Transcript Principles of Electronic Communication Systems

Principles of Electronic
Communication Systems
Second Edition
Louis Frenzel
© 2002 The McGraw-Hill Companies
Principles of Electronic
Communication Systems
Second Edition
Chapter 14
Antennas and Wave Propagation
©2003 The McGraw-Hill Companies
Antennas and Wave Propagation
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.
Topics Covered in Chapter 14
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Antenna Fundamentals
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Common Antenna Types
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Radio-Wave Propagation
Antenna Fundamentals
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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.
Magnetic Fields
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A magnetic field is an invisible force field created by
a magnet.
An antenna is a type of electromagnet.
A magnetic field is generated around a conductor
when current flows through it.
The SI unit for magnetic field strength is ampereturns per meter.
Magnetic Field Around a Conductor
Electric Field
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An electric field is an invisible force field produced
by the presence of a potential difference between two
conductors.
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.
Magnetic and Electric Fields in a
Transmission Line
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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 primarily 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.
The Nature of an Antenna
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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.
The Nature of an Antenna
(Continued)
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The magnetic fields no longer cancel, and in fact, 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 a
quarter wavelength long at the operating frequency.
This makes an antenna that is one-half wavelength
long.
Converting a Transmission Line
into an Antenna
By Definition…
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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.
Polarization refers to the orientation of magnetic and
electric fields with respect to the earth.
Antenna Reciprocity
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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.
Basic Antenna
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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.
Dipole Antenna
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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.
The Dipole Antenna
By Definition…
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The dipole has an impedance of 73 Ω at its center,
which is the radiation resistance.
An antenna is a frequency-sensitive device.
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.
If a dipole is used at a frequency different from its
design frequency, the SWR rises and power is lost.
Conical Antenna
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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
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.
Conical Antenna
Dipole Polarization
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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.
Radiation Pattern and Directivity
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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.
Typically that radiation is concentrated in a pattern
that has a recognizable geometric shape.
Three-Dimensional Pattern of a
Half-Wave Dipole
By Definition…
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The radiation pattern of any antenna is the shape of
the electromagnetic energy radiated from or received
by that antenna.
The measure of an antenna’s directivity is beamwidth,
the angle of the radiation pattern over which a
transmitter’s energy is directed or received.
The power radiated by an antenna with directivity and
therefore gain is called the effective radiated power
(ERP).
Folded Dipole
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A popular variation of the half-wave dipole is the
folded 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
The folded dipole is an effective, low-cost antenna
that can be used for transmitting and receiving.
Folded Dipole
Marconi or Ground-Plane Vertical
Antenna
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The quarter-wavelength vertical antenna, also called a
Marconi antenna is widely used.
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It is similar in operation to a vertically mounted
dipole antenna.
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The Marconi antenna offers major advantages
because it is half the length of a dipole antenna.
Radiation Pattern
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Vertical polarization and omnidirectional
characteristics can be achieved using a quarterwavelength vertical radiator.
It is usually fed with coaxial cable; the center
conductor is connected to the vertical radiator and the
shield is connected to earth ground.
With this arrangement, the earth acts as a type of
electrical “mirror,” effectively providing the other
quarter wavelength making it equivalent to a vertical
dipole.
Ground Plane Antenna
Directivity
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Directivity refers to the ability of an antenna 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.
A highly directional antenna acts as a type of filter to provide
selectivity.
Directional antennas provide greater efficiency of power
transmission.
Horizontal Radiation Pattern
Parasitic Array
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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.
Yagi Antenna
Driven Arrays
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A driven array is an antenna that has two or more
driven elements.
Each element receives RF energy from the
transmission line, and 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.
Collinear Antenna
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Collinear antennas usually consist of two or more
half-wave dipoles mounted end to end.
Collinear antennas use half-wave sections separated
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
and UHF bands because their length becomes
prohibited at the lower frequencies.
Radiation Pattern of a FourElement Collinear Antenna
Broadside Antenna
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A broadside array is, essentially, 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 which is broadside or perpendicular to the
plane of the array.
The broasdside antenna is bidirectional, but the
radiation pattern has a very narrow beamwidth and
high gain.
Broadside Array
End-Fire Antenna
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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 a narrower bandwidth and lower
gain.
The radiation is in the plane of the driven elements.
By careful selection of the optimal number of
elements with the appropriately related spacing, a
highly unidirectional antenna is created.
Bidirectional End-Fire Antenna
Impedance Matching
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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.
By Definition…
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A Q section, or matching stub, is a 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.
Matching Stub
Bifilar Toroidal Balun
Radio-Wave Propagation
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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.
Optical Characteristics of Radio
Waves
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Radio waves act much like light waves.
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Light waves can be reflected, refracted, diffracted,
and focused by other objects.
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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.
Reflection
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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 which states 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 degree phase shift.
Reflection of Radio Waves
Refraction
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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.
The relationship between the angles and the indices
of refraction are given by a formula known as Snell’s
law.
Diffraction
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Diffraction is the bending of waves around an object.
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Diffraction is explained by what is known as
Huygen’s principle.
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Huygens’ principle is based on the assumption that all
electromagnetic waves, light as well as radio waves,
radiate as spherical waveforms from a source.
Radio-Wave Propagation Through
Space
The three basic paths that a radio signal can take
through space are:
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Ground wave
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Sky wave
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Space wave
Ground Waves
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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 sky waves at night.
Sky Waves
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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.
Space Waves
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A direct, 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 linof-sight 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.
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 causes the received signal to vary in
amplitude, typically making it smaller.
Fading is caused by four factors: variation in distance
between transmitter and receiver, changes in the
environmental characteristics of the signal path, the
presence of multiple signal paths, and relative motion
between the transmitter and receiver.
Diversity
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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.