Lec microwave

2

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.

© 2008 The McGraw-Hill Companies

3

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.


4

Microwave Concepts
: Microwave frequency bands.


5

Microwave Concepts
Benefits of Microwaves
 Moving into higher frequency ranges has helped to
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.


6

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.


7

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.


8

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.


9


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.


10


Microwave Transistors
 The GaAs MESFET metal semiconductor field effect
transistor, 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.


11


Microwave transistors. (a) and (b) Low-power small signal. (c) FET power. (d) NPN
bipolar power.

12


Small-Signal Amplifiers: Transistor Amplifiers
 A low-noise transistor with a gain of about 10 to 25 dB
is 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.


13


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.


14


Small-Signal Amplifiers: Power Amplifiers
 A typical class A microwave power amplifier is designed
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 constant-
current sources.
 A single-stage FET power amplifier can achieve a
power output of 100 W in the high UHF and low
microwave region.


15
Waveguides

Waveguides
 Most microwave energy transmission above 6 GHz is
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.


16


17
Waveguides

Wave paths in a
waveguide at
various
frequencies.
(a) High
frequency.
(b) Medium
frequency.
(c) Low
frequency.
(d) Cutoff
frequency.


18
Waveguides

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.


19
Waveguides

A choke joint permits sections of waveguide to be interconnected with
minimum loss and radiation.

20
Microwave
Semiconductor Diodes
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


21
Microwave
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.


22
Microwave
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.


23
Microwave

Hot carrier or Schottky diode.

24
Microwave
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


25
Microwave
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.


26
Microwave
Oscillator Diodes: IMPATT and TRAPATT Diodes
 Two microwave diodes widely used as oscillators are
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.


27
Microwave
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.


28

Microwave Antennas
Horn Antenna
 Microwave antennas must be some extension of or
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.


29

Microwave Antennas

Basic horn antenna.

30


31

Microwave Antennas
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.


32

Microwave Antennas

Cross-sectional view of a parabolic dish antenna.

33

Microwave Antennas
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.


34

Microwave Antennas

Cassegrain feed.

35

Microwave Antennas
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.


36

Microwave Antennas

The helical antenna.

37


38

Microwave Antennas
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.


39

Microwave Antennas

The omnidirectional bicone antenna.

40

Microwave Antennas
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.


41

Microwave Antennas

Lens antenna operations. (a) Dielectric lens. (b) Zoned lens.

42

Microwave Antennas
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.


43


44

Microwave Antennas
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.


45

TV Smart Antenna Multi-Directional HDTV

Multiple–radio smart antenna platform

the Smart BRO antenna.


46

Microwave Antennas
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.


47

Microwave Antennas
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.


48

Microwave Antennas
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.


49

Microwave Applications
Major applications of
microwave radio.


50

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).


51

16-7: Microwave Applications
Radar
 There are two basic types of radar systems: pulsed and
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.


52

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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