnetic fields no longer cancel; they now aid one
another.
 The electric field spreads out from conductor to
conductor.
 Optimum radiation occurs if the segment of
transmission wire converted into an antenna is one
quarter wavelength long at the operating frequency.
 This makes an antenna that is one-half wavelength
long.
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14-1: Antenna Fundamentals
Figure 14-5: Converting a transmission line into an antenna. (a) An open transmission
line radiates a little. (b) Bending the open transmission line at right angles creates
an efficient radiation pattern.
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14-1: Antenna Fundamentals
Antenna Operation
 The ratio of the electric field strength of a radiated wave
to the magnetic field strength is a constant and is called
the impedance of space, or the wave impedance.
 The electric and magnetic fields produced by the
antenna are at right angles to one another, and are both
perpendicular to the direction of propagation of the
wave.
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14-1: Antenna Fundamentals
Antenna Operation
 Antennas produce two sets of fields, the near field and
the far field.
 The near field describes the region directly around
the antenna where the electric and magnetic fields
are distinct.
 The far field is approximately 10 wavelengths from
the antenna. It is the radio wave with the composite
electric and magnetic fields.
 Polarization refers to the orientation of magnetic and
electric fields with respect to the earth.
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14-1: Antenna Fundamentals
Antenna Reciprocity
 Antenna reciprocity means that the characteristics
and performance of an antenna are the same whether
the antenna is radiating or intercepting an
electromagnetic signal.
 A transmitting antenna takes a voltage from the
transmitter and converts it into an electromagnetic
signal.
 A receiving antenna has a voltage induced into it by the
electromagnetic signal that passes across it.
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14-1: Antenna Fundamentals
The Basic Antenna
 An antenna can be a length of wire, a metal rod, or a
piece of tubing.
 Antennas radiate most effectively when their length is
directly related to the wavelength of the transmitted
signal.
 Most antennas have a length that is some fraction of a
wavelength.
 One-half and one-quarter wavelengths are most
common.
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14-2: Common Antenna Types
The Dipole Antenna
 One of the most widely used antenna types is the half-
wave dipole.
 The half-wave dipole, also called a doublet, is formally
known as the Hertz antenna.
 A dipole antenna is two pieces of wire, rod, or tubing
that are one-quarter wavelength long at the operating
resonant frequency.
 Wire dipoles are supported with glass, ceramic, or
plastic insulators at the ends and middle.
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14-2: Common Antenna Types
Figure 14-10: The dipole antenna.
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14-2: Common Antenna Types
The Dipole Antenna
 The dipole has an impedance of 73 Ω at its center,
which is the radiation resistance.
 An antenna is a frequency-sensitive device.
 To get the dipole to resonate at the frequency of
operation, the physical length must be shorter than the
one-half wavelength computed by λ = 492/f.
 Actual length is related to the ratio of length to diameter,
conductor shape, Q, the dielectric (when the material is
other than air), and a condition known as end effect.
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14-2: Common Antenna Types
The Dipole Antenna
 End effect is a phenomenon caused by any support
insulators used at the ends of the wire antenna and has
the effect of adding capacitance to the end of each wire.
 The actual antenna length is only about 95 percent of
the computed length.
 If a dipole is used at a frequency different from its
design frequency, the SWR rises and power is lost.
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14-2: Common Antenna Types
The Dipole Antenna: Antenna Q and Bandwidth
 The bandwidth of an antenna is determined by the
frequency of operation and the Q of the antenna
according to the relationship BW = fr/Q.
 The higher the Q, the narrower the bandwidth.
 For an antenna, low Q and wider bandwidth are
desirable so that the antenna can operate over a wider
range of frequencies with reasonable SWR.
 In general, any SWR below 2:1 is considered good in
practical antenna work.
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14-2: Common Antenna Types
The Dipole Antenna: Antenna Q and Bandwidth
 The Q and thus the bandwidth of an antenna are
determined by the ratio of the length of the conductor to
the diameter of the conductor.
 Bandwidth is sometimes expressed as a percentage of
the resonant frequency of the antenna.
 A small percentage means a higher Q, and a narrower
bandwidth means a lower percentage.
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14-2: Common Antenna Types
The Dipole Antenna: Conical Antennas
 A common way to increase bandwidth is to use a
version of the dipole antenna known as the conical
antenna.
 The center radiation resistance of a conical antenna is
much higher than the 73 Ω usually found when straightwire or tubing conductors are used.
 The primary advantage of conical antennas is their
tremendous bandwidth.
 They can maintain a constant impedance and gain over
a 4:1 frequency range.
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14-2: Common Antenna Types
Figure 14-14: The conical dipole and its variation. (a) Conical antenna. (b) Broadside
view of conical dipole antenna (bow tie antenna) showing dimensions. (c) Open-grill
bow tie antenna.
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14-2: Common Antenna Types
The Dipole Antenna: Dipole Polarization
 Most half-wave dipole antennas are mounted
horizontally to the earth.
 This makes the electric field horizontal to the earth and
the antenna is horizontally polarized.
 Horizontal mounting is preferred at the lower
frequencies because the physical construction,
mounting, and support are easier.
 This mounting makes it easier to attach the
transmission line and route it to the transmitter or
receiver.
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14-2: Common Antenna Types
The Dipole Antenna: Radiation Pattern and Directivity
 The radiation pattern of any antenna is the shape of
the electromagnetic energy radiated from or received by
that antenna.
 Most antennas have directional characteristics that
cause them to radiate or receive energy in a specific
direction.
 The radiation is concentrated in a pattern that has a
recognizable geometric shape.
 The measure of an antenna’s directivity is beam width,
the angle of the radiation pattern over which a
transmitter’s energy is directed or received.
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14-2: Common Antenna Types
Figure 14-15: Three-dimensional pattern of a half-wave dipole.
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14-2: Common Antenna Types
The Dipole Antenna: Antenna Gain
 A directional antenna can radiate more power in a given
direction than a nondirectional antenna. In this “favored”
direction, it acts as if it had gain.
 Antenna gain of this type is expressed as the ratio of
the effective radiated output power Pout to the input
power Pin.
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14-2: Common Antenna Types
The Dipole Antenna: Antenna Gain
 Effective radiated power is the actual power that would
have to be radiated by a reference antenna (usually a
nondirectional or dipole antenna) to produce the same
signal strength at the receiver as the actual antenna
produces.
 The power radiated by an antenna with directivity and
therefore gain is called the effective radiated power
(ERP).
ERP = ApPt
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14-2: Common Antenna Types
The Dipole Antenna: Folded Dipole
 A popular variation of the half-wave dipole is the folded
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dipole.
The folded dipole is also one-half wavelength long.
It consists of two parallel conductors connected at the
ends with one side open at the center for connection to
the transmission line.
The impedance of this antenna is 300 Ω.
Folded dipoles usually offer greater bandwidth than
standard dipoles.
The folded dipole is an effective, low-cost antenna that
can be used for transmitting and receiving.
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14-2: Common Antenna Types
Figure 14-18: Folded dipole. (a) Basic configuration. (b) Construction with twin lead.
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14-2: Common Antenna Types
Marconi or Ground-Plane Vertical Antenna
 The one-quarter wavelength vertical antenna, also
called a Marconi antenna, is widely used.
 It is similar in operation to a vertically mounted dipole
antenna.
 The Marconi antenna offers major advantages because
it is half the length of a dipole antenna.
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14-2: Common Antenna Types
Marconi or Ground-Plane Vertical Antenna: Radiation
Pattern
 Vertical polarization and omnidirectional
characteristics can be achieved using a one-quarter
wavelength vertical radiator. This antenna is called a
Marconi or ground-plane antenna.
 It is usually fed with coaxial cable; the center conductor
is connected to the vertical radiator and the shield is
connected to earth ground.
 The earth then acts as a type of electrical “mirror,”
providing the other one-quarter wavelength making it
equivalent to a vertical dipole.
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14-2: Common Antenna Types
Figure 14-20: Ground-plane antenna. (a) One-quarter wavelength vertical antenna.
(b) Using radials as a ground plane.
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14-2: Common Antenna Types
Marconi or Ground-Plane Vertical Antenna: Ground
Plane, Radials, and Counterpoise
 When a good electrical connection to the earth has
been made, the earth becomes what is known as a
ground plane.
 If a ground plane cannot be made to earth, an
artificial ground can be constructed of several onequarter wavelength wires laid horizontally on the
ground or buried in the earth.
 These horizontal wires at the base of the antenna are
called radials, and the collection of radials is called a
counterpoise.
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14-2: Common Antenna Types
Marconi or Ground-Plane Vertical Antenna: Antenna
Length
 For many applications, e.g., with portable or mobile
equipment, it is not possible to make the antenna a full
one-quarter wavelength long.
 To overcome this problem, shorter antennas are used,
and lumped electrical components are added to
compensate for the shortening.
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14-2: Common Antenna Types
Marconi or Ground-Plane Vertical Antenna: Antenna
Length
 The practical effect of this design is a decreased
inductance. The antenna no longer resonates at the
desired operating frequency, but at a higher frequency.
 To compensate for this, a series inductor, called a
loading coil, is connected in series with the antenna
coil.
 The loading coil brings the antenna back into resonance
at the desired frequency.
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14-2: Common Antenna Types
Figure 14-22: Using a base leading coil to increase effective antenna length.
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14-2: Common Antenna Types
Directivity
 Directivity refers to an antenna’s ability to send or
receive signals over a narrow horizontal directional
range.
 The physical orientation of the antenna gives it a
highly directional response or directivity curve.
 A directional antenna eliminates interference from
other signals being received from all directions other
than the desired signal.
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14-2: Common Antenna Types
Directivity
 A highly directional antenna acts as a type of filter to
provide selectivity.
 Directional antennas provide greater efficiency of
power transmission.
 Directivity, because it focuses the power, causes the
antenna to exhibit gain, which is one form of
amplification.
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14-2: Common Antenna Types
Figure 14-25: Radiation pattern of a highly directional antenna with gain. (a) Horizontal
radiation pattern. (b) Three-dimensional radiation pattern.
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14-2: Common Antenna Types
Directivity
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To create an antenna with directivity and gain, two or
more antenna elements are combined to form an
array.
 Two basic types of antenna arrays are used to achieve
gain and directivity:
1. Parasitic arrays.
2. Driven arrays.
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14-2: Common Antenna Types
Parasitic Arrays
 A parasitic array consists of a basic antenna
connected to a transmission line plus one or more
additional conductors that are not connected to the
transmission line.
 These extra conductors are referred to as parasitic
elements and the antenna is called a driven element.
 A Yagi antenna is made up of a driven element and
one or more parasitic elements.
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14-2: Common Antenna Types
Figure 14-26: A parasitic array known as a Yagi antenna.
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14-2: Common Antenna Types
Driven Arrays
 A driven array is an antenna that has two or more
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driven elements.
Each element receives RF energy from the
transmission line.
Different arrangements of the elements produce
different degrees of directivity and gain.
The three basic types of driven arrays are the collinear,
the broadside, and the end-fire.
A fourth type is the wide-bandwidth log-periodic
antenna.
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14-2: Common Antenna Types
Driven Arrays: Collinear Antenna
 Collinear antennas usually consist of two or more half-
wave dipoles mounted end to end.
 Collinear antennas typically use half-wave sections
separated by shorted quarter-wave matching stubs
which ensure that the signals radiated by each halfwave section are in phase.
 Collinear antennas are generally used only on VHF and
UHF bands because their length becomes prohibited at
the lower frequencies.
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14-2: Common Antenna Types
Figure 14-29: Radiation pattern of a four-element collinear antenna.
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14-2: Common Antenna Types
Driven Arrays: Broadside Antenna
 A broadside array is a stacked collinear antenna
consisting of half-wave dipoles spaced from one
another by one-half wavelengths.
 This antenna produces a highly directional radiation
pattern that is broadside or perpendicular to the plane of
the array.
 The broadside antenna is bidirectional in radiation, but
the radiation pattern has a very narrow beam width and
high gain.
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14-2: Common Antenna Types
Figure 14-30: A broadside array.
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14-2: Common Antenna Types
Driven Arrays: End-Fire Antenna
 The end-fire array uses two half-wave dipoles spaced
one-half wavelength apart.
 The end-fire array has a bidirectional radiation pattern,
but with narrower beam widths and lower gain.
 The radiation is in the plane of the driven elements.
 A highly unidirectional antenna can be created by
careful selection of the optimal number of elements with
the appropriately related spacing.
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14-2: Common Antenna Types
Figure 14-31: End-fire antennas. (a) Bidirectional. (b) Unidirectional.
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14-2: Common Antenna Types
Driven Arrays: Log-Periodic Antennas
 A special type of driven array is the wide-bandwidth
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log-periodic antenna.
The lengths of the driven elements vary from long to
short and are related logarithmically. The spacing is also
variable.
The great advantage of the log-periodic antenna over a
Yagi or other array is its very wide bandwidth.
The driving impedance is constant over this range.
Most TV antennas in use today are of the log-periodic
variety so that they can provide high gain and directivity
on both VHF and UHF TV channels.
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14-2: Common Antenna Types
Figure 14-32: Log-periodic antenna.
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14-2: Common Antenna Types
Impedance Matching
 One of the most critical aspects of any antenna system
is ensuring maximum power transfer from the
transmitter to the antenna.
 When the characteristic impedance of the transmission
line matches the output impedance of the transmitter
and the impedance of the antenna, the SWR will be 1:1.
 When SWR is 1:1, maximum power transfer will take
place.
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14-2: Common Antenna Types
Impedance Matching
 A Q section, or matching stub, is a one-quarter
wavelength of coaxial or balanced transmission line of a
specific impedance that is connected between a load
and source and is used to match impedances.
 A balun is a transformer used to match impedances.
 An antenna tuner is a variable inductor, one or more
variable capacitors, or a combination of these
components connected in various configurations.
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14-2: Common Antenna Types
Figure 14-33: A one-quarter wavelength matching stub or Q section.
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14-2: Common Antenna Types
Figure 14-34: A bifilar toroidal balun for impedance matching.
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14-2: Common Antenna Types
Figure 14-36: An antenna tuner.
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14-3: Radio-Wave Propagation
 Once a radio signal has been radiated by an antenna,
it travels or propagates through space and ultimately
reaches the receiving antenna.
 The energy level of the signal decreases rapidly with
distance from the transmitting antenna.
 The electromagnetic wave is affected by objects that it
encounters along the way such as trees, buildings,
and other large structures.
 The path that an electromagnetic signal takes to a
receiving antenna depends upon many factors,
including the frequency of the signal, atmospheric
conditions, and time of day.
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14-3: Radio-Wave Propagation
Optical Characteristics of Radio Waves
 Radio waves act much like light waves.
 Light waves can be reflected, refracted, diffracted, and
focused by other objects.
 The focusing of waves by antennas to make them more
concentrated in a desired direction is comparable to a
lens focusing light waves into a narrower beam.
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14-3: Radio-Wave Propagation
Optical Characteristics of Radio Waves: Reflection
 Any conducting surface looks like a mirror to a radio
wave, and so radio waves are reflected by any
conducting surface they encounter.
 Radio-wave reflection follows the principles of lightwave reflection.
 The angle of reflection is equal to the angle of
incidence.
 The direction of the electric field approaching the
reflecting surface is reversed from that leaving the
surface. This is equivalent to a 180° phase shift.
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14-3: Radio-Wave Propagation
Figure 14-37: How a conductive surface reflects a radio wave.
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14-3: Radio-Wave Propagation
Optical Characteristics of Radio Waves: Refraction
 Refraction is the bending of a wave due to the physical
makeup of the medium through which the wave passes.
 Index of refraction is obtained by dividing the speed of
a light (or radio) wave in a vacuum and the speed of a
light (or radio) wave in the medium that causes the
wave to be bent.
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14-3: Radio-Wave Propagation
Optical Characteristics of Radio Waves: Refraction
 The relationship between the angles and the indices of
refraction is given by a formula known as Snell’s law:
n1 sin Θ1 = n2 sin Θ2
where
n1 = index of refraction of initial medium
n2 = index of refraction of medium into which wave
passes
Θ1 = angle of incidence
Θ2 = angle of refraction
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14-3: Radio-Wave Propagation
Figure 14-38: How a change in the index of refraction causes bending of a radio wave.
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14-3: Radio-Wave Propagation
Optical Characteristics of Radio Waves: Diffraction
 Diffraction is the bending of waves around an object.
 Diffraction is explained by Huygen’s principle:
 Assuming that all electromagnetic waves radiate as
spherical waveforms from a source, each point on a
wave front can be considered as a point source for
additional spherical waves.
 When the waves encounter an obstacle, they pass
around it, above it, and on either side.
 As the wave front passes the object, the point
sources of waves at the edge of the obstacle create
additional spherical waves that penetrate and fill in
the shadow zone.
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14-3: Radio-Wave Propagation
Figure 14-39: Diffraction causes waves to bend around obstacles.
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14-3: Radio-Wave Propagation
Radio-Wave Propagation Through Space
 The three basic paths that a radio signal can take
through space are:
 Ground wave
 Sky wave
 Space wave
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14-3: Radio-Wave Propagation
Radio-Wave Propagation Through Space: Ground
Waves
 Ground or surface waves leave an antenna and
remain close to the earth.
 Ground waves actually follow the curvature of the earth
and can travel at distances beyond the horizon.
 Ground waves must have vertical polarization to be
propagated from an antenna.
 Ground-wave propagation is strongest at the low- and
medium-frequency ranges.
 AM broadcast signals are propagated primarily by
ground waves during the day and by sky waves at night.
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14-3: Radio-Wave Propagation
Figure 14-40: Ground or surface wave radiation from an antenna.
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14-3: Radio-Wave Propagation
Radio-Wave Propagation Through Space: Sky Waves
 Sky-wave signals are radiated by the antenna into the
upper atmosphere, where they are bent back to earth.
 When a radio signal goes into the ionosphere, the
different levels of ionization cause the radio waves to be
gradually bent.
 The smaller the angle with respect to the earth, the
more likely it is that the waves will be refracted and sent
back to earth.
 The higher the frequency, the smaller the radiation
angle required for refraction to occur.
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14-3: Radio-Wave Propagation
Figure 14-41: Sky wave propagation.
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14-3: Radio-Wave Propagation
Radio-Wave Propagation Through Space: Space Waves
 A direct wave, or space wave, travels in a straight line
directly from the transmitting antenna to the receiving
antenna.
 Direct-wave radio signaling is often referred to as line-ofsight communication.
 Direct or space waves are not refracted, nor do they
follow the curvature of the earth.
 Line-of-sight communication is characteristic of most
radio signals with a frequency above 30 MHz, particularly
VHF, UHF, and microwave signals.
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14-3: Radio-Wave Propagation
Figure 14-42: Line-of-sight communication by direct or space waves.
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14-3: Radio-Wave Propagation
Radio-Wave Propagation Through Space: Space Waves
 Repeater stations extend the communication distance at
VHF, UHF, and microwave frequencies.
 A repeater is a combination of a receiver and a
transmitter operating on separate frequencies.
 The receiver picks up a signal from a remote transmitter,
amplifies it, and retransmits it (on another frequency) to a
remote receiver.
 Repeaters are widely used to increase the
communication range for mobile and handheld radio
units.
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14-3: Radio-Wave Propagation
Radio-Wave Propagation Through Space: Space Waves
 In a trunked repeater system, multiple repeaters are
under the control of a computer system that can transfer
a user from an assigned but busy repeater to another,
available repeater, thus spreading the communication
load.
 Communication satellites act as fixed repeater
stations.
 The receiver-transmitter combination within the satellite
is known as a transponder.
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14-3: Radio-Wave Propagation
Common Propagation Problems: Fading
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Fading is the variation in signal amplitude at the
receiver caused by the characteristics of the signal
path and changes in it.
Fading typically makes the received signal smaller.
Fading is caused by four factors:
1. Variation in distance between transmitter and receiver.
2. Changes in the environmental characteristics of the
signal path.
3. The presence of multiple signal paths.
4. Relative motion between the transmitter and receiver.
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14-3: Radio-Wave Propagation
Common Propagation Problems: Diversity System
 A diversity system uses multiple transmitters,
receivers, or antennas to mitigate the problems caused
by multipath signals.
 With frequency diversity, two separate sets of
transmitters and receivers operating on different
frequencies are used to transmit the same information
simultaneously.
 Space or spatial diversity uses two receive antennas
spaced as far apart as possible to receive the signals.
© 2008 The McGraw-Hill Companies