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Waveguide Components Questions and Answers

Direction for [ Question No: 1 To 1 ] :

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 \(T = \left(n + \frac{3}{4}\right)\) 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.

1.

The primary of the helix in a travelling wave tube is to

Exam Year(s)

UGC NET - 2014,

Answer: B

The helix reduces the axial velocity of the RF field. The electron beam travels at a speed that is a small fraction of the speed of light, so the RF wave must be slowed to about the beam velocity for them to interact over a distance. The helix does this because the wave follows the turns of the wire and so advances along the axis much more slowly.

Preventing the beam from spreading is the job of the focusing magnet. Broadband operation and the noise figure are features of the tube, but they are not the primary purpose of the helix.

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Direction for [ Question No: 2 To 2 ] :

Directions : The following items consist of two statements, one labelled the "Assertion (A)" and the other labelled the "Reason (R)". You are to examine these two statements carefully and decide if the Assertion (A) and the Reason (R) are individually true and if so, whether the Reason is a correct explanation of the Assertion. Select your answers to these items using the codes given below and mark your answer accordingly.

2.

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 \(\frac{1}{10}\) of the velocity of light.

Exam Year(s)

UGC NET - 2014,

Answer: D

The Assertion is false and the Reason is true. A travelling wave tube is a wide-band device, not a narrow-band one: its helix slows the RF wave to near the beam velocity over a wide range of frequencies. The Reason correctly describes how it works, a beam travelling through a slow-wave structure at a small fraction of the speed of light.

The options that call the Assertion true are wrong because a TWTA is valued for its large bandwidth. The two statements are therefore not a true pair.

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Direction for [ Question No: 3 To 7 ] :

Read the paragraph and answer the questions :

Microwave tubes are used as microwave amplifiers and oscillators. Three general type of microwaves tubes in which third type of tubes are useful. In this tube 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). Another group of tube in which a magnetic field ensures a constant electron beam - RF field interaction and this is complimented by the cross-field amplifier (CFA). Multicavity Klystron is used as an amplifier of microwaves and it consists of buncher cavity and cache cavity. The Reflex Klystron is a low power and low efficiency microwave oscillator. Reflex Klystron oscillator has been replaced by various semiconductor oscillators.

3.

One of the following is unlikely to be used as pulsed device. It is the

Exam Year(s)

UGC NET - 2013, UGC NET - 2014,

Answer: B

The device unlikely to be used as a pulsed device is the BWO, the backward wave oscillator.

The multicavity klystron, the cross-field amplifier and the travelling wave tube are amplifiers and are widely used in pulsed radar transmitters. The BWO is an oscillator tuned by its voltage, so it is used as a continuous tunable source and not as a pulsed device.

That is why (A), (C) and (D) are the wrong choices for this question.

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4.

One of the reason why vacuum tubes eventually fail at microwave frequencies is that their

Exam Year(s)

UGC NET - 2013,

Answer: A

Vacuum tubes fail at microwave frequencies partly because their noise figure increases.

At these frequencies transit-time effects, shot noise and induced grid noise all rise, so noise figure increases.

The other options state the opposite of what happens. The transit time becomes too long, not too short, because the electron transit time becomes comparable to the period. The shunt capacitive reactance becomes too small, not too large, and the series inductive reactance of the leads becomes too large, not too small.

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5.

The Multicavity Klystron

Exam Year(s)

UGC NET - 2013,

Answer: B

The multicavity klystron is not a good low-level amplifier because of noise.

Its electron beam and bunching process give a fairly high noise figure, so it is used as a high power amplifier rather than for weak signals.

A high repeller voltage belongs to the reflex klystron, which has a repeller, and a multicavity klystron has none, so (A) is wrong. It is suitable for pulsed operation, which makes (C) false. A short transit time is needed through the buncher cavity and not a long one, so (D) is wrong.

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6.

Indicate the false statement. Klystron amplifiers may use intermediate cavities to

Exam Year(s)

UGC NET - 2013,

Answer: D

The false statement is that intermediate cavities prevent the oscillations that occur in a two cavity klystron, option (D).

Intermediate cavities are added to increase the gain, the efficiency and the bandwidth of the klystron, by bunching the electrons more fully. They are not meant to suppress oscillation.

Statements (A), (B) and (C) are the genuine benefits, so they are true and are not the answer to a question that asks for the false one.

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7.

TWT is sometimes preferred to the multicavity Klystron amplifier because the former

Exam Year(s)

UGC NET - 2013,

Answer: B

A TWT is sometimes preferred to a multicavity klystron because the former has a greater bandwidth.

A klystron works with resonant cavities, so it has a narrow bandwidth. The travelling wave tube uses a non-resonant slow-wave helix, and amplifies over a wide band, often an octave or more.

The multicavity klystron is generally the more efficient and the higher power tube, so (A) and (D) are wrong. Having more modes is not an advantage, which rules out (C).

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Direction for [ Question No: 8 To 8 ] :


Assertion – Reason type questions :
The following items consist of two statements, one labelled the ‘Assertion (A)’ and the other labelled the ‘Reason (R)’. You are to examine these two statements and decide if the Assertion (A) and the Reason (R) are individually true and if so, whether the Reason is a correct explanation of the Assertion. Select your answers to these items using the codes given below and mark your answer sheet accordingly.

Codes :
(A) Both (A) and (R) are true and (R) is the correct explanation of (A).
(B) Both (A) and (R) are true but (R) is not the correct explanation of (A).
(C) (A) is true and (R) is false.
(D) (A) is false and (R) is true.

8.

Assertion (A) : Magic Tee is a 4-junction hybrid Tee and displays power dividing properties in E and H plane and has advantage of being fully matched.

Reason (R) : It has a hybrid junction about an imaginary plane bisecting two arms.

Exam Year(s)

UGC NET - 2012,

Answer: A

The key marks both statements true with the reason explaining the assertion. A magic tee is a four-port hybrid junction that combines the E-plane and H-plane tees, so it divides power in both planes, and when its ports are matched it has the advantage that nothing is reflected back. The hybrid junction of the magic tee is formed about an imaginary plane that bisects two of its arms, and that symmetry produces the isolation between the sum and difference ports and the power division. The answer is that both are true and (R) is the correct explanation of (A).

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9.

On which of the following principles does Klystron operate ?

Exam Year(s)

MH SET - 2019, SET Exam - 2019,

Answer: D

The klystron operates on velocity modulation.

In a klystron, the electron beam passes through the gap of an input (buncher) cavity, where the RF field speeds up some electrons and slows down others. Over the drift space the fast electrons catch up with the slow ones, so the beam forms bunches: velocity modulation is turned into current (density) modulation. The bunched beam then gives energy to the RF field in the output (catcher) cavity, producing amplification, or in a reflex klystron, oscillation.

Amplitude, frequency and pulse modulation are ways of impressing information on a carrier, not the mechanism by which a klystron works.

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10.

Duplexer in RADAR system is :

Exam Year(s)

MH SET - 2019, SET Exam - 2019,

Answer: B

The duplexer in a RADAR system is a microwave switch.

A radar using one antenna for both transmitting and receiving needs a duplexer to connect the antenna to the transmitter during the high-power pulse and to the receiver during the listening time between pulses, while protecting the sensitive receiver from the transmitted pulse. It does this with switching devices such as TR (transmit-receive) and ATR tubes, ferrite circulators or PIN diode switches.

It generates no signal, as an oscillator would, does not amplify, and is not an active filter.

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