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2008 The McGraw-Hill Com anies

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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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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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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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7/81 2008 The McGraw-Hill Com anies

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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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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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Figure 14-2: Electric field across the plates of a capacitor.



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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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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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Figure 14-3: (a) Magnetic and electric fields around a transmission line. (b) Electric

field. (c) Magnetic fields.



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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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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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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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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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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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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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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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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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Figure 14-10: The dipole antenna.

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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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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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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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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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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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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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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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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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Figure 14-15: Three-dimensional pattern of a half-wave dipole.

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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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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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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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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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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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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.

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