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    2008 The McGraw-Hill Com anies

    1

    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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    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 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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    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 becauseit 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-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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    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 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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    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 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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    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

    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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    14-2: Common Antenna Types

    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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    14-2: Common Antenna Types

    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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    14-2: Common Antenna Types

    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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    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 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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    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 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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    14-2: Common Antenna Types

    Figure 14-30: A broadside array.

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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-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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    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 takeplace.

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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 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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    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 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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    14-3: Radio-Wave Propagation

    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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    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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    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.