MAYURBHANJ SCHOOL OF ENGINEERING � LAXMIPOSI ,BARIPADA,757107
Prepared by Er. Viswanath Behera (Lecturer E & TC Engineering Department)
Subject – WAVE PROPAGATION & BROADBAND COMMUNICATION ENGINEERING
Chapter – 4 – MICROWAVE TUBES
Topic – Microwave Tube
Semester – 5th
Branch – Electronics & Telecommunication
AY-2021-2022, WINTER-2021
MICROWAVE TUBES
Pre-requisites for Microwave Tubes Topic :
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* Transmission media
* Wave guide theory and modes
Cavity Resonators
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A cavity resonator, is one in which the waves exist in a hollow space inside the device. Acoustic cavity resonators in which sound is produced by air vibrating in a cavity with one opening, are known as Helmholtz resonators.
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An illustration of the electric and magnetic field of one of the possible modes in a cavity resonator.
The cavity has interior surfaces which reflect a wave of a specific frequency. When a wave that is resonant with the cavity enters, it bounces back and forth within the cavity, with low loss. As more wave energy enters the cavity, it combines with and reinforces the standing wave, increasing its intensity
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A standing wave in a rectangular cavity resonator.
Rectangular cavity resonators
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Starting from a rectangular waveguide of cross section ‘a’ by ‘b’ metres, we can add short circuit walls in the y-z planes, along the direction of propagation.
This gives a rectangular box whose resonant frequency is given by ‘f’
where (f*λ) = c = 3*10^8, and
1/[λ]^2 = {m/2a}^2 + {n/2b}^2 + {p/2d}^2
Here, there are m half wavelength loops along x, n half wavelength loops along y, and p half wavelength loops along d. It is possible for just one only of the loop numbers m, n, and p to take the value zero.
The spacings of the walls are d along z, b along y, and a along x. We see
there are many modes of a rectangular cavity.
b
a
d
Conventional Vacuum Tube
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Frequency Limitations of Conventional Tubes
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Microwave tubes
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--- the dimensions of the tube structure in relation to the wavelength of the signal that it generates or amplifies, and the time during which the electrons interact with the microwave field.
Microwave tubes
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MW TUBES
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Applications of high power devices at millimeter wave frequency range
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Radar (long-range and high resolution) Communication (high information density) Electronic warfare
Directed energy weaponry Material processing Waste remediation
Ozone generation
Atmospheric purification of admixtures like freons that destroy ozone layer
Microwave Tubes
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Linear Beam Devices
Cross Field Devices
Magnetron CFA
Resonant Cavity
slow-wave structure (non-resonant)
Forward Wave
Backward Wave
Helix TWT
BWA BWO
Coupled Cavity TWT
Klystron Amplifier
Reflex Klystron
MICROWAVE SOURCES
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High Power Microwave Tubes
RF conversion efficiency = ratio of RF power output available to the dc power input
RF conversion efficiency of RF Power sources : 10% to 60%
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Two of the researchers instrumental in the initial development of the IOT, a pair of brothers named Sigurd and Russell Varian
Inductive Output Tube (IOT)
It was discovered in 1939 that a toroidal cavity made of conductive material called a cavity resonator surrounding an electron beam of oscillating intensity could extract power from the beam without actually intercepting the beam itself.
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The oscillating electric and magnetic fields associated with the beam "echoed" inside the cavity, in a manner similar to the sounds of traveling automobiles echoing in a roadside canyon, allowing radio-frequency energy to be transferred from the beam to a waveguide or coaxial cable connected to the resonator with a coupling loop.
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This input resonator acted as a pair of inductive grids to alternately "bunch" and release packets of electrons down the drift space of the tube, so the electron beam would be composed of electrons traveling at different velocities. This "velocity modulation" of the beam translated into the same sort of amplitude variation at the output resonator, where energy was extracted from the beam.
AuTghuest V2a9,ri2a0n17brothers called their invention a klystron.
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Two Cavity Klystron Amplifier
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It is not uncommon to see a klystron with a beam current of 25 THOUSAND VOLTS (that’s 25KV) at 5 Amps. Now if'n I done my math correctly, P=IE, so Power Out = 25,000 multiplied by 5. This tube would have a beam power of 125,000 Watts.
You don't have to touch anything! There is so much electrical potential built up in the surrounding air that your hair stands on end just being around that sort of voltage.
the beam must be carefully guided up through the drift tube until it reaches it's final resting place. This is usually done with electromagnetic coils. Magnet supply voltages are commonly in the 200 Volt range.
New and recent development of a special type of klystron using fixed permanent magnets, called a PPM Focused Klystron which was able to obtain power levels on the order of 50 Megawatts.
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Super power Klystron used at the Canberra Deep Space Communications Complex
Multi-cavity Klystron
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Electrons emitted from the heated cathode travel through the cavity grids toward the repeller plate, then are repelled and returned back the way they came (hence the name reflex) through the cavity grids. Self-sustaining oscillations would develop in this tube, the frequency of which could be changed by adjusting the repeller voltage. Hence, this tube operated as a voltage-controlled oscillator.
Reflex Klystron
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As a voltage-controlled oscillator, reflex klystron tubes served commonly as "local oscillators" for radar equipment and microwave receivers
Initially developed as low-power devices whose output required further amplification for radio transmitter use, reflex klystron design was refined to the point where the tubes could serve as power devices in their own right.
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Magnetron tube
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One microwave tube performs its task so well and so cost-effectively that it continues to reign supreme in the competitive realm of consumer electronics: the magnetron tube.
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This device forms the heart of every microwave oven, generating several hundred watts of microwave RF energy used to heat food and beverages, and doing so under the most grueling conditions for a tube: powered on and off at random times and for random durations.
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Magnetron tubes are representative of an entirely different kind of tube than the IOT and klystron. Whereas the latter tubes use a linear electron beam, the magnetron directs its electron beam in a circular pattern by means of a strong magnetic field:
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Magnetic flux runs perpendicular to the plane of the circular electron path. In other words, from the view of the tube shown in the diagram, you are looking straight at one of the magnetic poles.
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A cross-sectional diagram of a resonant cavity magnetron. Magnetic field is perpendicular to the plane of the diagram
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Magnetic flux runs perpendicular to the plane of the circular electron path. In other words, from the view of the tube shown in the diagram, you are looking straight at one of the magnetic poles.
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Cavity resonators are used as microwave-frequency "tank circuits," extracting energy from the passing electron beam inductively.
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Like all microwave-frequency devices using a cavity resonator, at least one of the resonator cavities is tapped with a coupling loop:
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A loop of wire magnetically coupling the coaxial cable to the resonant structure of the cavity, allowing RF power to be directed out of the tube to a load.
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In the case of the microwave oven, the output power is directed through a waveguide to the food or drink to be heated, the water molecules within acting as tiny load resistors, dissipating the electrical energy in the form of heat.
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Magnetrons have been used since the 1940s as pulsed microwave radiation sources for radar tracking.
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Because of their compactness and the high efficiency with which they can emit short bursts of megawatt peak output power, they have proved excellent for installation in aircraft as well as in ground radar stations.
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In continuous operation, a magnetron can produce a kilowatt of microwave power which is appropriate for rapid microwave cooking.
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Magnetron with magnet in its mounting box. The horizontal plates form a Heatsink, cooled by airflow from a fan
Magnetron with section removed (magnet is not shown)
Health hazards
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Among more speculative hazards, at least one in particular is well known and documented.
As the lens of the eye has no cooling blood flow, it is particularly prone to overheating when exposed to microwave radiation. This heating can in turn lead to a higher incidence of cataracts in later life. A microwave oven with a warped door or poor microwave sealing can be hazardous.
There is also a considerable electrical hazard around magnetrons, as they require a high voltage power supply. Operating a magnetron with the protective covers and interlocks bypassed should therefore be avoided.
Some magnetrons have ceramic insulators with a bit of beryllium oxide The beryllium in this ceramic is a serious chemical hazard if crushed and inhaled, or otherwise ingested. Single or chronic exposure can lead to berylliosis, an incurable lung condition. In addition, beryllia is listed as a confirmed human carcinogen by the IARC; therefore, broken ceramic insulators or magnetrons should not be directly handled.
Traveling Wave Tube (TWT)
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The traveling wave tube (TWT) is an electron tube used for amplification at microwave frequencies – generally identified as frequencies between 500 MHz and 300 GHz or to wavelengths measured from 30 cm to 1 mm.
The TWT is not a new device. Its remarkable capabilities and some of its potential applications have been known for nearly 60 years.
It was invented during the latter part of World War II by an Austrian refugee, Dr. Rudolf Kompfner, while working on microwave tubes for the British Admiralty.
Power generation capabilities range from watts to megawatts.
For helix TWTs, bandwidths may be as high as two octaves or more and power levels of tens to hundreds of watts
For coupled-cavity TWTs, bandwidths in the 10 – 20% range are common with power levels in the megawatt levels.
Components of a TWT
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Wave – Beam Interaction
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At the same time that the cylindrical electron beam is moving along the length of the tube axis, the RF signal to be amplified is fed into the slow-wave structure consisting, in this case, of a coiled wire called a helix.
The RF energy travels along the helix wire at the velocity of light. However, because of the helical path, the energy progresses along the axial length of the tube at a considerably lower axial velocity, determined primarily by the pitch and
dAiaugmusett2e9r, o2f01th7 e helix
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Specific Applications and TWT Design Trade-Offs
The design of a TWT originates with the requirements to provide certain amounts of gain and power over a certain frequency band
These considerations lead to trade-offs that affect each of the major subassemblies of the TWT. Those considerations include:
Type of slow-wave circuit to be used in meeting the power and bandwidth requirements, including the selection of cathode voltage and current to be
used in meeting those requirements.
It is important to note that the higher thermal dissipation capability in coupled-cavity TWT circuits can provide two orders of magnitude and greater power output capability than available from TWTs having helix circuits, at the penalty of increased size and weight;
Method to be employed for focusing the electron beam;
Method to be used for varying the beam current, including the method used for turning the TWT on and off as well as any modulation required during TWT operation;
contd…
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Crossed-Field Amplifier
A (CFA) is a specialized vacuum tube, first introduced in the mid-1950s and frequently used as a microwave amplifier in very-high-power transmitters.
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A CFA has lower gain and bandwidth than other microwave amplifier tubes (such as klystrons or traveling wave tubes); but it is more efficient and capable of much higher output power.
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Peak output powers of many megawatts and average power levels of tens of kilowatts can be achieved, with efficiency ratings in excess of 70 percent
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The electric and magnetic fields in a CFA are perpendicular to each other ("crossed fields"). This is the same type of field interaction used in a magnetron; as a result, the two devices share many characteristics (such as high peak power and efficiency) and they have similar physical appearances. However, a magnetron is an oscillator and a CFA is an amplifier; a CFA's RF circuit (or slow-wave structure) is similar to that in a coupled-cavity TWT.
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Raytheon engineer William C. Brown's work to adapt magnetron principles to create a new broadband amplifier is generally recognized as the first CFA, which he called an Amplitron. Other names that are sometimes used by CFA manufacturers include Platinotron or Stabilotron.
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Klystrons
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TWT
Peak Power : up to 30 MW, Average power : 700 KW Gain : 30 - 70 dB, Efficiency : 15 – 60 %
Bandwidth : 1 – 8 % (Klystron), 15 – 60 % (TWT)
Magnetron
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CFA