Chapter 16

© 2008 The McGraw-Hill Companies

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Principles of ElectronicPrinciples of ElectronicCommunication SystemsCommunication Systems

Third Edition

Louis E. Frenzel, Jr.


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Chapter 16Chapter 16

Microwave Communication


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Topics Covered in Chapter 16Topics 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 Concepts16-1: 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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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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Figure 16-1: Microwave frequency bands.


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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.


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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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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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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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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 microwave

components. 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: 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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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 16-2: Microwave Lines and DevicesDevices

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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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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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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Microstrip Tuned Circuits An important characteristic of microstrip is its

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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Figure 16-8: How a one-quarter wavelength microstrip can transform impedancesand reactances.


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Microstrip Tuned Circuits Microstrip can also be used to realize coupling from one

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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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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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 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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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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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 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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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.


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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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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 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.


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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 16-3: Waveguides and Cavity Resonatorsand Cavity Resonators

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.


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


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


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


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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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Figure 16-25: A choke joint permits sections of waveguide to be interconnected withminimum 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 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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Figure 16-30: Directional coupler.


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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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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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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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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 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 16-4: Microwave Semiconductor DiodesSemiconductor 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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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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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 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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Figure 16-36: Hot carrier or Schottky diode.


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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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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 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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Figure 16-37: A varactor frequency multiplier.


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Frequency-Multiplier Diodes: Step-Recovery Diodes A step-recovery diode or snap-off varactor is widely

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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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 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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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.


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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 Tubes16-5: 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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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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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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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 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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Figure 16-41: A traveling-wave tube (TWT).


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16-6: Microwave Antennas16-6: 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, 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. 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

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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16-7: Microwave Applications16-7: Microwave Applications

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

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

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