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Transcript of antennaswaveprofrenzel-121121205103-phpapp01
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Principles of ElectronicCommunication Systems
Third Edition
Louis E. Frenzel, Jr.
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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 thetransmitter 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
Figure 14-1: Magnetic field around a current-carrying conductor. Magnetic field strength
H in ampere-turns per meter = H = II(2 d).
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14-1: Antenna Fundamentals
Radio Waves: Electric Field
An electric fieldis an invisible force field produced by
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.
Permittivityis 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 aTransmission Line
At any given time in a two-wire transmission line, thewires have opposite polarities.
During one-half cycle of the ac input, one wire ispositive and the other is negative.
During the negative half-cycle, the polarity reverses.
The direction of the electric field between the wiresreverses once per cycle.
The direction of current flow in one wire is alwaysopposite that in the other wire. Therefore, the magneticfields 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 fieldconcentrated 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 radiationthat 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 conductorsso 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 toconductor.
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 fieldand
the far field.
The near fielddescribes the region directly aroundthe antenna where the electric and magnetic fields
are distinct.
The far fieldis approximately 10 wavelengths from
the antenna. It is the radio wave with the compositeelectric 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 reciprocitymeans that the characteristics
and performance of an antenna are the same whether
the antenna is radiating or intercepting anelectromagnetic 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 isdirectly 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 adoublet, is formallyknown 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 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 = 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 effectis 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 antennais 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 Qand 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 aversion of the dipole antenna known as the conicalantenna.
The center radiation resistance of a conical antenna ismuch higher than the 73 usually found when straight-wire 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 andthe antenna is horizontally polarized.
Horizontal mounting is preferred at the lowerfrequencies because the physical construction,mounting, and support are easier.
This mounting makes it easier to attach thetransmission line and route it to the transmitter orreceiver.
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14-2: Common Antenna Types
The Dipole Antenna: Radiation Pattern and Directivity
The radiation patternof any antenna is the shape ofthe electromagnetic energy radiated from or received bythat antenna.
Most antennas have directional characteristics thatcause them to radiate or receive energy in a specificdirection.
The radiation is concentrated in a pattern that has a
recognizable geometric shape. The measure of an antennas directivity is beam width,
the angle of the radiation pattern over which atransmitters 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
directionthan 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 radiatedoutput power Poutto the input
power Pin.
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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 samesignal 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 foldeddipole.
The folded dipole is also one-half wavelength long.
It consists of two parallel conductors connected at theends with one side open at the center for connection tothe transmission line.
The impedance of this antenna is 300 .
Folded dipoles usually offer greater bandwidth thanstandard dipoles.
The folded dipole is an effective, low-cost antenna thatcan be used for transmitting and receiving.
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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 becauseit is half the length of a dipole antenna.
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Marconi or Ground-Plane Vertical Antenna: Radiation
Pattern
Vertical polarization and omnidirectional
characteristics can be achieved using a one-quarterwavelength vertical radiator. This antenna is called a
Marconi or ground-plane antenna.
It is usually fed with coaxial cable; the center conductoris connected to the vertical radiator and the shield isconnected to earth ground.
The earth then acts as a type of electrical mirror,providing the other one-quarter wavelength making itequivalent to a vertical dipole.
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Figure 14-20: Ground-plane antenna. (a) One-quarter wavelength vertical antenna.
(b) Using radials as a ground plane.
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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 aground plane.
If a ground plane cannot be made to earth, an
artificial ground can be constructed of several one-
quarter wavelength wires laid horizontally on theground 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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Marconi or Ground-Plane Vertical Antenna: Antenna
Length
The practical effect of this design is a decreased
inductance. The antenna no longer resonates at thedesired 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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Figure 14-22: Using a base leading coil to increase effective antenna length.
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Directivity
Directivityrefers to an antennas ability to send orreceive signals over a narrow horizontal directionalrange.
The physical orientation of the antenna gives it ahighly directional response or directivity curve.
A directional antenna eliminates interference fromother signals being received from all directions other
than the desired signal.
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Directivity
A highly directional antenna acts as a type of filter to
provide selectivity.
Directional antennas provide greater efficiency ofpower transmission.
Directivity, because it focuses the power, causes the
antenna to exhibit gain, which is one form of
amplification.
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Directivity
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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Parasitic Arrays
A parasitic arrayconsists 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
elementsand the antenna is called a driven element.
AYagi antennais made up of a driven element andone or more parasitic elements.
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Figure 14-26: A parasitic array known as a Yagi antenna.
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Driven Arrays
A driven arrayis an antenna that has two or more
driven elements.
Each element receives RF energy from thetransmission 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 antennasusually consist of two or more half-
wave dipoles mounted end to end.
Collinear antennas typically use half-wave sectionsseparated by shorted quarter-wave matching stubs
which ensure that the signals radiated by each half-
wave section are in phase.
Collinear antennas are generally used only on VHF andUHF bands because their length becomes prohibited at
the lower frequencies.
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Figure 14-29: Radiation pattern of a four-element collinear antenna.
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Driven Arrays: Broadside Antenna
A broadside arrayis 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, butthe radiation pattern has a very narrow beam width and
high gain.
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Figure 14-30: A broadside array.
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Figure 14-31: End-fire antennas. (a) Bidirectional. (b) Unidirectional.
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Driven Arrays: Log-Periodic Antennas
A special type of driven array is the wide-bandwidthlog-periodic antenna.
The lengths of the driven elements vary from long to
short and are related logarithmically. The spacing isalso variable.
The great advantage of the log-periodic antenna over aYagi 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 directivityon both VHF and UHF TV channels.
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Figure 14-32: Log-periodic antenna.
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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 takeplace.
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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 balunis a transformer used to match impedances.
An antenna tuneris a variable inductor, one or more
variable capacitors, or a combination of thesecomponents connected in various configurations.
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Figure 14-33: A one-quarter wavelength matching stub or Q section.
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Figure 14-34: A bifilar toroidal balun for impedance matching.
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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 ultimatelyreaches 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 areceiving antenna depends upon many factors,including the frequency of the signal, atmosphericconditions, and time of day.
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Optical Characteristics of Radio Waves
Radio waves act much like light waves.
Light waves can be reflected, refracted, diffracted, andfocused by other objects.
The focusing of waves by antennas to make them more
concentrated in a desired direction is comparable to alens focusing light waves into a narrower beam.
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Figure 14-37: How a conductive surface reflects a radio wave.
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Optical Characteristics of Radio Waves: Refraction
Refractionis 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 ofa light (or radio) wave in a vacuum and the speed of alight (or radio) wave in the medium that causes thewave to be bent.
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Figure 14-38: How a change in the index of refraction causes bending of a radio wave.
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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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Radio-Wave Propagation Through Space: GroundWaves
Groundor surface wavesleave an antenna andremain close to the earth.
Ground waves actually follow the curvature of the earthand can travel at distances beyond the horizon.
Ground waves must have vertical polarization to bepropagated from an antenna.
Ground-wave propagation is strongest at the low- andmedium-frequency ranges.
AM broadcast signals are propagated primarily byground waves during the day and by sky waves at night.
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Figure 14-40: Ground or surface wave radiation from an antenna.
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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 begradually bent.
The smaller the angle with respect to the earth, themore 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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Figure 14-41: Sky wave propagation.
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Radio-Wave Propagation Through Space: Space Waves A direct wave,or spacewave,travels in a straight line
directly from the transmitting antenna to the receivingantenna.
Direct-wave radio signaling is often referred to as line-of-sight communication.
Direct or space waves are not refracted, nor do theyfollow the curvature of the earth.
Line-of-sight communication is characteristic of mostradio signals with a frequency above 30 MHz, particularlyVHF, UHF, and microwave signals.
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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 stationsextend the communication distance at
VHF, UHF, and microwave frequencies.
A repeateris a combination of a receiver and atransmitter 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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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 satellitesact as fixed repeater
stations.
The receiver-transmitter combination within the satellite
is known as a transponder.
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Common Propagation Problems: Fading Fadingis the variation in signal amplitude at the
receiver caused by the characteristics of the signalpath 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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Common Propagation Problems: Diversity System
A diversity systemuses 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 diversityuses two receive antennas
spaced as far apart as possible to receive the signals.