Read the paragraph and answer the questions : Microwave tubes are used as microwave amplifiers and oscillators. Three general type of microwave tubes in which third type tubes are important because in these tubes there is an interaction between an electron and an RF field is continuous. The Travelling Wave Tube (TWT) is the prime example of this interaction. It is an amplifier, whose oscillator counter part is called Backward Wave Oscillator (BWO). The second sub-group consists of tubes in which a magnetic field ensures a constant electron beam – RF field interaction, and this is complemented by the Cross-Field Amplifier (CFA). Multicavity Klystron is used as high and very high power amplifiers in the UHF and microwave ranges. The frequency range covered is from about 250 MHz to over 95 GHz. The reflex-Klystron is a low power microwave oscillator. It is assumed that oscillations are started by switching transients. For oscillations to be maintained the transient time in the repeller space cycle is given as \(T=\left(n+\dfrac{3}{4}\right)\) where n is an integer, each value of n is said to correspond to different reflex klystron mode. Reflex Klystrons with integral cavities are available in the frequency range 4 to 200 GHz.
The primary function of the helix in a travelling wave tube is to
reduce the axial velocity of the RF field
The helix is a slow-wave structure, and slowing the wave is exactly its purpose — option 2.
The problem it solves. An electron beam can be accelerated to perhaps a tenth of the speed of light before the required voltage becomes impractical — around 10 kV. An RF wave on an ordinary transmission line travels at nearly \(c\). The wave would therefore race past the electrons, alternately accelerating and retarding them, and the net energy transfer over the length of the tube would average to nothing.
How the helix fixes it. The wave must still travel along the wire at close to \(c\), but the wire is coiled, so its progress along the axis is slowed by the ratio of the pitch to the circumference of one turn:
\(v_{axial}\approx c\,\dfrac{p}{2\pi a}\approx\dfrac{c}{10}\)
Now wave and beam travel at nearly the same axial speed. Electrons stay in the same phase of the RF field for the whole transit, are continuously decelerated by it, and give up their kinetic energy to the wave — which grows exponentially along the tube.
| Option | Verdict |
|---|---|
| Prevent beam spreading | ✗ That is the job of the focusing magnet |
| Reduce axial velocity of the RF field | ✓ |
| Ensure broadband operation | A real consequence, not the function |
| Reduce noise figure | ✗ Unrelated |
Option 3 is the thoughtful distractor and deserves a clear answer. A TWT is broadband, and that follows from the helix being non-resonant, unlike a klystron's cavities. But breadth of band is a by-product of using a slow-wave transmission line; the reason a helix is there at all is to establish synchronism, without which the tube would not amplify at any frequency. The question asks for the primary function.
Option 1 names a genuine problem with the wrong solution. The beam does spread, since like charges repel, and it must be confined over a tube 20 to 50 cm long. That is done by an axial magnetic field — a solenoid, or more usually a periodic permanent-magnet stack. The helix carries the signal; it does not confine the beam.
Hence, the helix serves to reduce the axial velocity of the RF field.
Similar Questions
There are three general purpose microwave tubes. The first is ordinary gridded tube, having electrodes like vacuum tube diode and triodes. The second type are those in which interaction between the electron beam and RF field takes place. The klystron is the example of the second type of microwave tubes. The third category of the device is one in which interaction between an RF field and electron beam is continuous. TWT (Travelling Wave Tube) is the example of this category.
Read the paragraph and answer the questions :
Microwave tubes are used as microwave amplifiers and oscillators. Three general type of microwave tubes in which third type tubes are important because in these tubes there is an interaction between an electron and an RF field is continuous. The Travelling Wave Tube (TWT) is the prime example of this interaction. It is an amplifier, whose oscillator counter part is called Backward Wave Oscillator (BWO). The second sub-group consists of tubes in which a magnetic field ensures a constant electron beam – RF field interaction, and this is complemented by the Cross-Field Amplifier (CFA). Multicavity Klystron is used as high and very high power amplifiers in the UHF and microwave ranges. The frequency range covered is from about 250 MHz to over 95 GHz. The reflex-Klystron is a low power microwave oscillator. It is assumed that oscillations are started by switching transients. For oscillations to be maintained the transient time in the repeller space cycle is given as \(T=\left(n+\dfrac{3}{4}\right)\) where n is an integer, each value of n is said to correspond to different reflex klystron mode. Reflex Klystrons with integral cavities are available in the frequency range 4 to 200 GHz.
The correct sequence of sub-systems of Klystron amplifiers as they appear in the direction of flow of electron beam is
Assertion (A) : TWTA is a narrow band device and has a helical structure and it is used as amplifier and oscillator.
Reason (R) : In TWT, the electron beam travels through a slow-wave structure and speed of electron beam is \(\dfrac{1}{10}\) of the velocity of light.
The Multicavity Klystron
One of the reasons why vacuum tubes eventually fail at microwave frequencies is that their
Indicate the false statement. Klystron amplifiers may use intermediate cavities to
One of the following is unlikely to be used as a pulsed device. It is the
In a two cavity Klystron the secondary cavity is called
For a reflex Klystron :
Important Questions from Microwave Engineering
Which of the following frequency bands fall under microwave frequency?
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A uniformly spaced linear array of identical radiators having uniform amplitude of excitation and linear phase variation with non-zero gradient will produce
