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Principles of Electronic Communication Systems. Third Edition Louis E. Frenzel, Jr. Chapter 16. Microwave Communication. 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

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principles of electronic communication systems

Principles of ElectronicCommunication Systems

Third Edition

Louis E. Frenzel, Jr.

chapter 16

Chapter 16

Microwave Communication

topics covered in chapter 16
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
16 1 microwave concepts
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.
16 1 microwave concepts5
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.
16 1 microwave concepts6
16-1: Microwave Concepts

Figure 16-1: Microwave frequency bands.

16 1 microwave concepts7
16-1: 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.
16 1 microwave concepts8
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.
16 1 microwave concepts9
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.
16 1 microwave concepts10
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.
16 1 microwave concepts11
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.

16 1 microwave concepts12
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.

16 1 microwave concepts13
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.
16 1 microwave concepts14
16-1: Microwave Concepts

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.
16 1 microwave concepts15
16-1: Microwave Concepts

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

16 1 microwave concepts16
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.
16 1 microwave concepts17
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.
16 2 microwave lines and devices
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.
16 2 microwave lines and devices19
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.
16 2 microwave lines and devices20
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).

16 2 microwave lines and devices21
16-2: Microwave Lines and Devices

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

16 2 microwave lines and devices22
16-2: Microwave Lines and Devices

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.
16 2 microwave lines and devices23
16-2: Microwave Lines and Devices

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

and reactances.

16 2 microwave lines and devices24
16-2: Microwave Lines and Devices

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.
16 2 microwave lines and devices25
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.
16 2 microwave lines and devices26
16-2: Microwave Lines and Devices

Figure 16-12: A microstrip hybrid ring.

16 2 microwave lines and devices27
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.
16 2 microwave lines and devices28
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.
16 2 microwave lines and devices29
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.

16 2 microwave lines and devices30
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.
16 2 microwave lines and devices31
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 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.
16 2 microwave lines and devices32
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.
16 2 microwave lines and devices33
16-2: Microwave Lines and Devices

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

16 2 microwave lines and devices34
16-2: Microwave Lines and Devices

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.
16 2 microwave lines and devices35
16-2: Microwave Lines and Devices

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

16 2 microwave lines and devices36
16-2: Microwave Lines and Devices

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

16 2 microwave lines and devices37
16-2: Microwave Lines and Devices

Figure 16-18: An FET power amplifier.

16 3 waveguides and cavity resonators
16-3: Waveguides and 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.
16 3 waveguides and cavity resonators39
16-3: Waveguides and Cavity Resonators

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.
16 3 waveguides and cavity resonators40
16-3: Waveguides and Cavity Resonators

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

16 3 waveguides and cavity resonators41
16-3: Waveguides and Cavity Resonators

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.
16 3 waveguides and cavity resonators42
16-3: Waveguides and Cavity Resonators

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.
16 3 waveguides and cavity resonators43
16-3: Waveguides and Cavity Resonators

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

16 3 waveguides and cavity resonators44
16-3: Waveguides and Cavity Resonators

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.
16 3 waveguides and cavity resonators45
16-3: Waveguides and Cavity Resonators

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.
16 3 waveguides and cavity resonators46
16-3: Waveguides and Cavity Resonators

Figure 16-22: Wave paths in a waveguide at various frequencies.

  • High frequency.
  • Medium frequency.
  • Low frequency.
  • Cutoff frequency.
16 3 waveguides and cavity resonators47
16-3: Waveguides and Cavity Resonators

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.
16 3 waveguides and cavity resonators48
16-3: Waveguides and Cavity Resonators

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

16 3 waveguides and cavity resonators49
16-3: Waveguides and Cavity Resonators

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 the waveguide at the center.
  • A T section or T junction is used to split or combine two or more sources of microwave power.
16 3 waveguides and cavity resonators50
16-3: Waveguides and Cavity Resonators

Figure 16-25: A choke joint permits sections of waveguide to be interconnected with

minimum loss and radiation.

16 3 waveguides and cavity resonators51
16-3: Waveguides and Cavity Resonators

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 a high-power microwave signal to be sent to another circuit or piece of equipment.
16 3 waveguides and cavity resonators52
16-3: Waveguides and Cavity Resonators

Figure 16-30: Directional coupler.

16 3 waveguides and cavity resonators53
16-3: Waveguides and Cavity Resonators

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.
16 3 waveguides and cavity resonators54
16-3: Waveguides and Cavity Resonators

Figure 16-31: Cavity resonator made with waveguide. (b) Side view of cavity resonator showing coupling of energy by a probe.

16 3 waveguides and cavity resonators55
16-3: Waveguides and Cavity Resonators

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.
16 3 waveguides and cavity resonators56
16-3: Waveguides and Cavity Resonators

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.

16 3 waveguides and cavity resonators57
16-3: Waveguides and Cavity Resonators

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.
16 4 microwave semiconductor diodes
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
16 4 microwave semiconductor diodes59
16-4: Microwave Semiconductor Diodes

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.
16 4 microwave semiconductor diodes60
16-4: Microwave Semiconductor Diodes

Figure 16-35: A point-contact diode.

16 4 microwave semiconductor diodes61
16-4: Microwave Semiconductor Diodes

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.
16 4 microwave semiconductor diodes62
16-4: Microwave Semiconductor Diodes

Figure 16-36: Hot carrier or Schottky diode.

16 4 microwave semiconductor diodes63
16-4: Microwave Semiconductor Diodes

Frequency-Multiplier Diodes

  • Microwave diodes designed primarily for frequency-multiplier service include:
    • Varactor diodes
    • Step-recovery diodes
16 4 microwave semiconductor diodes64
16-4: Microwave Semiconductor Diodes

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.
16 4 microwave semiconductor diodes65
16-4: Microwave Semiconductor Diodes

Figure 16-37: A varactor frequency multiplier.

16 4 microwave semiconductor diodes66
16-4: Microwave Semiconductor Diodes

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.
16 4 microwave semiconductor diodes67
16-4: Microwave Semiconductor Diodes

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
16 4 microwave semiconductor diodes68
16-4: Microwave Semiconductor Diodes

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.
16 4 microwave semiconductor diodes69
16-4: Microwave Semiconductor Diodes

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.
16 4 microwave semiconductor diodes70
16-4: Microwave Semiconductor Diodes

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.
16 5 microwave tubes
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.
16 5 microwave tubes72
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.
16 5 microwave tubes73
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.
16 5 microwave tubes74
16-5: Microwave Tubes

Figure 16-40: A magnetron tube used as an oscillator.

16 5 microwave tubes75
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.
16 5 microwave tubes76
16-5: Microwave Tubes

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

16 6 microwave antennas
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.
16 6 microwave antennas78
16-6: Microwave Antennas

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.
16 6 microwave antennas79
16-6: Microwave Antennas

Figure 16-42: A corner reflector used with a dipole for low microwave frequencies.

16 6 microwave antennas80
16-6: 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.
16 6 microwave antennas81
16-6: Microwave Antennas

Figure 16-43: Basic horn antenna.

16 6 microwave antennas82
16-6: 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.
16 6 microwave antennas83
16-6: Microwave Antennas

Figure 16-48: Cross-sectional view of a parabolic dish antenna.

16 6 microwave antennas84
16-6: 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.
16 6 microwave antennas85
16-6: Microwave Antennas

Figure 16-51: Cassegrain feed.

16 6 microwave antennas86
16-6: 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.
16 6 microwave antennas87
16-6: Microwave Antennas

Figure 16-52: The helical antenna.

16 6 microwave antennas88
16-6: 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.
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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 λat 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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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.