A material that gives a scrutinized details on Microwave devices and how they operate.

1. High Power Microwave Devices &
Applications
By
Prof. Gloria Chukwudebe
Department of Electrical and Electronic Engineering
Federal University of Technology, Owerri
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2. Outline
• Introduction
• Properties of Microwaves
• Applications Of Microwaves
• Advantages of Microwave Communication
• Microwave semiconductor devices: Backward, Gunn, Impatt,
Schottky, Tunnel diodes, etc.
• Microwave Tubes: Klystron , Traveling Wave Tube, Magnetron, etc
• Waveguides.
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3. Introduction
• Microwaves have frequencies > 1 GHz approx.
• Stray reactances become important as frequency increases.
• Cable losses increase as frequency increases, waveguides often used instead.
• To generate such high-power microwaves:
• Semiconductor devices or
• Tubes are used.
• Semiconductor devices are mostly diodes made of Gallium Arsenide or
Indium Phosphide while Tubes are made of metallic cylinders that use CRT
technology
• The Microwave Tubes are designed to transform the energy of an intense electron
beam into electromagnetic radiation at microwave frequencies.
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Properties of Microwaves
1. Microwave is an electromagnetic radiation of short wavelength.
2. They can reflected by conducting surfaces just like optical waves
since they travel in straight line.
3. Microwave currents flow through a thin outer layer of an ordinary
cable.
4. Microwaves are easily attenuated within short distances.
5. They are not reflected by ionosphere

5. Applications Of Microwaves
• Wireless Communications (space, cellular phones, cordless phones, WLANs, Bluetooth, satellites etc.)
• Radar and Navigation (Airborne,vehicle, weather radars, GPS etc.)
• Remote sensing (Meteorology, mining, land surface, aviation and marine traffic etc.)
• RF Identification (Security, product tracking, animal tracking, toll collection etc.)
• Broadcasting (AM,FM radio, TV etc.)
• Heating (Baking, Food process, Ovens, Drying, Mining, rubber industry)
• Bio-medical application(Diagnostics)
Advantage of Microwave Communication
Power requirement is very less compared to LF signals
Larger Bandwidth : The band width of microwaves is larger than the low frequency signals - more
information can be transmitted using single carrier
Improved directive properties
Less Fading effect and more reliable
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6. Microwave Bands, Frequency Range and Applications
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7. Microwave Semiconductor Devices
• Conventional bipolar transistors are not suitable for microwave
frequencies.
• Electrons move faster than holes.
• Component leads introduce elevated reactance.
• XL increases and XC decreases therefore collector feedback becomes
worse as frequency increases.
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9. • Applications of Gunn Diode
• Gunn Diodes are used as oscillators and Amplifiers.
• They are used in radio communication, military and commercial radar sources.
• Gunn diodes are used as fast controlling equipment in microelectronics for modulation of
laser beams.
• It is used in tachometers.
• Gunn diode is used in sensors for detection in trespass detecting system, in-door opening
system, pedestrian safety systems etc.
• It is also used extensively in microwave relay data link transmitters.
• These are the advantages, disadvantages and applications of Gunn diode. These are
extensively used in sensors in the detection system.
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10. • Gunn diodes are also known as transferred electron devices, TED, are widely used in microwave RF
applications for frequencies between 1 and 100 GHz. •
• The Gunn diode is most commonly used for generating microwave RF signals - these circuits may also
be called a transferred electron oscillator or TEO. The Gunn diode may also be used for an amplifier in
what may be known as a transferred electron amplifier or TEA.
• 4-100hz
• The Gunn diode operation depends on the fact that it has a voltage controlled negative resistance.
• Gunn diodes are fabricated from a single piece of n-type semiconductor. The most common materials
are gallium Arsenide, GaAs and Indium Phosphide, InP. However other materials including Ge, CdTe,
InAs, InSb, ZnSe and others have been used. The device is simply an n-type bar with n+ contacts. It is
necessary to use n-type material because the transferred electron effect is only applicable to electrons
and not holes found in a p-type material
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12. Backward diode
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13. • A backward diode is a semiconductor device which works in reversed biased mode. It is designed by
providing variation in the design characteristics of Zener diode and tunnel diode. It is unilateral device
because its designing mechanism allows it to operate in one direction only.
• Structure of Backward Diode
• The construction of backward diode is similar to that of the tunnel diode. One side of the junction is lightly
doped and another side of the junction is heavily doped. The characteristics so generated resembles the
characteristics of the tunnel diode.
• The operation of the diode takes place in reverse biasing mode thus, it is called backward diode.
• Applications- oscilators, mixer,
• Application of Backward Diode
• Detector: It can be used as a detector up to the frequency of 40 GHz. It possesses low capacitance thus the
problem of charge storage is minimized in these diodes. Besides, it’s nonlinear characteristics for small signal
makes it appropriate for application of detector.
• Switch: The low capacitance of these diodes imparts an ability to the diode to switch from On state to off
state efficiently. Thus, it is used in switching circuitry.
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14. • The device exhibits a negative resistance region on its V/I curve as
seen below. This negative resistance area enables the Gunn diode to
amplify signals. This can be used both in amplifiers and oscillators.
However Gunn diode oscillators are the most commonly found
• This negative resistance region means that the current flow in diode
increases in the negative resistance region when the voltage falls - the
inverse of the normal effect in any other positive resistance element.
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15. • Sensors and measuring instruments ▫ Gunn diode oscillators are used
to generate microwave power for: airborne collision avoidance radar,
antilock brakes, sensors for monitoring the flow of traffic, car radar
detectors, pedestrian safety systems, "distance traveled" recorders,
motion detectors, "slowspeed" sensors (to detect pedestrian and
traffic movement up to 50 m.p.h), traffic signal controllers, automatic
door openers, automatic traffic gates, process control equipment to
monitor throughput, burglar alarms and equipment to detect
trespassers, sensors to avoid derailment of trains, remote vibration
detectors, rotational speed tachometers, moisture content monitors
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16. • IMPATT Diode • Impact Avalanche and Transit-Time (IMPATT) Diode, also
called Avalanche transit-time diodes • Multilayer diodes of several
different types used to generate microwave power
• In contrast to other devices in this class (tunnel diodes, thyristors, and
Gunn diodes), the negative resistance of avalanche-and-transit time diodes
appears only at superhigh frequencies. • Avalanche-and-transit time diodes
are used to generate oscillations in the frequency range from 1 to 300
gigahertz • Avalanche-and-transit time diodes can be made from structures
of the p +-n-i-n + type (the Read diode) or the p-i-n, p-n, p+-n, and p-n +
types
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17. • IMPATT Diode - Operation • As a result they collide with the crystal lattice and free other carriers. These newly freed carriers are
similarly accelerated and collide with the crystal lattice freeing more carriers. This process gives rise to what is termed avalanche
breakdown as the number of carriers multiplies very quickly. For this type of breakdown only occurs when a certain voltage is
applied to the junction. Below this the potential does not accelerate the carriers sufficiently.
• Once the carriers have been generated the device relies on negative resistance to generate and sustain an oscillation. The effect
does not occur in the device at DC, but instead, here it is an AC effect that is brought about by phase differences that are seen at
the frequency of operation. When an AC signal is applied the current peaks are found to be 180 degrees out of phase with the
voltage. This results from two delays which occur in the device: injection delay, and a transit time delay as the current carriers
migrate or drift across the device.
• The voltage applied to the IMPATT diode has a mean value that means the diode is on the verge of avalanche breakdown. The
voltage varies as a sine wave, but the generation of carriers does not occur in unison with the voltage variations. It might be
expected that it would occur at the peak voltage. This arises because the generation of carriers is not only a function of the electric
field but also the number of carriers already in existence.
• As the electric field increases so does the number of carriers. Then even after the field has reached its peak the number of carriers
still continues to grow as a result of the number of carriers already in existence. This continues until the field falls to below a
critical value when the number of carriers starts to fall. As a result of this effect there is a phase lag so that the current is about 90
degrees behind the voltage. This is known as the injection phase delay. • When the electrons move across the N+ region an
external current is seen, and this occurs in peaks, resulting in a repetitive waveform.
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19. • Tunnel Diode
• Definition: Tunnel diode is a heavily doped diode which possesses high conductivity due to the higher
concentration of impurity atoms. It is different from a conventional P-N junction in terms of doping density.
• Applications of Tunnel Diode
• It is used as a switch, oscillator, amplifier etc. It can be used as a high-frequency component because of its
fast response, but due to the availability of better devices, it is not preferred.
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20. • Schottky diode is a metal-semiconductor junction which does not store
charge carriers at the junction because it has no depletion layer. It finds its
application where fast switching is needed.
• Significance of Schottky diode
• When a P-N junction diode is forward biased, it starts conducting, but
when it is reverse biased, it stops conduction. But this transition from
conduction to insulation is not instant. Diode takes some time to reach a
steady state of no conduction when it is reverse biased.
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21. • Microwave Tubes are used for high power/high frequency applications.
• Tubes generate and amplify high levels of microwave power more
cheaply than semiconductor solid state devices.
Examples of Microwave Tubes :
• Klystron tubes,
• Traveling Wave Tube (TWT),
• Magnetron.
Microwave Tubes
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22. • Klystron tube is a vacuum tube that can be operated either as an
oscillator or as an amplifier at microwave frequencies.
• Two basic configurations of klystron tubes are :
1. Multi-cavity klystron which is used as a low power microwave
amplifier.
2. Reflex klystron which is used as a low power microwave oscillator.
Applications of Klystron tube
 As power output tubes
1. in UHF TV transmitters
2. in troposphere scatter transmitters
3. satellite communication ground station
4. radar transmitters
 As power oscillator (5 – 50 GHz), if used as a klystron oscillator
Klystron Tubes
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Klystron Tube Operation
• Electron beam moves down tube past several cavities.
• Input cavity is the buncher, output cavity is the catcher.
• Buncher modulates the velocity of the electron beam

23. Multi-cavity Klystron Two cavity Klystron Amplifier
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24. Mechanism of operation of Reflex Klystron
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A reflex klystron consists of an electron gun, an
accelerating grid, a single re-entrant cavity and a
repeller plate.
Electrons emitted from cathode is accelerated by the
grid and passes through the cavity-anode to the
repeller space.
Due to DC energy, RF noise is generated in the cavity.
Electrons passing through cavity gap experiences
Velocity Modulation.
• the velocity modulated electrons are bunched together
and lose their kinetic energy when they encounter the
positive peak of the cavity RF field.
This loss of energy is transferred to the cavity to
conserve total power.
When power delivered by the electrons is equal to the
power loss in the cavity- Microwave oscillation is
started.

25.  The reflex klystrons are used in
1. Radar receivers
2. Local oscillator in microwave receivers
3. Signal source in microwave generator of variable frequency
4. Portable microwave links
5. Pump oscillator in parametric amplifier
Applications of Reflex Klystrons
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26. • TWT is an amplifier that makes use of distributed interaction between electron
beam and a travelling wave.
• It is mainly used for amplification of high frequencies. i.e. 3000 MHz or above.
• Its principle feature is based on a slow wave structure.
• The RF wave propagate at the speed of light, while electron beam propagate at
much slow velocity.
• Therefore the mechanism that reduces RF wave phase velocity in a TWT is a
slow wave structure.
Traveling Wave Tube (TWT)
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27. Traveling-Wave Tube (TWT) features
The unique feature of the TWT is a helix or coil
that surrounds the length of the tube and the
electron beam passes through the centre or
axis of the helix.
The microwave signal to be amplified is applied
to the end of the helix near the cathode and
the output is taken from the end of the helix
near the collector (anode).
The purpose of the helix is to provide path for
RF signal (slow-wave structure)
Energy is transferred from electron beam to
microwaves.
The propagation of the RF signal along the helix
is made approximately equal to the velocity of
the electron beam from the cathode to the
anode.
Structure of Traveling-Wave Tube (TWT)
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Traveling-Wave Tube Operation
The passage of the microwave signal down the helix produces electric and magnetic
fields that will interact with the electron beam.
The electromagnetic field produced by the helix causes the electrons to be speeded up
and slowed down, this produces velocity modulation of the beam which produces
density modulation.
Density modulation causes bunches of electrons to group together one wavelength
apart and these bunch of electrons travel down the length of the tube toward the
collector.
The electron bunches induce voltages into the helix which reinforce the voltage already
present there. Due to that, the strength of the electromagnetic field on the helix
increases as the wave travels down the tube towards the collector.
At the end of the helix, the signal is considerably amplified.
Coaxial cable or waveguide structures are used to extract the energy from the helix.

29. Applications of TWT
1. Low noise RF amplifier in broad band microwave
receivers.
2. Repeater amplifier in wide band communication
links and long distance telephony.
3. Due to long tube life (50,000 hours against ¼th for
other types), TWT is used as power output tube
in communication satellites.
4. Continuous wave high power TWT’s are used in
troposcatter links (due to larger power and larger
bandwidths).
5. Used in Air borne and ship borne pulsed high
power radars.
Benefits of TWT
1. TWT has extremely wide bandwidth.
Hence, it can be made to amplify signals
from UHF to hundreds of gigahertz.
2. Most of the TWTs have a frequency range
of approximately 2:1 in the desired
segment of the microwave region to be
amplified.
3. The TWTs can be used in both continuous
and pulsed modes of operation with power
levels up to several thousands watts.
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30. • Magnetron is a high-power microwave
oscillator.
• Common in radar and microwave ovens.
• Cathode in center, anode around outside.
• Strong DC magnetic field around tube causes
electrons from cathode to spiral as they move
toward anode.
• Current of electrons generates microwaves in
cavities around outside.
Magnetron
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31. • Magnetron has a Slow-Wave Structure,
there are cavities all around the outside.
• Wave circulates from one cavity to the next
around the outside.
• Each cavity represents one-half period.
• Wave moves around tube at a velocity
much less than that of light.
• Wave velocity approximately equals
electron velocity.
Magnetron ctd
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Cavity Magnetrons

33. Waveguides
• Pipe through which waves propagate
• Can have various cross sections
• Rectangular
• Circular
• Elliptical
• Can be rigid or flexible
• Waveguides have very low loss
Modes
• Waves can propagate in various ways
• Time taken to move down the guide varies with the mode
• Each mode has a cutoff frequency below which it won’t propagate
• Mode with lowest cutoff frequency is dominant mode
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35. Thank you
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