Traveling Wave Tube
Table of Content « TWT »
- Physical construction
- Electron gun
- Surrounding magnet
- Slow wave structure
- Functional describing
- Characteristics of a TWT
- TWT's Power-amplification
- TWT's Bandwith
- TWT's Noise Figure
- Different slow wave structures
- History of
What is a traveling wave tube?
Figure 1: Physical construction of a TWT: ① Electron gun; ② Surrounding magnet; ③ Slow wave structure (here: Helix); ④ Collector;
Figure 1: Physical construction of a TWT
Figure 1: Physical construction of a TWT: ① Electron gun; ② Surrounding magnet; ③ Slow wave structure (here: Helix); ④ Collector;(interactive picture)
Traveling Wave Tube
Traveling Wave Tubes (abbr.: TWT, pronounced: “twit”) are vacuum tubes used as high-gain, low-noise, wide-bandwidth microwave wideband amplifiers. A TWT is capable of gains from 40to70dB with bandwidths exceeding two octaves. (A bandwidth of 1octave is one in which the upper frequency is twice the lower frequency.) TWTs have been designed for frequencies as low as 300Megahertz and as high as 100Gigahertz. Power level range from a few watts to 10 MW. The TWT is primarily a voltage amplifier. Together with the klystrons they form a special group of linear-beam tubes in context of velocity-modulated tubes. There are two different main types of TWT:
- low-power Helix TWT
occurs as a highly sensitive, low-noise and wideband amplifier in radar receivers and measurement equipment;
- high-power Coupled-Cavity TWT
are used as a power-amplifier for high-power transmitters, e.g. as pre-amplifier for crossed field amplifiers (CFA). They have significant higher output-power but less bandwidth (10 … 20 percent).
Both types have the same operating principles and they both incorporate the basic components shown in Figure1. They mostly differ in the construction of the slow-wave structure. The wide-bandwidth and low-noise characteristics make the TWT ideal for use as an RF amplifier in microwave equipment. On reason of the special low-noise characteristic they are widely used as an active RFamplifier element in microwave receivers and transmitters in radar systems and in space communications.
The physical construction of a typical TWT is shown in Figure 1. It consists of four basic elements:
- Electron gun which produces and then accelerates an electron beam along the axis of the tube;
- Magnetic electron beam focusing system which provides a magnetic field along the axis of the tube to focus the electrons into a tight beam;
- Slow wave structure as RF- interaction circuit, e.g. a coiled wire (Helix) at the center of the tube, that provides a low-impedance transmission line for the RF energy within the tube;
- Collector. The electron beam is received at the collector after it has passed through the slow wave structure.
All components of the TWT are held under a very high vacuum. The RF input and output may couple onto and removed from the helix by waveguide directional couplers that have no physical connection to the helix.
Figure 2: Variants of magnets: a) solenoid; b) permanent magnet; c) periodic permanent magnets
Figure 2: Variants of magnets: a)solenoid; b)permanent magnet; c)periodic permanent magnets
The electron gun is similar in construction as in all cathode ray tubes. It consists of a indirect heated cathode, that must be heated to a temperature between 850° and 1 100°Celsius (≙1 500° to 2 000°Fahrenheit) to produce appreciable electron emission. A focusing grid with the same potential as the cathode (or a small negative bias up to −20 Volts relative to the cathode) directs the electrons in the desired direction. One or more anodes are used to generate the requisite electron velocity. The beam passes the anodes through a hole or a grid and travels through the slow wave structure.
The electron gun is covered by a shielding box to prevent hazardous radiation.
The surrounding magnet provides a magnetic field along the axis of the tube to focus the electrons into a tight beam. This magnet may be either a permanent magnet or a solenoid (electromagnetic) focusing element (see Figure 2a). A permanent magnet doesn't need a power supply and ensures that the magnetic field is always present. The disadvantage is that a permanent magnet doesn't provide an adjustment of the magnetic field to optimize the tubes performance.
If a single permanent magnet (see Figure 2b) is replaced by a number of smaller magnets then the size and total weight of the magnet structure is reduced (see Figure 2c).
The housing is usually made of aluminum to prevent the disturbing influence of ferromagnetic materials. Extrinsic magnetic materials can interfere with the uniform magnetic field and destroy the traveling wave tube. Therefore, the packaging of a traveling wave tube has oversized dimensions often.
RF induced into Helix
Figure 3: Amplified helix signal
RF induced into Helix
Figure 3: Amplified helix signal
Slow wave structure
Since the electron beam into the tube must obviously travel slower than the speed of light, there must be some means of slowing down the forward velocity of the electromagnetic wave. The electron beam speed of a TWT is about 10to 50percent of the speed of light. The speed depends on the cathode voltage that may be between 4 to 120 Kilovolts. The slowdown is done by means of a slow wave structure, on which the electromagnetic wave propagates.
The collector is a voltage electrode of the TWT. It's the same potential as the body of the tube, and this is usually on ground. In the absence of an input signal, the entire beam energy must be dissipated in the collector. Forced air-cooling or liquid cooling of the collector is necessary at high-power TWTs. High-power TWTs often use multi-stage collectors as shown in Figure1.
Figure 4: Electron-beam bunching
Figure 4: Electron-beam bunching
The input voltage creates an additional axial electric field that moves as fast as the electron beam on the wire of the helix. This electric field accelerates (in the positive half-wave) or decelerates (in the negative half-wave) the electrons in the electron beam. This process is called velocity modulation. If the electrons of the beam were accelerated to travel faster than the waves traveling on the wire, electron bunching would occur through the effect of velocity modulation. (see Figure4)
By delivering energy to the electron beam, the power of the traveling wave decreases. The additional attenuator causes a decreasing to zero. This one attenuator also prevents any reflected waves from traveling back down the helix.
Figure 5: Repelling of the electrons in the wire of the helix
Figure 5: Repelling of the electrons in the wire of the helix
However, the velocity modulation is still effective in the electron beam. The faster electrons catch up with the slower electrons and bunching occurs. The electron- beam bunching already starts at the beginning of the helix and reaches its highest expression on the end of the helix. The electron bunches in the beam give up energy to the wire of the slow wave structure. They repel the electrons in the wire and generate a new one traveling wave in the helix. The energy from bunches would increase the amplitude of the traveling wave in a progressive action that would take place all along the length of the TWT.
The injection of the wave in the slow wave structure (as shown in Figure5) causes a phase shift of −90 degrees relative to the initial waveform. When the electrons deliver their energy to the wave in the helix, they slow down. In some TWTs the helix is made narrower at the end of the tube therefore. This slows down the speed of the electromagnetic wave in the slow wave structure as well.
Characteristics of a TWT
Figure 6: characteristic of a traveling wave tube
Figure 6: characteristic of a traveling wave tube
The attainable power-amplification is essentially dependent on the following factors:
- constructive details (e.g. structure and length of the helix)
- electron beam diameter (adjustable by the density of the focusing magnetic field)
- power input (see Figure 6)
- voltage UA2 on the helix
As shown in Figure 6, the gain of a given TWT has got linear characteristic of about 26dB at small input power. If you increase the input power, the output power doesn't increase for the same gain. So you can prevent a saturation of e.g the following mixer stage in radar receiver. The relatively low efficiency of the TWT partially offsets the advantages of high gain and wide bandwidth.
The gain of a TWT is affected by the interaction of the electrons with the electric field caused by the wave in the slow wave structure. The effectiveness depends on the frequency response of the slow wave structure. A helix may have a bandwidth of more than two octaves. If the slow wave structure contains resonant parts, then the bandwidth depends on its frequency response. The bandwidth of commonly used Coupled-Cavity TWTs is about 10 … 20 percent of the center frequency.
The most important parameter for the use of the traveling wave tube as a pre-amplifier in radar receivers is the noise figure of the traveling wave tube. This determines the sensitivity of the receiver and thus the maximum range of the radar. The noise figure of recently used TWTs is 3 … 10dB. There are three unavoidable sources of noise in a traveling wave tube:
- Shot noise results from the random emission of electrons of the cathode
- Velocity noise arises from different velocities of the emitted electrons.
- Johnson–Nyquist noise is the electronic noise generated by the thermal agitation of the electrons.
The noise figure depends on the size of most supply voltages of the traveling wave tube. For example, if the voltages at the electrodes are 5% less than the optimum values, the noise figure approximately doubles.
Figure 7: Contra-wound Helix slow wave structure
Different Slow Wave Structures
The previously described helix may be replaced by some other slow wave structure such as a ring-bar, ring loop, or coupled cavity structure. The structure is chosen to give the characteristic appropriate to the desired gain/bandwidth and power characteristics.
A contra-wound helix uses two helices wound in opposite directions. Both helices must be identical in dimensions. A contra-wound helix is less sensitive to backward waves interactions and therefore allows higher operating voltages, currents and power. The penalty for these advantages is that the bandwidth is less than that of a single helix.
Figure 8: Ring-Loop slow wave structure
A Ring Loop TWT uses loops as slow wave structure to tie the rings together. These devices are capable of higher power levels than conventional helix TWTs, but have significantly less bandwidth of 5…15percent and lower cut-off frequency of 18GHz.
The feature of the ring-loop slow wave structure is high coupling impedance and low harmonic wave components. Therefore ring-loop traveling wave tube has advantages of high gain (40…60Decibels), small dimension, higher operating voltage and less danger of the backward wave oscillation.
Figure 9: Ring-Bar slow wave structure
The Ring-Bar TWT was developed from the contra-wound helix and has got the same characteristics likely the Ring-Loop TWT. This one slow wave structure is very easy to make by precise laser cuts in a thin copper pipe.
Figure 10: Coupled-cavity slow wave structure
The Coupled-cavity TWT uses a slow wave structure of a series of cavities coupled to one another. The resonant cavities are coupled together with a transmission line. The electron beam (shown in Figure 9 as red beam) is velocity modulated by an RF input signal at the first resonant cavity. This RF energy (displayed as blue arrow) travels along the cavities and induces RF voltages in each subsequent cavity.
If the spacing of the cavities is correctly adjusted, the voltages at each cavity induced by the modulated beam are in phase and travel along the transmission line to the output, with an additive effect, so that the output power is much greater than the power input.
A Travelling Wave Tube amplifies a modulated electromagnetic wave in order to transmit data. Inside the vacuum envelope, the electromagnetic wave interacts with an electron beam. Because both travel at almost the same speed, electrons transmit their kinetic energy to the wave, an effect known as the Cherenkov effect.What are the two basic types of traveling wave tubes? ›
There are two types of vacuum TWTs, the first is a helix based TWT and the other is a Coupled Cavity based TWT. The Coupled Cavity TWT is more common for military aircraft use. A TWT is a specialized cylindrical vacuum tube that amplifies microwave signals of a relatively wide frequency range.What is Travelling wave tube in microwave? ›
A traveling-wave tube (TWT, pronounced "twit") or traveling-wave tube amplifier (TWTA, pronounced "tweeta") is a specialized vacuum tube that is used in electronics to amplify radio frequency (RF) signals in the microwave range.What are the characteristics of traveling-wave tube? ›
Traveling-wave tubes are designed to emphasize particular inherent characteristics (e.g., gain, bandwidth, peak/average power, linearity, efficiency) for specific applications. Principle application areas include radar, electronic countermeasures, telecommunications, and satellite communications.What is the primary purpose of the helix in a Travelling wave tube? ›
Explanation: A helix in TWT plays an important role as a delay line, slowing down the RF signals travelling at a speed of light near the same speed along the tube as the electron beam.What is the traveling wave formula? ›
v = λf = ω/k = speed. If kx and ωt have the same sign, the wave travels in the negative x-direction. If they have opposite signs, the wave travels in the positive x-direction.What is another name for Travelling wave tube? ›
A Traveling-Wave Tube or TWT Amplifier is a high power, high-frequency amplifier that is built using traveling wave tubes. A traveling-wave tube is a type of vacuum tube used to amplify high-frequency signals. The RF signal is amplified by absorbing power from a beam of electrons as it goes through the tube.What is the efficiency of a traveling-wave tube? ›
For space-based applications, where efficiency is critical, a traveling-wave tube may use up to a five-stage depressed collector to achieve narrow-band electronic efficiencies approaching 80%.What is the efficiency of a TWT? ›
Uniform structure TWTs are usually operated such that the beam velocity is equal to or greater than the cold phase velocity in the supporting slow-wave structure. Typical efficiencies for synchronous structures are about 25%, which can be in- creased to 30-35% by use of a structure with a lower phase velocity.What is the difference between klystron and Travelling wave tube? ›
The main difference between the operation of a TWT and a klystron is that while the TWTs' circuit operates in a non-resonant mode, the klystrons' circuit operates in a resonant mode. Therefore, the electronic conversion efficiency is relatively low for TWTs and fairly high for klystrons.
Magnetron tubes are representative of an entirely different kind of tube than the 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.What are the five types of microwave tubes? ›
- Rectangular waveguide.
- Circular waveguide.
- Elliptical waveguide.
- Single-ridged waveguide.
- Double-ridged waveguide.
A wave is a disturbance that propagates, or moves from the place it was created. There are three basic types of waves: mechanical waves, electromagnetic waves, and matter waves.What are the advantages of traveling wave antenna? ›
An advantage of traveling wave antennas is that since they are nonresonant they often have a wider bandwidth than resonant antennas. Common types of traveling wave antenna are the Beverage antenna, axial-mode helical antenna, and rhombic antenna.What is the difference between travelling wave and stationary wave? ›
Travelling waves transport energy from one area of space to another, whereas standing waves do not transport energy.Which type of amplifier is Travelling wave tube? ›
A traveling wave tube (TWT) or TWT amplifier (TWTA) is a high-power, high-frequency amplifier that is built using traveling wave tubes. A traveling wave tube is a type of vacuum tube used to amplify high-frequency signals.What is the purpose of the attenuator in a Travelling wave tube? ›
The role of the RF attenuator placed in the center portion of the slow-wave circuit is to prevent feedback oscillation in the TWT. After passing over the slow-wave circuit, the electron beam reaches the collector, and the electron energy is converted to heat and dissipated.Why does the Travelling wave tube need slow wave structure? ›
Since the electron beam can be accelerated only to velocities that are about a fraction of the velocity of light, a slow wave structure must be incorporated in the microwave devices so that the phase velocity of the microwave signal can keep pace with that of the electron beam for effective interactions.What is the frequency of a traveling wave? ›
The wave's frequency is the number of waves that pass through a point per unit time and is equal to f=1/T. f = 1 / T . The period can be expressed using any convenient unit of time but is usually measured in seconds; frequency is usually measured in hertz (Hz), where 1Hz=1s−1.What is an example of a traveling wave? ›
The best example for longitudinal waves is sound waves moving through the air when you hear a loudspeaker playing in the distance. There is a second way to characterize the waves by types of matter they are able to move or travel through. Electromagnetic Waves - this type of wave can travel easily through a vacuum.
λ is called the wavelength and it can be measured, for example, as the distance between two adjacent crests. Note that this is a function of (x − vt) and so, in the (x,y) frame, it is a wave travelling to the right with speed v. yx=0 = − A sin (2πvt/λ) .Are traveling wave tubes still used? ›
In 2020, the traveling wave tube amplifier was added to the Space Foundation's Space Technology Hall of Fame. And as of 2022, the TWT on Voyager 2 is still plugging away, transmitting data as it continues to make the journey through interstellar space.What is a material through which a wave can travel through called? ›
A medium is a substance through which a wave can travel. A medium can be a solid, a liquid, or a gas.What are the disadvantages of Travelling wave tube? ›
Following are the disadvantages of TWT: ➨It operates at lower efficiencies. ➨In coupled cavity TWT, coupling effect takes place between the cavities. ➨Helix TWT has limitation on high peak power due to helix wire thickness.Does sound travel faster in a tube? ›
Since sound waves involve the transfer of kinetic energy between adjacent molecules, the closer those molecules are to each other, the faster the sound travels. Therefore, sound travels much faster through solids than through liquids or gas.What is the major advantage of Travelling wave tube over klystron? ›
TWT has wider bandwidth of operation. Klystron has smaller bandwidth of operation.What is the advantage and disadvantage of TWT? ›
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What is a TWT Amplifier? TWT amplifiers, also known as TWT power amplifiers, use vacuum tube technology to amplify RF and microwave signals to either strengthen a weak signal or simulate a signal's effect on a device. Traveling wave tubes make for high-gain, low-noise and wide-bandwidth RF amplifiers.Why is the TWT is sometimes preferred to the magnetron as a radar transmitter output tube? ›
TWT has the advantage of producing a high duty cycle.What are the three types of microwave tubes? ›
There are three types of Microwave Tubes - TWT (Travelling Wave Tubes), Klystron Tubes and Magnetrons.
|It can be used both as amplifier and oscillator.||It can only be used as oscillator.|
In simpler terms, rather than turn the magnetron on and off to control power/temperature, an inverter allows the magnetron to continuously run while only altering the amount of radiation that is coming out for your selected setting.What is the best microwave antenna? ›
MIMO microwave antennas are some of the best in smart antenna technology because they can transmit parallel data streams and they are also useful in applications that require highly effective antenna directivity.
The cover is actually a wave guide cover (mica plate) which is an important accessory in microwaving. Please leave the cover inside the microwave and do not use the product without it.Why conventional tubes are not used in microwave? ›
Conventional low frequency tubes like triodes fail to operate at microwave frequencies (MF) because the electron transit time from cathode to grid becomes do large that it cannot produce microwave oscillations.What is a travelling wave also known as? ›
In mathematics, a periodic travelling wave (or wavetrain) is a periodic function of one-dimensional space that moves with constant speed. Consequently, it is a special type of spatiotemporal oscillation that is a periodic function of both space and time.What is the difference between resonant and Travelling wave antenna? ›
In traveling wave antennas, the current and voltage can be represented using traveling waves in the same direction. Traveling wave antennas are non-resonant antennas. Standing wave antennas are bi-directional traveling wave antennas and are otherwise known as resonant antennas.Is a full wave antenna better than a half wave? ›
No. A half wave dipole has a low input impedance. A center-fed full wave antenna will have a very high input impedance and will require a matching network between it and the feedline or transmitter. A half-wave antenna is a complete, fundamental radiator, and is the basic resonant length.What is the difference between antenna and waveguide? ›
Transmission lines and waveguides are devices used to transmit signals in the form of guided electromagnetic waves from a source (generator) to a load. Antennas may be used to transmit signals from a source to a load in the form of directed but unguided waves.How do you know if a wave is a traveling wave? ›
Traveling waves are observed when a wave is not confined to a given space along the medium.
There are two basic types of waves: transverse and longitudinal. In a transverse wave the displacement of the medium is at right angles to the direction of wave propagation (Figure 19-1). In a longitudinal wave, the displacement of the medium is in the direction of the wave propagation (Figure 19-2).What is the concept of wave theory? ›
wave theory. noun. : a theory in physics: light is transmitted from luminous bodies to the eye and other objects by an undulatory movement. called also undulatory theory.What is the traveling wave theory quizlet? ›
Traveling wave theory: For each inward and outward movement of the footplate of the stapes, there is a downward and upward movement of the basilar membrane, produced by disturbance of the endolymph.What is wave theory in simple terms? ›
Also called undulatory theory. Physics. the theory that light is transmitted as a wave, similar to oscillations in magnetic and electric fields.What is an example of wave theory? ›
A simple way to answer is to say that light is a type of wave that causes objects to be visible to human eyes. The sun produces light, and that light bounces off objects and into our eyes. This makes it so that we can see things, because the brain can interpret that light and tell us what's out there.What are the three concepts of wave theory? ›
It has three unbreakable rules that define its formation: Wave two cannot retrace more than 100% of the first wave. The third wave can never be the shortest of waves one, three, and five. Wave four can't go beyond the third wave at any time.Is Elliott Wave theory legit? ›
The Elliott Wave Principle, as popularly practiced, is not a legitimate theory, but a story, and a compelling one that is eloquently told by Robert Prechter. The account is especially persuasive because EWP has the seemingly remarkable ability to fit any segment of market history down to its most minute fluctuations.Which statement is true about a traveling wave? ›
The main characteristic of the traveling wave is that it transfers energy from one point to another and it doesn't transfer mass along with the waves. Thus, the statement (a) is true about the traveling wave.What are traveling waves examples? ›
Examples include gamma rays, X-rays, ultraviolet waves, visible light, infrared waves, microwaves, and radio waves. Electromagnetic waves can travel through a vacuum at the speed of light, v=c=2.99792458×108m/s. v = c = 2.99792458 × 10 8 m/s .What is travelling wave concept with step response? ›
What is step response of traveling waves? The unique characteristic of the short circuit is that is impossible to develop any voltage across it. Thus when a travelling wave of voltage reaches a short circuit the reflected voltage wave must precisely cancel out the incident wave so that the refracted wave is zero.
These particles are known as photons. Hence, the wave theory of light cannot explain the photoelectric and Compton effect.What was one problem of the wave theory? ›
The wave theory was challenged by those who supported the particle theory. They attacked the wave theory for several reasons, leading to heated debate over the first half of the 19th century. One problem was that analysing the waves mathematically was extremely complex.What is quantum wave theory? ›
Quantum theory describes that matter, and light consists of minute particles that have properties of waves that are associated with them. Light consists of particles known as photons and matter are made up of particles known as protons, electrons, and neutrons.