Download Principles of Electronic Communication Systems

1
Principles of Electronic
Communication Systems
Third Edition
Louis E. Frenzel, Jr.
© 2008 The McGraw-Hill Companies
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Chapter 16
Microwave Communication
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Topics Covered in Chapter 16
 16-1: Microwave Concepts
 16-2: Microwave Lines and Devices
 16-3: Waveguides and Cavity Resonators
 16-4: Microwave Semiconductor Diodes
 16-5: Microwave Tubes
 16-6: Microwave Antennas
 16-7: Microwave Applications
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16-1: Microwave Concepts
 Microwaves are the ultrahigh, superhigh, and
extremely high frequencies directly above the lower
frequency ranges where most radio communication
now takes place and below the optical frequencies
that cover infrared, visible, and ultraviolet light.
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16-1: Microwave Concepts
Microwave Frequencies and Bands
 The practical microwave region is generally considered
to extend from 1 to 30 GHz, although frequencies could
include up to 300 GHz.
 Microwave signals in the 1- to 30-GHz have
wavelengths of 30 cm to 1 cm.
 The microwave frequency spectrum is divided up into
groups of frequencies, or bands.
 Frequencies above 40 GHz are referred to as
millimeter (mm) waves and those above 300 GHz are
in the submillimeter band.
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16-1: Microwave Concepts
Figure 16-1: Microwave
frequency bands.
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16-1: Microwave Concepts
Benefits of Microwaves
 Moving into higher frequency ranges has helped to
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solve the problem of spectrum crowding.
Today, most new communication services are assigned
to the microwave region.
At higher frequencies there is a greater bandwidth
available for the transmission of information.
Wide bandwidths make it possible to use various
multiplexing techniques to transmit more information.
Transmission of high-speed binary information requires
wide bandwidths and these are easily transmitted on
microwave frequencies.
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16-1: Microwave Concepts
Disadvantages of Microwaves
 The higher the frequency, the more difficult it becomes
to analyze electronic circuits.
 At microwave frequencies, conventional components
become difficult to implement.
 Microwave signals, like light waves, travel in perfectly
straight lines. Therefore, communication distance is
limited to line-of-sight range.
 Microwave signals penetrate the ionosphere, so
multiple-hop communication is not possible.
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16-1: Microwave Concepts
Microwave Communication Systems
 Like any other communication system, a microwave
communication system uses transmitters, receivers,
and antennas.
 The same modulation and multiplexing techniques used
at lower frequencies are also used in the microwave
range.
 The RF part of the equipment, however, is physically
different because of the special circuits and
components that are used to implement the
components.
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16-1: Microwave Concepts
Microwave Communication Systems: Transmitters
 Like any other transmitter, a microwave transmitter
starts with a carrier generator and a series of amplifiers.
 It also includes a modulator followed by more stages of
power amplification.
 The final power amplifier applies the signal to the
transmission line and antenna.
 A transmitter arrangement could have a mixer used to
up-convert an initial carrier signal with or without
modulation to the final microwave frequency.
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16-1: Microwave Concepts
Figure 16-3: Microwave transmitters. (a) Microwave transmitter using frequency
multipliers to reach the microwave frequency. The shaded stages operate in the
microwave region.
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16-1: Microwave Concepts
Figure 16-3: Microwave transmitters. (b) Microwave transmitter using up-conversion
with a mixer to achieve an output in the microwave range.
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16-1: Microwave Concepts
Microwave Communication Systems: Receivers
 Microwave receivers, like low-frequency receivers, are
the superheterodyne type.
 Their front ends are made up of microwave
components.
 Most receivers use double conversion.
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16-1: Microwave Concepts
Microwave Communication Systems: Receivers
 The antenna is connected to a tuned circuit, which
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could be a cavity resonator or microstrip or stripline
tuned circuit.
The signal is then applied to a special RF amplifier
known as a low-noise amplifier (LNA).
Another tuned circuit connects the amplified input signal
to the mixer.
The local oscillator signal is applied to the mixer.
The mixer output is usually in the UHF or VHF range.
The remainder of the receiver is typical of other
superheterodynes.
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16-1: Microwave Concepts
Figure 16-4: A microwave receiver. The shaded areas denote microwave circuits.
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16-1: Microwave Concepts
Microwave Communication Systems: Transmission
Lines
 Coaxial cable, most commonly used in lower-frequency
communication has very high attenuation at microwave
frequencies and conventional cable is unsuitable for
carrying microwave signals.
 Special microwave coaxial cable that can be used on
bands L, S, and C is made of hard tubing. This low-loss
coaxial cable is known as hard line cable.
 At higher microwave frequencies, a special hollow
rectangular or circular pipe called waveguide is used
for the transmission line.
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16-1: Microwave Concepts
Microwave Communication Systems: Antennas
 At low microwave frequencies, standard antenna types,
including the simple dipole and one-quarter wavelength
vertical antenna, are still used.
 At these frequencies antennas are very small; for
example, a half-wave dipole at 2 GHz is about 3 in.
 At higher microwave frequencies, special antennas are
generally used.
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16-2: Microwave Lines and Devices
 Although vacuum and microwave tubes like the
klystron and magnetron are still used, most microwave
systems use transistor amplifiers.
 Special geometries are used to make bipolar
transistors that provide voltage and power gain at
frequencies up to 10 GHz.
 Microwave FET transistors have also been created.
 Monolithic microwave integrated circuits (MMICs) are
widely used.
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16-2: Microwave Lines and Devices
Microstrip Tuned Circuits
 At higher frequencies, standard techniques for
implementing lumped components such as coils and
capacitors are not possible.
 At microwave frequencies, transmission lines,
specifically microstrip, are used.
 Microstrip is preferred for reactive circuits at the higher
frequencies because it is simpler and less expensive
than stripline.
 Stripline is used where shielding is necessary.
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16-2: Microwave Lines and Devices
Figure 16-6: Microstrip transmission line used for reactive circuits. (a) Perspective
view. (b) Edge or end view. (c) Side view (open line). (d) Side view (shorted line).
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16-2: Microwave Lines and Devices
Figure 16-7: Equivalent circuits of open and shorted microstrip lines.
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16-2: Microwave Lines and Devices
Microstrip Tuned Circuits
 An important characteristic of microstrip is its
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impedance.
The characteristic impedance of a transmission line
depends on its physical characteristics.
The dielectric constant of the insulating material is also
a factor.
Most characteristic impedances are less than 100 Ω.
One-quarter wavelength transmission line can be used
to make one type of component look like another.
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16-2: Microwave Lines and Devices
Figure 16-8: How a one-quarter wavelength microstrip can transform impedances
and reactances.
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16-2: Microwave Lines and Devices
Microstrip Tuned Circuits
 Microstrip can also be used to realize coupling from one
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circuit.
One microstrip line is simply placed parallel to another
segment of microstrip.
The degree of coupling between the two depends on
the distance of separation and the length of the parallel
segment.
The closer the spacing and the longer the parallel run,
the greater the coupling.
Microstrip patterns are made directly onto printed-circuit
boards.
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16-2: Microwave Lines and Devices
Microstrip Tuned Circuits
 A special form of microstrip is the hybrid ring.
 The unique operation of the hybrid ring makes it very
useful for splitting signals or combining them.
 Microstrip can be used to create almost any tuned
circuit necessary in an amplifier, including resonant
circuits, filters, and impedance-matching networks.
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16-2: Microwave Lines and Devices
Figure 16-12: A microstrip hybrid ring.
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16-2: Microwave Lines and Devices
Microwave Transistors
 The primary differences between standard lower-
frequency transistors and microwave types are internal
geometry and packaging.
 To reduce internal inductances and capacitances of
transistor elements, special chip configurations known
as geometries are used.
 Geometries permit the transistor to operate at higher
power levels and at the same time minimize distributed
and stray inductances and capacitances.
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16-2: Microwave Lines and Devices
Microwave Transistors
 The GaAs MESFET, a type of JFET using a Schottky
barrier junction, can operate at frequencies above 5
GHz.
 A high electron mobility transistor (HEMT) is a
variant of the MESFET and extends the range beyond
20 GHz by adding an extra layer of semiconductor
material such as AlGaAs.
 A popular device known as a heterojunction bipolar
transistor (HBT) is making even higher-frequency
amplification possible in discrete form and in integrated
circuits.
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16-2: Microwave Lines and Devices
Figure 16-14: Microwave transistors. (a) and (b) Low-power small signal. (c) FET
power. (d) NPN bipolar power.
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16-2: Microwave Lines and Devices
Small-Signal Amplifiers
 A small-signal microwave amplifier can be made up of a
single transistor or multiple transistors combined with a
biasing circuit and any microstrip circuits or components
as required.
 Most microwave amplifiers are of the tuned variety.
 Another type of small-signal microwave amplifier is a
multistage integrated circuit, a variety of MMIC.
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16-2: Microwave Lines and Devices
Small-Signal Amplifiers: Transistor Amplifiers
 A low-noise transistor with a gain of about 10 to 25 dB is
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typically used as a microwave amplifier.
Most microwave amplifiers are designed to have input
and output impedances of 50 Ω.
The transistor is biased into the linear region for class A
operation.
RFCs are used in the supply leads to keep the RF out
of the supply and to prevent feedback paths that can
cause oscillation and instability in multistage circuits.
Ferrite beads (FB) are used in the collector supply lead
for further decoupling.
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16-2: Microwave Lines and Devices
Small-Signal Amplifiers: MMIC Amplifiers
 A common monolithic microwave integrated circuit
(MMIC) amplifier is one that incorporates two or more
stages of FET or bipolar transistors made on a common
chip to form a multistage amplifier.
 The chip also incorporates resistors for biasing and
small bypass capacitors.
 Physically, these devices look like transistors.
 Another form of MMIC is the hybrid circuit, which
combines an amplifier IC connected to microstrip
circuits and discrete components.
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16-2: Microwave Lines and Devices
Figure 16-15: A single-stage class A RF microwave amplifier.
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16-2: Microwave Lines and Devices
Small-Signal Amplifiers: Power Amplifiers
 A typical class A microwave power amplifier is designed
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with microstrip lines used for impedance matching and
tuning.
Input and output impedances are 50 Ω.
Typical power-supply voltages are 12, 24, and 28 volts.
Most power amplifiers obtain their bias from constantcurrent sources.
A single-stage FET power amplifier can achieve a
power output of 100 W in the high UHF and low
microwave region.
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16-2: Microwave Lines and Devices
Figure 16-16: A class A microwave power amplifier.
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16-2: Microwave Lines and Devices
Figure 16-17: A constant-current bias supply for a linear power amplifier.
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16-2: Microwave Lines and Devices
Figure 16-18: An FET power amplifier.
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16-3: Waveguides
and Cavity Resonators
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Waveguides
 Most microwave energy transmission above 6 GHz is
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handled by waveguides.
Waveguides are hollow metal conducting pipes
designed to carry and constrain the electromagnetic
waves of a microwave signal.
Most waveguides are rectangular.
Waveguides are made from copper, aluminum or brass.
Often the insides of waveguides are plated with silver to
reduce resistance and transmission losses.
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16-3: Waveguides
and Cavity Resonators
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Waveguides: Signal Injection and Extraction
 A microwave signal to be carried by a waveguide is
introduced into one end of the waveguide with an
antennalike probe.
 The probe creates an electromagnetic wave that
propagates through the waveguide.
 The electric and magnetic fields associated with the
signal bounce off the inside walls back and forth as the
signal progresses down the waveguide.
 The waveguide totally contains the signal so that none
escapes by radiation.
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16-3: Waveguides
and Cavity Resonators
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Figure 16-19: Injecting a sine wave into a waveguide and extracting a signal.
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16-3: Waveguides
and Cavity Resonators
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Waveguides: Signal Injection and Extraction
 Probes and loops can be used to extract a signal from a
waveguide.
 When the signal strikes a probe or a loop, a signal is
induced which can then be fed to other circuitry through
a short coaxial cable.
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16-3: Waveguides
and Cavity Resonators
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Waveguides: Waveguide Size and Frequency.
 The frequency of operation of a waveguide is
determined by the inside width of the pipe (dimension
(a) in the figure following).
 This dimension is usually made equal to one-half
wavelength, a bit below the lowest frequency of
operation. This frequency is known as the waveguide
cutoff frequency.
 At its cutoff frequency and below, a waveguide will not
transmit energy.
 Above the cutoff frequency, a waveguide will propagate
electromagnetic energy.
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16-3: Waveguides
and Cavity Resonators
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Figure 16-20: The dimensions of a waveguide determine its operating frequency range.
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16-3: Waveguides
and Cavity Resonators
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Waveguides: Signal Propagation
 In a waveguide, when the electric field is at a right angle
to the direction of wave propagation, it is called a
transverse electric (TE) field.
 When the magnetic field is transverse to the direction of
propagation, it is called a transverse magnetic (TM)
field.
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16-3: Waveguides
and Cavity Resonators
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Waveguides: Signal Propagation
 The angles of incidence and reflection depend on the
operating frequency.
 At high frequencies, the angle is large and the path
between the opposite walls is relatively long.
 As the operating frequency decreases, the angle also
decreases and the path between the sides shortens.
 When the operating frequency reaches the cutoff
frequency of the waveguide, the signal bounces back
and forth between the sidewalls of the waveguide. No
energy is propagated.
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16-3: Waveguides
and Cavity Resonators
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Figure 16-22: Wave
paths in a
waveguide at
various
frequencies.
(a) High
frequency.
(b) Medium
frequency.
(c) Low
frequency.
(d) Cutoff
frequency.
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16-3: Waveguides
and Cavity Resonators
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Waveguides: Signal Propagation
 When a microwave signal is launched into a waveguide
by a probe or loop, electric and magnetic fields are
created in various patterns depending upon the method
of energy coupling, frequency of operation, and size of
waveguide.
 The pattern of the electromagnetic fields within a
waveguide takes many forms. Each form is called an
operating mode.
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16-3: Waveguides
and Cavity Resonators
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Figure 16-23: Electric (E ) and magnetic (H) fields in a rectangular waveguide.
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16-3: Waveguides
and Cavity Resonators
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Waveguide Hardware and Accessories
 Waveguides have a variety of special parts, such as
couplers, turns, joints, rotary connections, and
terminations.
 Most waveguides and their fittings are precision-made
so that the dimensions match perfectly.
 A choke joint is used to connect two sections of
waveguide. It consists of two flanges connected to the
waveguide at the center.
 A T section or T junction is used to split or combine
two or more sources of microwave power.
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16-3: Waveguides
and Cavity Resonators
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Figure 16-25: A choke joint permits sections of waveguide to be interconnected with
minimum loss and radiation.
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16-3: Waveguides
and Cavity Resonators
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Waveguide Hardware and Accessories: Directional
Couplers
 One of the most commonly used waveguide
components is the directional coupler.
 Directional couplers are used to facilitate the
measurement of microwave power in a waveguide and
the SWR.
 They can also be used to tap off a small portion of a
high-power microwave signal to be sent to another
circuit or piece of equipment.
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16-3: Waveguides
and Cavity Resonators
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Figure 16-30: Directional coupler.
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16-3: Waveguides
and Cavity Resonators
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Cavity Resonator
 A cavity resonator is a waveguide-like device that acts
like a high-Q parallel resonant circuit.
 A simple cavity resonator can be formed with a short
piece of waveguide one-half wavelength long.
 Energy is coupled into the cavity with a coaxial probe at
the center.
 The internal walls of the cavity are often plated with
silver or some other low-loss material to ensure
minimum loss and maximum Q.
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16-3: Waveguides
and Cavity Resonators
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Figure 16-31: Cavity resonator made with waveguide. (b) Side view of cavity
resonator showing coupling of energy by a probe.
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16-3: Waveguides
and Cavity Resonators
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Circulators
 A circulator is a three-port microwave device used for
coupling energy in only one direction around a closed
loop.
 Microwave energy is applied to one port and passed to
another with minor attenuation, however the signal will
be greatly attenuated on its way to a third port.
 The primary application of a circulator is a diplexer,
which allows a single antenna to be shared by a
transmitter and receiver.
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16-3: Waveguides
and Cavity Resonators
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Figure 16-31 Cavity resonator made with waveguide. (a) A section of rectangular
waveguide used as a cavity resonator. (b) Side view of cavity resonator showing
coupling of energy by a probe.
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16-3: Waveguides
and Cavity Resonators
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Isolators
 Isolators are variations of circulators, but they have one
input and one output.
 They are configured like a circulator, but only ports 1
and 2 are used.
 Isolators are often used in situations where a mismatch,
or the lack of a proper load, could cause reflection so
large as to damage the source.
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16-4: Microwave
Semiconductor Diodes
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Small Signal Diodes
 Diodes used for signal detection and mixing are the
most common microwave semiconductor devices.
 Two types of widely used microwave diodes are:
 Point-contact diode
 Schottky barrier or hot-carrier diode
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16-4: Microwave
Semiconductor Diodes
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Small Signal Diodes: Point-Contact Diode
 The oldest microwave semiconductor device is the
point-contact diode, also called a crystal diode.
 A point-contact diode is a piece of semiconductor
material and a fine wire that makes contact with the
semiconductor material.
 Point-contact diodes are ideal for small-signal
applications.
 They are widely used in microwave mixers and
detectors and in microwave power measurement
equipment.
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16-4: Microwave
Semiconductor Diodes
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Figure 16-35: A point-contact diode.
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16-4: Microwave
Semiconductor Diodes
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Small Signal Diodes: Hot Carrier Diodes
 For the most part, point-contact diodes have been
replaced by Schottky diodes, sometimes referred to as
hot carrier diodes.
 Like the point-contact diode, the Schottky diode is
extremely small and has a tiny junction capacitance.
 Schottky diodes are widely used in balanced
modulators and mixers.
 They are also used as fast switches at microwave
frequencies.
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16-4: Microwave
Semiconductor Diodes
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Figure 16-36: Hot carrier or Schottky diode.
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16-4: Microwave
Semiconductor Diodes
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Frequency-Multiplier Diodes
 Microwave diodes designed primarily for frequency-
multiplier service include:
 Varactor diodes
 Step-recovery diodes
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16-4: Microwave
Semiconductor Diodes
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Frequency-Multiplier Diodes: Varactor Diodes
 A varactor diode is basically a voltage variable
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capacitor.
When a reverse bias is applied to the diode, it acts like
a capacitor.
A varactor is primarily used in microwave circuits as a
frequency multiplier.
Varactors are used in applications in which it is difficult
to generate microwave signals.
Varactor diodes are available for producing relatively
high power outputs at frequencies up to 100 GHz.
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16-4: Microwave
Semiconductor Diodes
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Figure 16-37: A varactor frequency multiplier.
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16-4: Microwave
Semiconductor Diodes
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Frequency-Multiplier Diodes: Step-Recovery Diodes
 A step-recovery diode or snap-off varactor is widely
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used in microwave frequency-multiplier circuits.
A step-recovery diode is a PN-junction diode made with
gallium arsenide or silicon.
When it is forward-biased, it conducts as any diode, but
a charge is stored in the depletion layer.
When reverse bias is applied, the charge keeps the
diode on momentarily and then turns off abruptly.
This snap-off produces a high intensity reverse-current
pulse that is rich in harmonics.
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16-4: Microwave
Semiconductor Diodes
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Oscillator Diodes
 Three types of diodes other than the tunnel diode that
can oscillate due to negative resistance characteristics
are:
 Gunn diode
 IMPATT diode
 TRAPATT diode
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16-4: Microwave
Semiconductor Diodes
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Oscillator Diodes: Gunn Diodes
 Gunn diodes, also called transferred-electron
devices (TEDs), are not diodes in the usual sense
because they do not have junctions.
 A Gunn diode is a thin piece of N-type gallium arsenide
(GaAs) or indium phosphide (InP) semiconductor which
forms a special resistor when voltage is applied to it.
 The Gunn diode exhibits a negative-resistance
characteristic.
 Gunn diodes oscillate at frequencies up to 150 GHz.
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Semiconductor Diodes
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Oscillator Diodes: IMPATT and TRAPATT Diodes
 Two microwave diodes widely used as oscillators are
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the IMPATT and TRAPATT diodes.
Both are PN-junction diodes made of silicon, GaAs, or
InP.
They are designed to operate with a high reverse bias
that causes them to avalanche or break down.
IMPATT diodes are available with power ratings up to
25 W to frequencies as high as 300 GHz.
IMPATT are preferred over Gunn diodes if higher power
is required.
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Semiconductor Diodes
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PIN Diodes
 A PIN diode is a special PN-junction diode with an I
(intrinsic) layer between the P and the N sections.
 The P and N layers are usually silicon, although GaAs is
sometimes used and the I layer is a very lightly doped
N-type semiconductor.
 PIN diodes are used as switches in microwave circuits.
 PIN diodes are widely used to switch sections of
quarter- or half-wavelength transmission lines to provide
varying phase shifts in a circuit.
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16-5: Microwave Tubes
 Vacuum tubes are devices used for controlling a
large current with a small voltage to produce
amplification, oscillation, switching, and other
operations.
 Vacuum tubes are used in microwave transmitters
requiring high output power.
 Special microwave tubes such as the klystron, the
magnetron, and the traveling-wave tube are widely
used for microwave power amplification.
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16-5: Microwave Tubes
Klystrons
 A klystron is a microwave vacuum tube using cavity
resonators to produce velocity modulation of an electron
beam that produces amplification.
 Klystrons are no longer widely used in most microwave
equipment.
 Gunn diodes have replaced the smaller reflex klystrons
in signal-generating applications because they are
smaller and lower in cost.
 The larger multicavity klystrons are being replaced by
traveling-wave tubes in high-power applications.
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16-5: Microwave Tubes
Magnetrons
 A widely used microwave tube is the magnetron, a
combination of a simple diode vacuum tube with built-in
cavity resonators and an extremely powerful permanent
magnet.
 Magnetrons are capable of developing extremely high
levels of microwave power.
 When operated in a pulsed mode, magnetrons can
generate several megawatts of power.
 A typical application for a continuous-wave magnetron
is for heating purposes in microwave ovens.
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16-5: Microwave Tubes
Figure 16-40: A magnetron tube used as an oscillator.
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16-5: Microwave Tubes
Traveling-Wave Tubes
 One of the most versatile microwave RF power
amplifiers is the traveling-wave tube (TWT), which can
generate hundreds and even thousands of watts of
microwave power.
 The main advantage of the TWT is an extremely wide
bandwidth.
 Traveling-wave tubes can be made to amplify signals in
a range from UHF to hundreds of gigahertz.
 A common application of TWTs is as power amplifiers in
satellite transponders.
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16-5: Microwave Tubes
Figure 16-41: A traveling-wave tube (TWT).
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16-6: Microwave Antennas
 Because of the line-of-sight transmission of microwave
signals, highly directive antennas are preferred
because they do not waste the radiated energy and
because they provide an increase in gain, which helps
offset noise at microwave frequencies.
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Low-Frequency Antennas
 At low microwave frequencies, less than 2 GHz,
standard antennas are commonly used, including the
dipole and its variations.
 The corner reflector is a fat, wide-bandwidth, halfwave dipole fed with low-loss coaxial cable.
 The overall gain of a corner reflector antenna is 10 to 15
dB.
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Figure 16-42: A corner reflector used with a dipole for low microwave frequencies.
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Horn Antenna
 Microwave antennas must be some extension of or
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compatible with a waveguide.
Waveguide are not good radiators because they
provide a poor impedance match with free space. This
results in standing waves and reflected power.
This mismatch can be offset by flaring the end of the
waveguide to create a horn antenna.
Horn antennas have excellent gain and directivity.
The gain and directivity of a horn are a direct function of
its dimensions; the most important dimensions are
length, aperture area, and flare angle.
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Figure 16-43: Basic horn antenna.
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Parabolic Antennas
 A parabolic reflector is a large dish-shaped structure
made of metal or screen mesh.
 The energy radiated by the horn is pointed at the
reflector, which focuses the radiated energy into a
narrow beam and reflects it toward its destination.
 Beam widths of only a few degrees are typical with
parabolic reflectors.
 Narrow beam widths also represent extremely high
gains.
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Figure 16-48: Cross-sectional view of a parabolic dish antenna.
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Parabolic Antennas: Feed Methods
 A popular method of feeding a parabolic antenna is an
arrangement known as a Cassegrain feed.
 The horn antenna is positioned at the center of the
parabolic reflector.
 At the focal point is another small reflector with either a
parabolic or a hyperbolic shape.
 The electromagnetic radiation from the horn strikes the
small reflector, which then reflects the energy toward
the large dish which radiates the signal in parallel
beams.
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Figure 16-51: Cassegrain feed.
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Helical Antennas
 A helical antenna, as its name suggests, is a wire helix.
 A center insulating support is used to hold heavy wire or
tubing formed into a circular coil or helix.
 The diameter of the helix is typically one-third
wavelength, and the spacing between turns is
approximately one-quarter wavelength.
 The gain of a helical antenna is typically in the 12- to
20-dB range and beam widths vary from approximately
12° to 45°.
 Helical antennas are favored in many applications
because of their simplicity and low cost.
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Figure 16-52: The helical antenna.
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Bicone Antennas
 One of the most widely used omnidirectional microwave
antennas is the bicone.
 The signals are fed into bicone antennas through a
circular waveguide ending in a flared cone.
 The upper cone acts as a reflector, causing the signal to
be radiated equally in all directions with a very narrow
vertical beam width.
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Figure 16-53: The omnidirectional bicone antenna.
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Slot Antennas
 A slot antenna is a radiator made by cutting a one-half
wavelength slot in a conducting sheet of metal or into
the side or top of a waveguide.
 The slot antenna has the same characteristics as a
standard dipole antenna, as long as the metal sheet is
very large compared to λ at the operating frequency.
 Slot antennas are widely used on high-speed aircraft
where the antenna can be integrated into the metallic
skin of the aircraft.
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Figure 16-54: Slot antennas on a waveguide. (a) Radiating slots. (b) Nonradiating
slots.
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Dielectric (Lens) Antennas
 Dielectric or lens antennas use a special dielectric
material to collimate or focus the microwaves from a
source into a narrow beam.
 Lens antennas are usually made of polystyrene or some
other plastic, although other types of dielectric can be
used.
 Their main use is in the millimeter range above 40 GHz.
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Figure 16-57: Lens antenna operations. (a) Dielectric lens. (b) Zoned lens.
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Patch Antennas
 Patch antennas are made with microstrip on PCBs.
 The antenna is a circular or rectangular area of copper
separated from the ground plane on the bottom of the
board by the PCB’s insulating material.
 Patch antennas are small, inexpensive, and easy to
construct.
 Their bandwidth is directly related to the thickness of
the PCB material.
 Their radiation pattern is circular in the direction
opposite to that of the ground plane.
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Phased Arrays
 A phased array is an antenna system made up of a
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large group of similar antennas on a common plane.
Patch antennas on a common PCB can be used, or
separate antennas like dipoles can be mounted
together in a plane.
The basic purpose of an array is to improve gain and
directivity.
Arrays also offer better control of directivity, since
individual antennas in an array can be turned off or on,
or driven through different phase shifters.
Most phased arrays are used in radar systems, but they
are finding applications in some cell phone systems and
in satellites.
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Figure 16-59: An 8 × 8 phase array using patch antennas. (Feed lines are not
shown.)
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Printed-Circuit Antennas
 Because antennas are so small at microwave
frequencies, they can be conveniently made right on a
printed-circuit board that also holds the transmitter
and/or receiver ICs and related circuits.
 No separate antenna structure, feed line, or connectors
are needed.
 In addition to the patch and slot antennas, the loop, the
inverted-F, and the meander line antennas are also
used.
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Intelligent Antenna Technology
 Intelligent antennas or smart antennas are antennas
that work in conjunction with electronic decision-making
circuits to modify antenna performance to fit changing
situations.
 They adapt to the signals being received and the
environment in which they transmit.
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Intelligent Antenna Technology
 Also called adaptive antennas, these new designs
greatly improve transmission and reception in multipath
environments and can also multiply the number of users
of a wireless system.
 Some popular adaptive antennas today use diversity,
multiple-input multiple-output, and automatic beam
forming.
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Adaptive Beam Forming
 Adaptive antennas are systems that automatically
adjust their characteristics to the environment.
 They use beam-forming and beam-pointing techniques
to zero in on signals to be received and to ensure
transmission under noisy conditions.
 Beam-forming antennas use multiple antennas such as
phase arrays.
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Adaptive Beam Forming
 There are two kinds of adaptive antennas: switched
beam arrays and adaptive arrays.
 Both switched beam arrays and adaptive arrays are
being employed in some cell phone systems and in
newer wireless LANs.
 They are particularly beneficial to cell phone systems
because they can boost the system capacity.
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Figure 16-64: Major
applications of microwave
radio.
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Radar
 The electronic communication system known as radar
(radio detection and ranging) is based on the principle
that high-frequency RF signals are reflected by
conductive targets.
 In a radar system, a signal is transmitted toward the
target and the reflected signal is picked up by a receiver
in the radar unit.
 The radar unit can determine the distance to a target
(range), its direction (azimuth), and in some cases, its
elevation (distance above the horizon).
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Radar
 There are two basic types of radar systems: pulsed and
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continuous-wave (CW).
The pulsed type is the most commonly used radar
system.
Signals are transmitted in short bursts or pulses.
The time between transmitted pulses is known as the
pulse repetition time (PRT).
In continuous-wave (CW) radar, a constant-amplitude
continuous microwave sine wave is transmitted.
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Radar: UWB
 The newest form of radar is called ultrawideband
(UWB) radar.
 It is a form of pulsed radar that radiates a stream of
very short pulses several hundred picoseconds long.
 The very narrow pulses give this radar extreme
precision and resolution of small objects and details.
 The low power used restricts operation to short
distances.
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Radar: UWB
 The circuitry used is simple, so it is possible to make
inexpensive, single-chip radars.
 These are used in short-range collision detection
systems in airplanes and soon will be in automobiles for
automatic braking based upon distance from the vehicle
ahead.
 Another application of UWB radar is personnel
detection on the battlefield. These radars can penetrate
walls to detect the presence of human beings.
© 2008 The McGraw-Hill Companies