A traveling-wave pipe (TWT) is an electronic device used to amplify radio occurrence signals to high ability, usually within an electronic assemblage known as a traveling-wave tube amplifier (TWTA).
The bandwidth of your broadband TWT can be as high as three octaves, although tuned (narrowband) types exist, and operating frequencies range between 300 MHz to 50 GHz. The voltage gain of the pipe can be of the order of 70 decibels.
Traveling-Wave Tubes Traveling-wave tubes (TWTs) are high-gain, low- sound, huge and width microwave amplifiers, capable of increases of 40 dB or more, with bandwidths of over an octave. (A bandwidth of 1 1 octave is one where the upper frequency is twice the low occurrence. ) TWTs have been suitable for frequencies as low as 300 MHz so when high as 50 GHz. The principal use for
TWTs is voltage amplification (although high-power TWTs, with characteristics a lot like those of a power klystron, have been developed). Their huge bandwidth and low-noise characteristics make them ideal for use as RF amplifiers.
CONSTRUCTION:
The device can be an elongated vacuum pipe with an electron firearm (a heated up cathode that emits electrons) at one end. A magnetic containment field across the tube focuses the electrons into a beam, which then passes down the center of a cable helix that exercises from the RF source to the RF outcome, the electron beam finally striking a collector at the other end. A directional coupler, which can be either a waveguide or an electromagnetic coil, fed with the low-powered radio sign that is to be amplified, is positioned nearby the emitter, and induces an up-to-date in to the helix.
The helix serves as a hold off line, in which the RF signal journeys at near to the same speed across the tube as the electron beam. The electromagnetic field because of the RF transmission in the helix interacts with the electron beam, creating bunching of the electrons (an effect called speed modulation), and the electromagnetic field because of the beam current then induces more current back to the helix (i. e. the current builds up and therefore is amplified as it passes down).
A second directional coupler, located near the collector, obtains an amplified version of the suggestions indication from the even end of the helix. An attenuator placed on the helix, usually between the input and result helicies, prevents reflected wave from exploring back again to the cathode.
Higher power TWT's usually contain beryllium oxide ceramic as both a helix support rod and in some cases, as an electron collector for the TWT due to its special electrical, mechanical, and thermal properties.
OPERATION AND WORKING
While the electron beam in a klystron journeys primarily in locations free from RF electric fields, the beam in a TWT is regularly inter- performing with an RF electric field propagating along an external circuit surrounding the beam. To obtain amplification, the TWT must propagate a influx whose phase speed is nearly synchronous with the dc velocity of the electron beam. It really is difficult to accelerate the beam to greater than roughly one- fifth the velocity of light. Therefore, the front speed of the RF field propagating across the helix must be reduced to almost that of the beam. The phase speed in a waveguide, which is standard in the direction of propagation, is always higher than the velocity of light. However, this velocity can be reduced below the velocity of light by producing a periodic deviation of the circuit in the direction of propagation. The easiest form of deviation is obtained by wrapping the circuit in the form of a helix, whose pitch is equal to the required slowing factor.
TWT MIXER. - A TWT is also used as a micro- wave mixing machine. By virtue of its extensive bandwidth, the TWT can accommodate the frequencies made by the heterodyning process (so long as the frequencies have been chosen to be within the number of the pipe). The desired frequency is decided on through a filter on the outcome of the helix. A TWT mixing machine has the added benefit of providing gain as well as simply acting as a mixing machine.
TWT MODULATION. - A TWT can be modulated through the use of the modulating indication to a modulator grid. The modulator grid may be used to turn the electron beam on / off, such as pulsed microwave applications, or to control the denseness of the beam and its own ability to transfer energy to the touring wave. Thus, the grid can be used to amplitude modulate the result.
TWT OSCILLATOR. - A forward-wave TWT can be built to serve as a microwave oscillator. Physically, a TWT amplifier and an oscillator differ in two major ways. The helix of the oscillator is much longer than that of the amplifier, and there is no input connection to the oscillator. TWT oscillators tend to be called backward-wave oscillators (BWOs) or carcintrons.
The Traveling-Wave Tube The TRAVELING-WAVE TUBE (twt) is a high-gain, low-noise, wide-bandwidth microwave amplifier. It really is capable of benefits higher than 40 dB with bandwidths exceeding an octave. (A bandwidth of just one 1 octave is one in which the upper regularity is twice the lower rate of recurrence. ) Traveling-wave tubes have been suitable for frequencies only 300 megahertz and since high as 50 gigahertz. The twt is generally a voltage amplifier. The wide-bandwidth and low-noise characteristics make the twt well suited for use as an RF amplifier in microwave equipment. The physical construction of a typical twt is shown in amount 2-13.
Fig-2
The twt has an electron gun which produces and then accelerates an electron beam across the axis of the pipe. The encompassing magnet provides a magnetic field along the axis of the tube to target the electrons into a tight beam. The HELIX, at the center of the pipe, is a coiled cable that delivers a low-impedance transmitting series for the RF energy within the tube. The RF insight and end result are coupled onto and removed from the helix by directional couplers which have no physical connection to the helix. If the RF energy is transported on coaxial cables, the coaxial couplers are wound in a helical manner similar compared to that shown in physique 2. If the RF energy is transported in waveguides, waveguide directional couplers are being used. The attenuator stops any reflected waves from journeying back off the helix. Physical building of an twt. A simplified version of twt procedure is shown in fig below. In the figure, an electron beam is transferring along a nonresonant transmitting line represented by the straight line. The insight to the transmission line can be an RF influx which travels on the line from suggestions to end result. The lines will transport an array of RF frequencies if it's terminated in the quality impedance of the series. The electromagnetic waves vacationing down the road produce electric domains that interact with the electrons of the beam.
Fig:-3
If the electrons of the beam were accelerated to visit faster than the waves touring on the line, bunching would happen through the effect of speed modulation. Velocity modulation would be induced by the relationship between the traveling-wave areas and the electron beam. Bunching would cause the electrons to give up energy to the journeying influx if the fields were of the correct polarity to slow down the bunches. The from the bunches would boost the amplitude of the journeying influx in a progressive action that could take place all along the space of the twt, as shown in physique. However, because the waves travel along the line at the speed of light, the simple twt shown in body 3 won't work. At the moment no way is known to accelerate an electron beam to the speed of light. Since the electron beam cannot travel faster than the influx on the wire, bunching will not happen and the pipe will not work. The twt is therefore designed with a delay framework to decrease the traveling influx right down to or below the speed of the electrons in the beam. A common twt delay composition is a line, wound in the form of a long coil or helix, as shown in amount, view (A). The condition of the helix slows the effective speed of the influx along the normal axis of the helix and the tube to about one-tenth the swiftness of light. The wave still trips down the helix line at the rate of light, however the coiled shape causes the wave to travel a much increased total distance than the electron beam. The swiftness at which the wave journeys down the pipe can be varied by changing the number of changes or the diameter of the changes in the helix line. The helical wait structure works well because it gets the added benefit of causing a big proportion of electric domains that are parallel to the electron beam. The parallel areas provide maximum relationship between the domains and the electron beam.
In an average twt, the electron beam is aimed down the center of the helix while, at the same time, an RF sign is coupled onto the helix. The electrons of the beam are velocity-modulated by the electric areas produced by the RF signal. Amplification begins as the electron bunches form and release energy to the signal on the helix. The just a little amplified signal causes a denser electron bunch which, subsequently, amplifies the transmission even more. The amplification process is constant as the RF wave and the electron beam travel down the distance of the pipe. Any portion of the twt productivity signal that reflects back again to the input will cause oscillations within the tube which results in a decrease in amplification. Attenuators are located along the length of the helix to avoid reflections from achieving the suggestions. The attenuator causes a loss in amplitude, as can be seen in body, view (B), but it could be placed so as to minimize loss while still isolating the input from the outcome. The relatively low efficiency of the twt partially offsets the advantages of high gain and large bandwidth. The inner attenuator reduces the gain of the tube, and the power required to energize the focusing magnet can be an operational loss that can't be retrieved. The twt also produces high temperature which must be dissipated by either air-conditioning or liquid-cooling systems. Many of these factors reduce the overall efficiency of the twt, however the benefits of high gain and wide bandwidth are usually enough to beat the cons.
THE MAGNETRON
The MAGNETRON, shown in amount 4-A, is a self-contained microwave oscillator that functions diversely from the linear-beam pipes, including the twt and the klystron. Physique 4-B is a simplified pulling of the magnetron. CROSSED-ELECTRON and MAGNETIC domains are being used in the magnetron to create the high-power outcome required in radar and communications equipment.
Figure 4. A. -Magnetron
Figure4 b. -Magnetron
The magnetron is classed as a diode since it has no grid. A magnetic field located in the space between the plate (anode) and the cathode serves as a grid. The bowl of a magnetron does not have the same physical appearance as the plate of a typical electron tube. Since classic inductive- capacitive (LC) networks become impractical at microwave frequencies, the dish is fabricated into a cylindrical copper block filled with resonant cavities which serve as tuned circuits. The magnetron basic differs considerably from the conventional tube basic. The magnetron platform is short long and has large diameter leads that are carefully sealed into the pipe and shielded. The cathode and filament are at the guts of the tube and are backed by the filament leads. The filament leads are large and rigid enough to keep the cathode and filament structure fixed in position. The output business lead is usually a probe or loop extending into one of the tuned cavities and combined into a waveguide or coaxial lines. The plate framework, shown in physique 5, is a solid stop of copper. The cylindrical openings around its circumference are resonant cavities. A narrow slot works from each cavity into the central part of the tube dividing the internal structure into as many sections as there are cavities. Alternate segments are strapped jointly to put the cavities in parallel in regards to to the outcome. The cavities control the outcome frequency. The straps are circular, metal rings that are placed across the top of the block at the access slot machine games to the cavities. Since the cathode must operate at high power, it must be rather large and must also have the ability to tolerate high operating temperature. It must also have good emission characteristics, specifically under return bombardment by the electrons. It is because almost all of the output electricity is provided by the large number of electrons that are emitted when high-velocity electrons return to hit the cathode. The cathode is indirectly heated up and is constructed of a high- emission material. The available space between your plate and the cathode is called the Connections SPACE. In such a space the electric and magnetic domains interact to exert make after the electrons.
Figure 5. -Cutaway view of a magnetron
The magnetic field is usually provided by a solid, permanent magnet mounted around the magnetron so the magnetic field is parallel with the axis of the cathode. The cathode is installed in the center of the connection space. BASIC MAGNETRON OPERATION. -Magnetron theory of procedure is based on the movement of electrons under the influence of blended electric and magnetic fields. The following information presents the laws governing this motion. The course of a power field is from the positive electrode to the negative electrode. Regulations governing the action of your electron within an electric field (E field) expresses: The push exerted by an electric field with an electron is proportional to the strength of the field. Electrons tend to move from a spot of negative probable toward a good potential.
This is shown in amount 6. In other words, electrons tend to move from the E field. When an electron has been accelerated by an E field, as shown in figure 6, energy is taken from the field by the electron.
Figure 6. -Electron movement within an electric field
The legislations of motion of your electron in a magnetic field (H field) areas: The power exerted with an electron in a magnetic field reaches right perspectives to both field and the road of the electron. The path of the make is in a way that the electron trajectories are clockwise when seen in direction of the magnetic field. That is shown in number 7.
Figure 7. -Electron action in a magnetic field
In physique 7, assume a southern pole is below the shape and a north pole is above the amount so that the magnetic field is certainly going into the newspaper. When an electron is moving through space, a magnetic field develops about the electron just as it could around a wire when electrons are flowing through a line. In body 7 the magnetic field across the moving electron adds to the permanent magnetic field on the still left part of the electron's journey and subtracts from the long lasting magnetic field on the right part. This step weakens the field on the right area; therefore, the electron course bends to the right (clockwise). If the strength of the magnetic field is increased, the road of the electron will have a sharper flex. Similarly, if the velocity of the electron rises, the field around it increases and the path will bend more sharply. A schematic diagram of a simple magnetron is shown in shape 8A. The pipe consists of a cylindrical dish with a cathode positioned along the center axis of the plate. The tuned circuit comprises of cavities in which oscillations take place and are literally found in the plate. When no magnetic field is out there, heat the cathode leads to a even and direct movement of the field from the cathode to the plate, as illustrated in physique 8B. However, as the magnetic field adjoining the tube is increased, an individual electron is influenced, as shown in shape 9. In figure 9, view (A), the magnetic field has been risen to a point where the electron proceeds to the plate in a curve rather than direct path.
Figure 8A. -Basic magnetron. SIDE VIEW
Figure 9. -Result of the magnetic field on a single electron
In view (B) of body 9, the magnetic field has reached a value great enough to cause the electron to just miss the plate and return to the filament in a round orbit. This value is the CRITICAL VALUE of field strength. In view (C), the value of the field durability has been risen to a spot beyond the critical value; the electron is made to happen to be the cathode in a round route of smaller diameter. View (D) of amount 9. shows how the magnetron plate current varies consuming the varying magnetic field. In view (A), the electron movement reaches the plate, so a huge amount of plate current is streaming. However, when the critical field value is come to, as shown in view (B), the electrons are deflected from the dish and the plate current then drops quickly to an extremely small value. When the field strength is made still higher, as shown in view (C), the dish current drops to zero. If the magnetron is altered to the cutoff, or critical value of the plate current, and the electrons just neglect to reach the plate in their circular motion, it can produce oscillations at microwave frequencies. These oscillations are triggered by the currents induced electrostatically by the moving electrons. The consistency depends upon the time it requires the electrons to visit from the cathode toward the dish and again. A transfer of microwave energy to lots is manufactured possible by linking an exterior circuit between your cathode and the plate of the magnetron. Magnetron oscillators are split into two classes: NEGATIVE-RESISTANCE and ELECTRON-RESONANCE MAGNETRON OSCILLATORS. A negative-resistance magnetron oscillator is handled by way of a static negative level of resistance between its electrodes. This oscillator has a occurrence add up to the frequency of the tuned circuit linked to the tube. An electron-resonance magnetron oscillator is operated by the electron transit time required for electrons to visit from cathode to dish. This oscillator is with the capacity of generating large peak electricity outputs at frequencies in the a large number of megahertz. Although its average electricity output more than a time frame is low, it provides very high-powered oscillations in short bursts of pulses.
