Chapter 16 Microwave Communications

7/13/2019 Chapter 16 Microwave Communications

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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 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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Microwavesare the ultrahigh, superhigh, and

extremely high frequencies directly above the lower

frequency ranges where most radio communication

now takes place and below the optical frequenciesthat cover infrared, visible, and ultraviolet light.


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Microwave Frequencies and Bands

The practical microwave region is generally consideredto extend from 1 to 30 GHz, although frequencies couldinclude up to 300 GHz.

Microwave signals in the 1- to 30-GHz havewavelengths of 30 cm to 1 cm.

The microwave frequency spectrum is divided up intogroups of frequencies, or bands.

Frequencies above 40 GHz are referred to asmillimeter (mm) wavesand those above 300 GHz arein the submillimeterband.


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Figure 16-1: Microwave

frequency bands.


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Benefits of Microwaves

Moving into higher frequency ranges has helped tosolve the problem of spectrum crowding.

Today, most new communication services are assignedto the microwave region.

At higher frequencies there is a greater bandwidthavailable 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 requireswide bandwidths and these are easily transmitted onmicrowave frequencies.



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Disadvantages of Microwaves

The higher the frequency, the more difficult it becomes

to analyze electronic circuits.

At microwave frequencies, conventional componentsbecome 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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Microwave Communication Systems

Like any other communication system, a microwavecommunication system uses transmitters, receivers,and antennas.

The same modulation and multiplexing techniques usedat lower frequencies are also used in the microwaverange.

The RF part of the equipment, however, is physically

different because of the special circuits andcomponents that are used to implement thecomponents.



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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 ofpower amplification.

The final power amplifier applies the signal to the

transmission line and antenna.

A transmitter arrangement could have a mixer used toup-convert an initial carrier signal with or without

modulation to the final microwave frequency.



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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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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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Microwave Communication Systems: Receivers

Microwave receivers, like low-frequency receivers, are

the superheterodyne type.

Their front ends are made up of microwavecomponents.

Most receivers use double conversion.



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Microwave Communication Systems: Receivers

The antenna is connected to a tuned circuit, which

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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Figure 16-4: A microwave receiver. The shaded areas denote microwave circuits.

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Microwave Communication Systems: TransmissionLines

Coaxial cable, most commonly used in lower-frequencycommunication has very high attenuation at microwavefrequencies and conventional cable is unsuitable forcarrying microwave signals.

Special microwave coaxial cable that can be used onbands 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 waveguideis usedfor the transmission line.

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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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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 bipolartransistors 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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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 expensivethan stripline.

Stripline is used where shielding is necessary.

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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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Figure 16-7: Equivalent circuits of open and shorted microstrip lines.

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An important characteristic of microstrip is its

impedance.

The characteristic impedance of a transmission linedepends on its physical characteristics.

The dielectric constant of the insulating material is also

a factor.

Most characteristic impedances are less than 100 . One-quarter wavelength transmission line can be used

to make one type of component look like another.

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Figure 16-8: How a one-quarter wavelength microstrip can transform impedances

and reactances.

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Microstrip can also be used to realize coupling from onecircuit.

One microstrip line is simply placed parallel to anothersegment of microstrip.

The degree of coupling between the two depends onthe distance of separation and the length of the parallelsegment.

The closer the spacing and the longer the parallel run,the greater the coupling.

Microstrip patterns are made directly onto printed-circuitboards.

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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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Figure 16-12: A microstrip hybrid ring.

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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 geometriesare used.

Geometries permit the transistor to operate at higherpower levels and at the same time minimize distributed

and stray inductances and capacitances.

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Microwave Transistors

The GaAs MESFET, a type of JFET using a Schottkybarrier junction, can operate at frequencies above 5GHz.

A high electron mobility transistor (HEMT)is avariant of the MESFET and extends the range beyond20 GHz by adding an extra layer of semiconductormaterial such as AlGaAs.

A popular device known as a heterojunction bipolartransistor (HBT)is making even higher-frequencyamplification possible in discrete form and in integratedcircuits.

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Figure 16-14: Microwave transistors. (a) and (b) Low-power small signal. (c) FET

power. (d) NPN bipolar power.

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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 componentsas 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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Small-Signal Amplifiers: Transistor Amplifiers

A low-noise transistor with a gain of about 10 to 25 dBis typically used as a microwave amplifier.

Most microwave amplifiers are designed to have inputand output impedances of 50 .

The transistor is biased into the linear region for class Aoperation.

RFCs are used in the supply leads to keep the RF out

of the supply and to prevent feedback paths that cancause oscillation and instability in multistage circuits.

Ferrite beads (FB) are used in the collector supply leadfor further decoupling.

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Small-Signal Amplifiers: MMIC Amplifiers

A common monolithic microwave integrated circuit(MMIC)amplifier is one that incorporates two or morestages of FET or bipolar transistors made on a common

chip to form a multistage amplifier.

The chip also incorporates resistors for biasing andsmall bypass capacitors.

Physically, these devices look like transistors.

Another form of MMIC is the hybrid circuit,whichcombines an amplifier IC connected to microstripcircuits and discrete components.

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Figure 16-15: A single-stage class A RF microwave amplifier.

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Small-Signal Amplifiers: Power Amplifiers

A typical class A microwave power amplifier is designedwith microstrip lines used for impedance matching andtuning.

Input and output impedances are 50 .

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 apower output of 100 W in the high UHF and lowmicrowave region.

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Figure 16-16: A class A microwave power amplifier.

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Figure 16-17: A constant-current bias supply for a linear power amplifier.

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Figure 16-18: An FET power amplifier.

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16-3: Waveguides

and Cavity Resonators

Waveguides

Most microwave energy transmission above 6 GHz ishandled by waveguides.

Waveguides are hollow metal conducting pipesdesigned to carry and constrain the electromagneticwaves 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 toreduce resistance and transmission losses.

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16-3: Waveguides


Waveguides: Signal Injection and Extraction

A microwave signal to be carried by a waveguide isintroduced into one end of the waveguide with anantennalike probe.

The probe creates an electromagnetic wave thatpropagates through the waveguide.

The electric and magnetic fields associated with thesignal 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


Figure 16-19: Injecting a sine wave into a waveguide and extracting a signal.

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16-3: Waveguides


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 isinduced which can then be fed to other circuitry through

a short coaxial cable.

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16-3: Waveguides


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


Figure 16-20: The dimensions of a waveguide determine its operating frequency range.

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16-3: Waveguides


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.

Whenthe magnetic field is transverse to the direction of

propagation, it is called a transverse magnetic (TM)

field.

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16-3: Waveguides



The angles of incidence and reflection depend on the

operating frequency.

At high frequencies, the angle is large and the pathbetween 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 cutofffrequency 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


Figure 16-22: Wavepaths in awaveguide atvarious

frequencies.(a) Highfrequency.

(b) Mediumfrequency.

(c) Lowfrequency.

(d) Cutofffrequency.

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16-3: Waveguides



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


Figure 16-23: Electric (E ) and magnetic (H) fields in a rectangular waveguide.

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16-3: 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 jointis used to connect two sections of

waveguide. It consists of two flanges connected to thewaveguide at the center.

A T sectionor T junctionis used to split or combine

two or more sources of microwave power.

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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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Waveguide Hardware and Accessories: Directional

Couplers

One of the most commonly used waveguide

components is the directional coupler. Directional couplersare 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 ahigh-power microwave signal to be sent to another

circuit or piece of equipment.

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16-3: Waveguides


Figure 16-30: Directional coupler.

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16-3: Waveguides


Cavity Resonator

A cavity resonator is a waveguide-like device that acts

like a high-Qparallel resonant circuit.

A simple cavity resonator can be formed with a shortpiece 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 withsilver or some other low-loss material to ensure

minimum loss and maximum Q.

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16-3: Waveguides


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


Circulators

A circulatoris 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


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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Isolators

Isolators are variations of circulators, but they have one

input and one output.

They are configured like a circulator, but only ports 1and 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

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


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 semiconductormaterial and a fine wire that makes contact with thesemiconductor material.

Point-contact diodes are ideal for small-signalapplications.

They are widely used in microwave mixers anddetectors and in microwave power measurementequipment.

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Figure 16-35: A point-contact diode.

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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 isextremely small and has a tiny junction capacitance.

Schottky diodes are widely used in balancedmodulators and mixers.

They are also used as fast switches at microwavefrequencies.

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Figure 16-36: Hot carrier or Schottky diode.

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16-4: Microwave


Frequency-Multiplier Diodes

Microwave diodes designed primarily for frequency-

multiplier service include:

Varactor diodes Step-recovery diodes

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Frequency-Multiplier Diodes: Varactor Diodes

A varactor diode is basically a voltage variable

capacitor.

When a reverse bias is applied to the diode, it acts likea capacitor.

A varactor is primarily used in microwave circuits as afrequency multiplier.

Varactors are used in applications in which it is difficult

to generate microwave signals.

Varactor diodes are available for producing relativelyhigh power outputs at frequencies up to 100 GHz.

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Figure 16-37: A varactor frequency multiplier.

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Frequency-Multiplier Diodes: Step-Recovery Diodes

A step-recovery diodeor snap-off varactoris widely

used in microwave frequency-multiplier circuits.

A step-recovery diode is a PN-junction diode made withgallium 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 thediode 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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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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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 whichforms 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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Oscillator Diodes: IMPATT and TRAPATT Diodes

Two microwave diodes widely used as oscillators arethe 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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PIN Diodes

A PIN diodeis 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 dopedN-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 providevarying phase shifts in a circuit.

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Vacuum tubesare 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 widelyused for microwave power amplification.

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Klystrons A klystronis a microwave vacuum tube using cavity

resonators to produce velocity modulation of an electronbeam that produces amplification.

Klystrons are no longer widely used in most microwaveequipment.

Gunn diodes have replaced the smaller reflex klystronsin 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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Magnetrons A widely used microwave tube is the magnetron, a

combination of a simple diode vacuum tube with built-incavity 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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Figure 16-40: A magnetron tube used as an oscillator.

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Traveling-Wave Tubes One of the most versatile microwave RF power

amplifiers is the traveling-wave tube (TWT),which cangenerate 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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Figure 16-41: A traveling-wave tube (TWT).

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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 reflectoris a fat, wide-bandwidth, half-

wave 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

compatible with a waveguide.

Waveguide are not good radiators because they

provide a poor impedance match with free space. Thisresults in standing waves and reflected power.

This mismatch can be offset by flaring the end of thewaveguide to create a horn antenna.

Horn antennas have excellent gain and directivity.

The gain and directivity of a horn are a direct function ofits dimensions; the most important dimensions arelength, aperture area, and flare angle.

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Figure 16-43: Basic horn antenna.

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Parabolic Antennas

A parabolic reflectoris a large dish-shaped structure

made of metal or screen mesh.

The energy radiated by the horn is pointed at thereflector, 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 thesmall 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 ortubing formed into a circular coil or helix.

The diameter of the helix is typically one-thirdwavelength, and the spacing between turns isapproximately one-quarter wavelength.

The gain of a helical antenna is typically in the 12- to

20-dB range and beam widths vary from approximately12to 45.

Helical antennas are favored in many applicationsbecause 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 antennais a radiator made by cutting a one-half

wavelength slot in a conducting sheet of metal or intothe side or top of a waveguide.

The slot antenna has the same characteristics as astandard dipole antenna, as long as the metal sheet isvery large compared to at the operating frequency.

Slot antennas are widely used on high-speed aircraft

where the antenna can be integrated into the metallicskin 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 Dielectricor lens antennasuse a special dielectric

material to collimate or focus the microwaves from asource into a narrow beam.

Lens antennas are usually made of polystyrene or someother plastic, although other types of dielectric can beused.

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 antennasare made with microstrip on PCBs.

The antenna is a circular or rectangular area of copperseparated from the ground plane on the bottom of the

board by the PCBs 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 arrayis an antenna system made up of a

large group of similar antennas on a common plane.

Patch antennas on a common PCB can be used, orseparate antennas like dipoles can be mountedtogether in a plane.

The basic purpose of an array is to improve gain anddirectivity.

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 theyare finding applications in some cell phone systems andin 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 antennasare also

used.

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Intelligent Antenna Technology

Intelligent antennasor smart antennasare 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 antennasare 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 arraysand adaptive arrays.

Both switched beam arrays and adaptive arrays are

being employed in some cell phone systems and innewer wireless LANs.

They are particularly beneficial to cell phone systems

because they can boost the system capacity.

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16 7: Microwave Applications

Figure 16-64:Majorapplications of microwave

radio.

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Radar The electronic communication system known as radar

(radio detection and ranging) is based on the principlethat 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 receiverin the radar unit.

The radar unit can determine the distance to a target(range), its direction (azimuth), and in some cases, itselevation (distance above the horizon).

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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.

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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.

Chapter 16 Microwave Communications

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Transcript of Chapter 16 Microwave Communications