Arun Microwave

8/9/2019 Arun Microwave

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In the two-chamber klystron, the electron beam is injected into a resonant cavity. The

electron beam, accelerated by a positive potential, is constrained to travel through a

cylindrical drift tube in a straight path by an axial magnetic field. While passing through the

first cavity, the electron beam is velocity modulated by the weak RF signal. In the moving

frame of the electron beam, the velocity modulation is equivalent to aplasma oscillation.

Plasma oscillations are rapid oscillations of the electron density in conducting media such as

plasmas ormetals.(The frequency only depends weakly on the wavelength). So in a quarter of

one period of the plasma frequency, the velocity modulation is converted to density

modulation, i.e. bunches of electrons. As the bunched electrons enter the second chamber

they induce standing waves at the same frequency as the input signal. The signal induced in

the second chamber is much stronger than that in the first.

Two-cavity klystron oscillator

The two-cavity amplifier klystron is readily turned into an oscillator klystron by providing a

feedbackloop between the input and output cavities. Two-cavity oscillator klystrons have the

advantage of being among the lowest-noise microwave sources available, and for that reason

have often been used in the illuminator systems ofmissile targeting radars. The two-cavity

oscillator klystron normally generates more power than the reflex klystrontypically watts

of output rather than milliwatts. Since there is no reflector, only one high-voltage supply is

necessary to cause the tube to oscillate, the voltage must be adjusted to a particular value.

This is because the electron beam must produce the bunched electrons in the second cavity inorder to generate output power. Voltage must be adjusted to vary thevelocity of the electron

beam (and thus the frequency) to a suitable level due to the fixed physical separation betweenthe two cavities. Often several "modes" of oscillation can be observed in a given klystron.

Reflex klystron

In the reflex klystron (also known as a 'Sutton' klystron after its inventor), the electron beam

passes through a single resonant cavity. The electrons are fired into one end of the tube by an

electron gun. After passing through the resonant cavity they are reflected by a negatively

charged reflector electrode for another pass through the cavity, where they are then collected.

The electron beam is velocity modulated when it first passes through the cavity.The

formation of electron bunches takes place in the drift space between the reflector and the

cavity. The voltage on the reflector must be adjusted so that the bunching is at a maximum asthe electron beam re-enters the resonant cavity, thus ensuring a maximum of energy is

transferred from the electron beam to the RF oscillations in the cavity. The voltage shouldalways be switched on before providing the input to the reflex klystron as the whole function

of the reflex klystron would be destroyed if the supply is provided after the input. Thereflector voltage may be varied slightly from the optimum value, which results in some loss

of output power, but also in a variation in frequency. This effect is used to good advantagefor automatic frequency control in receivers, and infrequency modulation for transmitters.

The level of modulation applied for transmission is small enough that the power outputessentially remains constant. At regions far from the optimum voltage, no oscillations are

obtained at all. This tube is called a reflex klystron because it repels the input supply orperforms the opposite function of a klystron.


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There are often several regions of reflector voltage where the reflex klystron will oscillate;

these are referred to as modes. The electronic tuning range of the reflex klystron is usually

referred to as the variation in frequency between half power pointsthe points in the

oscillating mode where the power output is half the maximum output in the mode. The

frequency of oscillation is dependent on the reflector voltage, and varying this provides a

crude method of frequency modulating the oscillation frequency, albeit with accompanying

amplitude modulation as well.

Modern semiconductortechnology has effectively replaced the reflex klystron in most

applications.

Multicavity klystron

Large klystrons as used in the storage ring of the Australian Synchrotron to maintain theenergy of the electron beam

In all modern klystrons, the number of cavities exceeds two. A larger number of cavities may

be used to increase the gain of the klystron, or to increase the bandwidth.

Tuning a klystron

Some klystrons have cavities that are tunable. Tuning a klystron is delicate work which, if not

done properly, can cause damage to equipment or injury to the technician. By adjusting the

frequency of individual cavities, the technician can change the operating frequency, gain,

output power, or bandwidth of the amplifier. The technician must be careful not to exceed the

limits of the graduations, or damage to the klystron can result.

Manufacturers generally send a card with the unique calibrations for a klystron's performance

characteristics, that lists the graduations to be set to attain any of a set of listed frequencies.

No two klystrons are exactly identical (even when comparing like part/model number

klystrons), and so every card is specific to the individual unit. Klystrons have serial numberson each of them to uniquely identify each unit, and for which manufacturers may (hopefully)

have the performance characteristics in a database. If not, loss of the calibration card may be

an insoluble problem, making the klystron unusable or perform marginally un-tuned.

Other precautions taken when tuning a klystron include using nonferrous tools. Some

klystrons employ permanent magnets. If a technician uses ferrous tools, (which are

ferromagnetic), and comes too close to the intense magnetic fields that contain the electronbeam, such a tool can be pulled into the unit by the intense magnetic force, smashing fingers,

injuring the technician, or damaging the unit. Special lightweight nonmagnetic (akadiamagnetic) tools made ofberyllium alloy have been used for tuning U.S. Air Force

klystrons.


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Precauti are routi el taken when transporting kl stron devices in aircraft as the intense

magnetic field can interfere with magnetic navigation equipment Special overpacks are

designed to help limitthis field "in the field," and thus allow such devices to be transported

safel

Opti l l tron

In an optical kl stron the cavities are replaced with undulators. Very high voltages areneeded. The electron gun, the drifttube and the collector are still used.

Floating dri t tube kl tron

The floating drifttube klystron has a single cylindrical chamber containing an electrically

isolated centraltube. Electrically, this is similarto the two cavity oscillator klystron with a lot

of feedback between the two cavities. Electrons exiting the source cavity are velocity

modulated by the electric field as they travelthrough the drifttube and emerge atthe

destination chamberin bunches, delivering powerto the oscillation in the cavity. This type of

oscillator klystron has an advantage overthe two-cavity klystron on which itis based. It only

needs one tuning elementto effect changes in frequency. The drifttube is electricallyinsulated from the cavity walls, and DC bias is applied separately. The DC bias on the drift

tube may be adjusted to alterthe transittime through it, thus allowing some electronic tuning

ofthe oscillating frequency. The amount oftuning in this manneris notlarge and is normally

used for frequency modulation when transmitting.

Collector

Afterthe R energy has been extracted from the electron beam, the beam is destroyed in a

collector. Some klystrons include depressed collectors, which recover energy from the beam

before collecting the electrons, increasing efficiency. Multistage depressed collectors enhance

the energy recovery by "sorting"the electrons in energy bins.

Applications

Klystrons produce microwave power farin excess ofthat developed by solid state. In modern

systems, they are used from UHF (100's ofMHz) up through hundreds of gigahertz (as in the

Extended Interaction Klystrons in the CloudSat satellite). Klystrons can be found at workin

radar, satellite and wideband high-power communication (very common in television

broadcasting and EHF satelliteterminals), medicine (radiation oncology), and high-energy

physics (particle accelerators and experimental reactors). AtSLAC, for example, klystrons

are routinely employed which have outputs in the range of 50 megawatts (pulse) and 50

kilowatts (time-averaged) at frequencies nearing 3 GHz Popular Science's"Best of What's

New 2007"[3][4]included a company GlobalResource Corporation using a klystron to convertthe hydrocarbonsin everyday materials, automotive waste, coal, oil shale, and oil sandsinto

natural gas and diesel fuel.[


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The Gunn oscillatorPrinciplesMicrowave power, i.e. electromagnetic powerin the GHz frequency range, may be generatedusing quite different physical phenomena.Some examples of this are vacuum-tube oscillatorssuch as the klystron and the magnetron,or semiconductor oscillators such as the FEToscillator,the Gunn oscillator and the impatt oscillator. Some simple experiments for understandingGunn oscillators are described below.Gunn effectIn some semiconductor materials, such as Gallium Arsenide (GaAs), the mobility of the electrons(= the quotient of the drift velocity vand the electrical field strength E) decreases above

a threshold value ETH of the electrical fieldstrength (see Fig. 1.1, left side). This is because,as the field strength increases more and more electrons transfer to a state in which theireffective mass becomes greater, thus decreasing their velocity. For field strengths where E>ETH the electrons have a negative differential mobility,i.e. an increase in the field strength resultsin a decrease in the drift velocity.When the electrical field strength in a homogeneously doped GaAs block (no barrier layer!)is greater than the threshold value ETH space charge instabilities occur as a result of thenegative differential mobility. While any random local surplus or deficiencyof electrons will disappear by itself when a positive differential mobility is present, this surplusor deficiency will increase under a negative differential mobility. In the upper rightsection of Fig. 1.1, a random surplus of electrons is assumed. The resulting increase in fieldstrength on the anode side leads to a decrease in the drift velocity v2 on the anode side relativeto the drift velocity v1 on the cathode side (decrease in field strength). This causes bunching

and yields a carrier enhancement layer.The analogous effect occurs in the event of a random deficiency of electrons, in which casea depletion layer occurs (see Fig. 1.1, middleright section). When the enhanced and depletedlayers approach each other, they attract each other and jointly pass through the diode inthe form of a domain (see Fig. 1.1, lower right section). The field in the interior of thedomain can be so high that the field outside it falls below the threshold value ETH. Thus nonew domain can be formed until the existing one has disappeared at the anode.If the Gunn element were not connected to a resonator (tuned circuit), the frequency of themicrowave power generated would be determined by the time it takes for the domains (velocityapproximately 107 cm/s) to pass through the diode (transit frequency). If, however, the Gunndiode is operated with a resonator, the resonator frequency can be imposed upon the Gunndiode. There are several operating modes here (delayed mode,quenched mode, LSA mode).The delayed domain mode is briefly described here as an example. It occurs when the resonatorfrequency is lower than the transit frequency. At the moment in which the domain reaches the

anode, the momentary value of the diode voltage (= bias voltage + RF voltage) is less than thethreshold value. The formation of a new domain is delayed until the voltage exceeds thethreshold value, thus imposing the oscillator frequency of the resonator on the Gunn element.


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Radar Basic Principles

The following figure shows the operating principle of a primary radar set. The radar antenna

illuminates the target with a microwave signal, which is then reflected and picked up by a

receiving device. The electrical signal picked up by the receiving antenna is called echo or

return. The radar signal is generated by a powerful transmitter and received by a highly

sensitive receiver.

Figure 1: Block diagram of a primary radar

All targets produce a diffuse reflection i.e. it is reflected in a wide number of directions. Thereflected signal is also called scattering.Backscatter is the term given to reflections in the

opposite direction to the incident rays.

Radar signals can be displayed on the traditional plan position indicator(PPI) or other more

advanced radar display systems. A PPI has a rotating vector with the radar at the origin,

which indicates the pointing direction of the antenna and hence the bearing of targets.


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A run time measurementisn't necessary for speed gauges, the actual range ofthe delinquent

car doesn't have a consequence. If you need range information, then the time measurement

can be realized by a frequency modulation or phase keying ofthe transmitted power.

A CW-radartransmitting a unmodulated power can measure the speed only by using the

Doppler- effect. It cannot measure a range and it cannot differ between two reflecting objects.

SpeedGauges

Figure: TRAFFIPAX SpeedoPhot

Speed gauges are very specialized CW-radars. A speed gauge uses the Doppler- frequency

for measurement ofthe speed. Since the value ofthe Doppler- frequency depends on thewavelength, these radar sets use a very high frequency band. The figure shows the speed

gauge Traffipax Speedophot produced by ROBOT Visual Systems GmbH. This radar usesa frequency of 24.125 gigahertz.

It can measure the speed ofthe incoming and the outgoing traffic, from the right orleft

border ofthe street. The radar can be mounted in a car or on a tripod. The traffic offence canbe circumstantiated by a photo camera with high resolution.


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Block DiagramofanCW-

adar

Simple CW Doppler- Radar sets have the following design:

Figure 3: schematic diagram of an CW Doppler- Radar

A mixer stage with a following narrowband filter generates the stable local-oscillator-

frequency as sum ofthe transmitter's frequency and a generated IF-frequency.

Frequency-Modulated Continuous-Wave Radar

Figure 1:Ranging with an FMCW system

tx-signal

Echo

Figure 1:Ranging with an FMCW system


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tx-s

c

CW radars have the disadvantage that they cannot measure distance, because there are nopulses to time. In order to correct for this problem, frequency shifting methods can be used.

In the frequency shifting method, a signal that constantly changes in frequency around a fixedreference is used to detect stationary objects. When a reflection is received the frequencies

can be examined, and by knowing when in the past that particular frequency was sent out,

you can do a range calculation similar to using a pulse. It is generally not easy to make abroadcaster that can send out random frequencies cleanly, so instead theseFrequency-

Modulated ContinuousWave radars (FMCW), use a smoothly varying ramp of

frequencies up and down.

R =

c0 t c0 = s ee of

light = 3108 m

/s

t = mes

e

time-

iffee

ce

[s]

R = distance

altimete to

te ain [m]

(1)

2

This kind of radar is used as radar altimeter often. The radar altimeter is used to measure

the exact height during the landing procedure of aircraft.Radar altimeters are also a

component of terrain avoidance warning systems, telling the pilot that the aircraft is flying

too low or that terrain is rising to meet the aircraft.

Functional block diagram

Trans onder

Receiver

Decoder

Transmitter

Coder

Interrogator


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Coder

Transmitter

Decoder

Receiver

PPI-screen

Synchronizer of

Primary Radar

Interrogation Path(Uplink)

Replay Path

(Downlink)

Figure 1: Block diagram of a secondary radar

Transponder

Receiver

Decoder

Transmitter


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Coder

Interrogator

Coder

Transmitter

Decoder

Receiver

PPI-screen

Synchronizer of

Primary Radar

Interrogation Path

(Uplink)

Replay Path

(Downlink)

Figure 1:Block diagram of a secondary radar

In the interrogatoron theground:

The secondary radar unit needs a synchronous impulse ofthe (analogous) primary radar unit

to the synchronization ofthe indication.

y The chosen mode is encoded in the Coder. (By the different modes different questions can

be defined to the airplane.)

y The transmittermodulates these impulses with the RF frequency. Because another

frequency than on the replay path is used on the interrogation path, an expensive duplexer

can be renounced.

y The antenna is usually mounted on the antenna of the primary radar unit and turns

synchronously to the deflection on the monitor therefore.

In theaircrafts transponder:

A receiving antenna and a transponderare in the airplane.

y The receiver amplifies and demodulates the interrogation impulses.

y The decoder decodes the question according to the desired information and induces the

coder to prepare the suitable answer.

y The coder encodes the answer.

y The transmitteramplifies the replay impulses and modulates these with the RF reply-

frequency.

Againin the interrogatoron theground:

y The receiver amplifies and demodulates the replay impulses. Jamming or interfering signals

are filtered out as well as possible at this.


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y From the informations Modeand Codethed

od

decodestheanswer.

y The monitor ofthe primary radar representstheadditional interrogator information.

Perhapsadditional numbers must beshown onanextra display.

Wa es and Frequency Ranges

The spectrum of the electric magnetic waves shows frequencies up to 102

Hz. This very

large complete range is subdivided because of different physical qualities in differentsubranges.

The division of the frequencies to the different ranges was competed on criteria formerly,

which arose historically and a new division of the wavebands which is used internationally isout-dated and arose so in the meantime. The traditional waveband name is partly still used in

the literature, however.

An overview shows the following figure:

Figure 1: Waves and frequency ranges used by radar.

Figure 1: Waves and frequency ranges used by radar.

Since without that the correct frequency is known, a transformation isn't always possible into

the new wavebands. Often in the manufacturers documents are published the traditional

wavebands. So I take on and commentn't these informations.

Figure 2: some radars and its frequency band


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Figure 2: some radars and its frequency band

Radar systems workin a wide band oftransmitted frequencies. The higherthe frequency of aradar system, the more itis affected by weather conditions such as rain or clouds. Butthe

higherthe transmitted frequency, the betteris the accuracy ofthe radar system.

The figure shows the frequency bands used by e.g. radarsystems.

A- andB- Band(HF- undVHF-

adar)

These radar bands below 300 MHz have a long historically tradition because these

frequencies represented the frontier of radio technology atthe time during the World War II.

Today these frequencies are used for early warning radars and so called Over The Horizon(OTH) Radars. Using these lower frequencies itis easierto obtain high-powertransmitters.

The attenuation ofthe electro-magnetic waves is lowerthan using higher frequencies. On the

other hand the accuracy is limited, because a lower frequency requires antennas with verylarge physical size which determines angle accuracy and angle resolution. These frequency-

bands are used by other communications and broadcasting services too, therefore thebandwidth ofthe radaris limited (atthe expense of accuracy and resolution again).

These frequency bands are currently experiencing a comeback, while the actually used

Stealth technologies don't have the desired effect at extremely low frequencies.


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C- Band(UHF-

adar)

There are some specialized Radar sets developed forthis frequency band

(300 MHz to1 GHz). Itis a good frequency forthe operation of radars forthe detection andtracking of satellites and ballistic missiles over a long range. These radars operate for early

warning and target acquisition like the surveillance radar forthe Medium Extended AirDefense System (MEADS). Some weather radar-applications e.g. wind profilers work with

these frequencies because the electromagnetic waves are very low affected by clouds andrain.

The new technology ofUltrawideband (UWB) Radars uses all frequencies from A- to C-

Band. UWB- radars transmit very low pulses in all frequencies simultaneously. They are used

fortechnically material examination and as Ground Penetrating Radar (GPR) for

archaeological explorations.

D- Band(L-Band

adar)

This frequency band (1 to 2 GHz) is preferred forthe operation oflong-range air-surveillance

radars outto 250 NM (400 km). They transmit pulses with high power, broad bandwidth

and an intrapulse modulation often. Due to the curvature ofthe earth the achievable

maximum range is limited fortargets flying with low altitude. These objects disappear very

fast behind the radar horizon.

In Air Traffic Management (ATM) long-range surveillance radars like the AirRoute

Surveillance Radar (ARSR) works in this frequency band. Coupled with a MonopulseSecondary Surveillance Radar (MSSR) they use a relatively large, but slower rotating

antenna. The designatorL-Band is good as mnemonic rhyme as large antenna orlong range.

E/F-Band(S-Band

adar)

The atmospheric attenuation is higherthan in D-Band. Radar sets need a considerably higher

transmitting powerthan in lower frequency ranges to achieve a good maximum range. As

example given the Medium PowerRadar (MPR) with a pulse power of up to 20 MW. In this

frequency range the influence of weather conditions is higherthan in D-band. Therefore acouple of weather radars workin E/F-Band, but more in subtropic and tropic climaticconditions, because here the radar can see beyond a severe storm.

Special Airport Surveillance Radars (ASR) are used at airports to detect and display the

position of aircraftin the terminal area with a medium range up to 5060 NM (100 km).

An ASRdetects aircraft position and weather conditions in the vicinity of civilian and

military airfields. The designatorS-Band (contrary to L-Band) is good as mnemonic rhyme as

smaller antenna orshorter range.

G- Band(C-Band

adar)

In G- Band there are many mobile military battlefield surveillance, missile-control and

ground surveillance radar sets with short or medium range. The size ofthe antennas provides

an excellent accuracy and resolution, butthe relatively small-sized antennas don't bother afast relocation. The influence of bad weather conditions is very high. Therefore air-

surveillance radars use an antenna feed with circular polarization often. This frequency bandis predetermined for mosttypes of weather radar used to locate precipitation in temperate

zone like Europe.

I/J- Band(X-andKu- Band

adars)

In this frequency-band (8 to 12 GHz) the relationship between used wave length and size of

the antenna is considerably betterthan in lower frequency-bands. The I/J- Band is a relatively


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popular radar band for military applications like airborne radars for performing the roles of

interceptor, fighter, and attack of enemy fighters and of ground targets. A very small antenna

size provides a good performance. Missile guidance systems at I/J- band are of a convenient

size and are, therefore, ofinterest for applications where mobility and light weight are

important and very long range is not a major requirement.

This frequency band is wide used for maritime civil and military navigation radars. Very

small and cheap antennas with a high rotation speed are adequate for a fair maximum range

and a good accuracy. Slotted waveguide and small patch antennas are used as radar antenna,under a protective radome mostly.

This frequency band is also popular for spaceborne or airborne imaging radars based on

Synthetic Aperture Radar (SAR) both for military electronic intelligence and civil geographicmapping. A special Inverse Synthetic Aperture Radar (ISAR) is in use as a maritime airborne

instrument of pollution control.

K- Band(K-andKa- Band

adars)

The higherthe frequency, the higheris the atmospheric absorption and attenuation ofthe

waves. Otherwise the achievable accuracy and the range resolution rise too. Radar

applications in this frequency band provide short range, very high resolution and high data

renewing rate. In ATMthese radar sets are called Surface MovementRadar (SMR) or (as p.o.) Airport Surface Detection Equipment (ASDE). Using of very shorttransmitting pulses of

a few nanoseconds affords a range resolution, that outline ofthe aircraft can be seen on the

radars display.

V-Band

By the molecular dispersion (here this is the influence ofthe air humidity), this frequency

band stay for a high attenuation. Radar applications are limited for a short range of a couple

of meters here.

W-Band

Here are two phenomena visible: a maximum of attenuation at about 75 GHz and a relative

minimum at about 96 GHz. Both frequency ranges are in use practically. In automotive

engineering small builtin radar sets operate at 7576 GHz for parking assistants, blind spot

and brake assists. The high attenuation (here the influence ofthe oxygen molecules O2)

enhances the immunity to interference ofthese radar sets.

There are radar sets operating at 96 to 98 GHz as laboratory equipments yet. These

applications give a preview for a use of radarin extremely higher frequencies as 100 GHz.


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Traveling wave tube, TWT

- overview, tutorial information and theory about the basics of the travelling wave tube,

TWT, and the traveling wave tube amplifier, TWTA, used for very high frequency, high

power RF amplifiers.

This overview ofthermionic valve or vacuum tube technology is splitinto several pages,

each addressing a different aspect ofthermionic valve or vacuum tube technology:

[1] Vacuum tube / thermionic valve basics [2] Valve / tube electrodes [3] Valve / tube numberng

systems [4] Valve / tube equivalents [5] Common valve / tube pin connections [6] TWT - travelling

wave tube [7] Photomultiplier

The traveling wave tube is a form ofthermionic valve ortube thatis still used for high power

microwave amplifier designs. The travelling wave tube can be used for wideband RF

amplifier designs where even now it performs well against devices using newertechnologies.

TWTs are used in applications including broadcasting, radar and in satellite transponders.

The TWT is still widely used despite the factthat semiconductortechnology is advancing all

the time.

The traveling wave tube, TWT, has a high bandwidth compared to many other designs and it

can operate over bands of up to an octave, although itis possible to utilise narrow band

designs when the applications require this. The TWT is essentially a microwave amplifier and

TWT amplifiers may be used on frequencies ranging from around 300 MHz up to 40 or 50

GHz and providing gain levels of up to 40 dBin a single device.

TWT amplifiers, TWTAs may consist oftwo types. Broadband types generally use helix

TWT amplifiers and they are typically capable of developing powers of up to 2.5 kW. Fornarrowband RF amplifier applications itis possible to use coupled cavity TWTs and these

can deliver powerlevels of up to 15 kW.

TWT history and development

The travelling wave tube (traveling wave tube) can trace its origins backto the earliest days

ofthermionic valve or vacuum tube development. Withoutthe development ofthese devices

the travelling wave tube would not be possible. The tube itself was born out ofthe need for

efficient high power microwave RF amplifiers in the Second World War.

The TWT was actually invented and first developed by an engineer named by RudolfKompfner at a government based British radar research laboratory. He developed the basic

TWT theory and the practical device. Both the TWT theory and the tube itself were later

refined by Kompfner and John Pierce atBell Labs in the USA.

TWT Construction

The travelling wave tube, TWT, can be splitinto a number of separate major elements:

y Vacuum tube

y Electron gun

y Magnet and focussing structure

y RF input

y Helix


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y RF output

y Collector

Traveling wave tube operation

The travelling wave tube, TWT is contained within a glass vacuum tube. This obviously

maintains the vacuum thatis required forthe operation ofthe TWT.

Within the travelling wave tube the first elementis the electron gun comprising primarily of a

heated cathode and grids. This produces and then accelerates a beam of electrons thattravels

along the length ofthe tube.

In orderthatthe electrons are made to travel as a tight or narrow beam along the length ofthe

travelling wave tube, a magnet and focussing structure is included. The field from the magnet

keeps the beam as narrow as required and in this way ensures thatthe beam travels along the

length ofthe TWT.

The RF input consists of a direction coupler which may either be in the form of a waveguide

or an electromagnetic coil. This is positioned nearthe electron gun emitter and itinduces

currentinto the helix (see below).

A helix is an essential part ofthe traveling wave tube. It acts as a delay line, in which the RFsignaltravels at nearthe same speed along the tube as the electron beam. The electromagnetic

field due to the currentin the helix interacts with the electron beam, causing bunching ofthe

electrons in an effect known as velocity modulation and the electromagnetic field resulting

from the beam currentthen induces more current backinto the helix. In this way the currentbuilds up and the signalis therefore amplified.

The RF output from the traveling wave tube consists of a second directional coupler. Again

this may either be an electromagnetic coil of a waveguide. This is positioned nearthe

collector and it receives the amplified version ofthe signal from the far end ofthe helix from

the electron gun or emitter.

An attenuatoris included on the helix, usually between the input and output sections oftheTWT helix. This is essentialto preventthe reflected wave from travelling backto the cathode

ofthe electron gun.

The collector finally collects and absorbs the electron beam. Itis in this area that high levels

of power may be dissipated and therefore this section ofthe travelling wave tube can become

very hot and will require cooling.

Travelling wave tube applications

There are many areas in which TWT amplifiers are used. They are an ideal form ofRFamplifier for satellites and as a resultthey are extensively used for satellite transponders

where low levels signals are received and need to be retransmitted at much higherlevels. Inaddition to this TWT amplifiers are used in microwave radar systems where they are able to

produce the high levels of power required. Traveling wave tube, TWT technology is alsoused for electronic warfare applications. In these applications the grid on the travelling wave

tube may be used to pulse the transmission.

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