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Travelling Wave Tube
A travelling wave tube is a specialized vacuum tube that is used in electronics to amplify radio frequency (RF) signal in the microwave range. Travelling wave tube are abbreviated as TWT.
Travelling Wave Tube are non-resonant structure that offer continuous interaction of applied RF field with the electron beam over the entire length of the tube. Due to this reason, it provide wider operating bandwidth.
Construction of Travelling Wave Tube
Travelling wave tube is a cylindrical structure that contains an electron gun, which is used to produce a narrow, constant velocity electron beam. It also has anode plates, a helix and a collector. The RF input is sent to one end of the helix and the output is drawn from the other end of the helix.
The electron gun focusses an electron beam with the velocity of light. A magnetic field guides and focuses the beam, without scattering. The RF field propagates at a reduced velocity due to the helix. The Helix acts as a slow wave structure, slowing down the RF signal compared to the speed of light.
The applied RF signal, propagated through the helix, produces an electric field at the center of the helix. The resultant electric field due to applied RF signal, travels with the velocity of light multiplied by the ratio of helix pitch to helix circumference. The velocity of electron beam, travelling through the helix, interact with the RF waves on the helix.
Helix is a loosely wound, conducting helical wire, which acts as slow wave structure. The signal to be amplified is applied to the end of the helix adjustment to the electron gun. The amplified signal appears at the output, which is taken from the other end of the helix.
Need of Slow Wave Tube
The input RF field produces an electric field at the centre of the helix with the velocity of light. The velocity of the em wave is higher than the phase velocity of electron beam. The velocity of RF field is retarded by multiplying it with the ratio of helix pitch to helix circumferences.
USE OF ATTENUATOR : It is used to attenuate any reflected waves due to impedence mismatch at the end. In order to restrict the generation of oscillation inside the tube, it is placed over a part of the helix near the output end.
Vp = cP/√ (P)^2 + (πd)^2
c = velocity of light (3*10^8 m/s), V = phase velocity in m/s, P = pitch of helix in m, d = diameter of the helix in m
Working of Travelling Wave Tube
The applied RF signal produces an electric field inside the tube. During the positive half of the signal, the moving electron beam experiences accelerative force. However, the negative half of the input applies a de-accelerative force on the moving electrons. This is said to be velocity modulation because the electrons of the beam experience different velocities inside the tube.
The slowly travelling wave inside the tube continuous interacts with the electron beam. Due to this interaction, the high-speed electrons transfer their energy to the wave inside the tube and thus slow down. As a result, the rise in the amplitude of the wave. the velocity of electrons reduces and they start to bunch together inside the tube.
The growing of the wave resultantly causes more bunching of electrons while reaching the end from the beginning. This process amplifies the RF wave inside the tube. By the time the wave reaches the end, the signal is amplified. A positive potential at the end collects the electron bunches at the collector. The magnetic field inside the tube prevents the electron beam from spreading out.
Types of Travelling Wave Tube
Although there are various types of TWT, but two major categories are:
- Helix TWT: In which the radio waves interact with the electron beam while traveling down a wire helix that surround the beam.
- Coupled cavity TWT: In which the radio wave interacts with the beam in a series of the beam passes. These function as narrowband power amplifiers.
Characteristics of Travelling Wave Tube
- Its operating frequency range from 300 MHz to 50 GHz.
- Power gain 40 to 70 decibels.
- Output power ranges from few watt to mega watts.
- Efficiency: 20 to 40 %.
- Bandwidth: 0.8 GHz.
Applications of Travelling Wave Tube
There are many applications of a travelling wave tube are :
- TWT is used in microwave receivers as a low noise RF amplifier.
- TWT are also used in wide band communication links and co-axial cables as repearter amplifier or intermediated amplifier to amplify low signals.
- TWTs have a long tube life, due to which they are used as power output tubes in communication satellites.
- TWTs are used in high power pulsed radars and ground based radars.
- TWTs are highly used in continuous wave radar systems.
Frequently Asked Questions (FAQs)
What are the advantages of the TWT?
A major advantages of the TWT over some other microwave tube are:
1. Its ability to amplify a wide range of frequencies
2. Wide bandwidth
3. High gain and reliability
4. High output powerHow does a TWT amplifier work?
The TWT amplifier consists of an electron gun that emits a beam of electrons, which travels along a helix or a metal conductor. This electron beam interacts with the RF signal, amplifying it. The RF signal is transferred to the electrons, and the amplified signal is extracted at the output.
What are the limitations of TWT amplifiers?
1. Size : TWTs are relatively large, marking them less suitable for compact applications.
2. Heat generation : The electron beam in TWT amplifiers can generate significant heat, requiring effective cooling mechanism
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