InterviewSolution
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InterviewSolution
This section includes InterviewSolutions, each offering curated multiple-choice questions to sharpen your knowledge and support exam preparation. Choose a topic below to get started.
| 101. |
A quarter wave line open circuited at far end behaves as |
| Answer» A quarter wave line o.c. at far end behaves as a series tuned circuit. | |
| 102. |
For a 50 Ω resistor for 3 GHz application, the stray capacitance should be less than |
| Answer» XC should be at least ten times greater than R. Therefore for 50 Ω resistor at 3 GHz, C should be less than 0.1 pF. | |
| 103. |
In a travelling wave tube distributed interaction between an electron beam and a travelling wave takes place |
| Answer» In a klystron the resonant structure limits the bandwidth. A TWT is a broadband device. Its main components are electron gun (to produce the electron beam) and a structure supporting the slow electromagnetic wave. The velocity of wave propagation along the helix structure is less than velocity of light. The beam and wave travel along the structure at the same speed. Thus interaction occurs between beam and wave and the beam delivers energy to the RF wave. Therefore the signal gets strengthened and amplified output is delivered at the other end of tube. The main features of TWT are : 1. Frequency range - 0.5 GHz to 90 GHz 2. Power output - 5 mW at low frequencies(less than 20 GHz) 250 kW (continuous wave) at 3 GHz 10 MW (pulsed) at 3 GHz 3. Efficiency - about 5 to 20% 4. Noise - about 5 dB for low power TWT 25 dB for high power TWT TWT is used as RF amplifier in broadband microwave receivers, repeater amplifier in broad band communication systems, communication satellites etc. | |
| 104. |
A line has Z = 300 ∠ 0° Ω. If = 150 ∠ 0° Ω, reflection coefficient is |
| Answer» Reflection coefficient | |
| 105. |
In a microstrip transmission the signal can be easily redirected by changing the centre strip. |
| Answer» A Microstrip line has a single dielectric substratc with ground plane on one side and a strip on the other face. Its special feature is easy access to top surface so that minor adjustments can be made after circuit fabrication. The high dielectric constant of the substrate reduces guide wavelength and circuit dimensions. A microstrip line is the most commomly used transmission structure for microwave integrated circuits. | |
| 106. |
The frequency of oscillation in a backward wave oscillator can be changed by |
| Answer» It is somewhat similar to TWT and can deliver microwave power over a wide frequency band. It has an electron gun and a helix structure. However the interaction between electron beam and RF wave is different than in TWT. The growing RF wave travels in opposite direction to the electron beam. The frequency of wave can be changed by changing the voltage which controls the beam velocity. Moreover the amplitude of oscillations can be decreased continuously to zero by changing the beam current. It features are: 1. Frequency range - 1 GHz to 1000 GHz. 2. Power output - 10 mV to 150 mW (continuous wave) 250kW (pulsed). It is used as signal source in transmitters and instruments. | |
| 107. |
A line has a phase constant of 29.8 rad/. At 1000 MHz the wavelength is |
| Answer» . | |
| 108. |
The fabrication of microstrip line is done by |
| Answer» Printed circuit techniques is used for micro strip line. | |
| 109. |
The condition of minimum distortion in a transmission line is |
| Answer» The condition for minimum distortion is found by equating The result is . | |
| 110. |
Microwave resonators are used in |
| Answer» They are used in all the devices. | |
| 111. |
In a TWT the amplitude of resultant wave travelling down the helix |
| Answer» In a klystron the resonant structure limits the bandwidth. A TWT is a broadband device. Its main components are electron gun (to produce the electron beam) and a structure supporting the slow electromagnetic wave. The velocity of wave propagation along the helix structure is less than velocity of light. The beam and wave travel along the structure at the same speed. Thus interaction occurs between beam and wave and the beam delivers energy to the RF wave. Therefore the signal gets strengthened and amplified output is delivered at the other end of tube. The main features of TWT are : 1. Frequency range - 0.5 GHz to 90 GHz 2. Power output - 5 mW at low frequencies(less than 20 GHz) 250 kW (continuous wave) at 3 GHz 10 MW (pulsed) at 3 GHz 3. Efficiency - about 5 to 20% 4. Noise - about 5 dB for low power TWT 25 dB for high power TWT TWT is used as RF amplifier in broadband microwave receivers, repeater amplifier in broad band communication systems, communication satellites etc. | |
| 112. |
The velocity factor of a transmission line depends on |
| Answer» ∈r of a dielectric changes with humidity. | |
| 113. |
A loss less line of characteristic impedance Z is terminated in pure reactance of - value. VSWR is |
| Answer» A pure reactance does not absorb any power. Therefore VSWR is infinite because |rv| = 1. | |
| 114. |
In a klystron amplifier the input cavity is called |
| Answer» A Klystron is a vacuum tube used for generation/amplification of microwaves. An electron beam is produced by oxide coated indirectly heated cathode and is focussed and accelerated by focussing electrode. This beam is transmitted through a glass tube. The input cavity where the beam enters the glass tube is called buncher. As electrons move ahead they see an accelerating field for half cycle and retarding field for the other half cycle. Therefore, some electrons are accelerated and some are retarded. This process is called velocity modulation. The velocity modulation causes bunching of electrons. This bunching effect converts velocity modulation into density modulation of beam. The input is fed at buncher cavity and output is taken at catcher cavity. In a two cavity klystron only buncher and catcher cavity are used. In multi cavity klystron one or more intermediate cavities are also used. The features of a multicavity klystron are : 1. Frequency range - 0.25 GHz to 100 GHz 2. Power output - 10 kW to several hundred kW 3. Power gain - 60 dB (nominal value) 4. Efficiency - about 40%. A multicavity klystron is used in UHF TV transmitters, Radar transmitter and satellite communication. | |
| 115. |
In a circular waveguide the dominant mode is |
| Answer» Some applications require dual polarization capability. Circular waveguide has this capability. These analysis uses cylindrical coordinates. In circular waveguide TE11 mode has the lowest cut off frequency and is the dominant mode. If D is diameter of waveguide λc = 1.706 D for TE11 mode λc = 1.029 D for TE21 mode λc = 0.82 D for TE01 mode λc = 1.306 D for TM01 mode. | |
| 116. |
The reflection coefficient on a line is 0.2 ∠45°. The SWR is |
| Answer» | |
| 117. |
Reflex klystron oscillator is essentially a low power device |
| Answer» It uses a single cavity resonator for generating microwave oscillations. Its parts are electron gun, resonator, repeller and output coupling. It operates on the principle of positive feed back. The repeller electrode is at negative potential and sends the partially bunched electron beam back to resonator cavity. This positive feedback supports oscillations. Its feature are: 1. Frequency range - 2 to 100 GHz 2. Power output - 10 MW to about 2 W 3. Efficiency - 10 - 20 % Its applications include radar receivers, local oscillator in microwave devices, oscillator for microwave measurements in laboratories etc. | |
| 118. |
A 10 km long line has a characteristic impedance of 400 ohms. If line length is 100 km, the characteristic impedance is |
| Answer» and is independent of line length. | |
| 119. |
A matched generator (Z = Z) with 1 Ω available. If all ports are matched, the power delivered at ports 1, 2, 3 respectively are |
| Answer» Since all ports matched, input SWR = 1. No power is reflected and Pin = P4 = 1 W. This power splits between ports 1 and 2. Therefore P1 = P2 = 0.5 Ω, P3 = 0. | |
| 120. |
Which TM mode in rectangular waveguide has lowest cutoff frequency? |
| Answer» Hz = 0 Ex = ± ZTM Hy Ey = ± ZTM Hx where E0 is the amplitude of the wave. The expressions for β, λc, fc, λg, a are the same as for TE waves. For TM wave the lowest cut off frequency in rectangular wave guide is for TM11 mode. If , the cutoff frequency for TM11 mode is about 12% more than that for TE20 mode. | |
| 121. |
A waveguide section in a microwave circuit acts as |
| Answer» It allows high frequencies. | |
| 122. |
In a klystron amplifier dc electron velocity is and dc electron charge density is the dc beam current is |
| Answer» = coulomb |sec|m2 = current density. | |
| 123. |
A resistive microwave load with Z = 150 Ω is connected to 50 Ω coaxial line. SWR is |
| Answer» | |
| 124. |
In a turnstile antenna two-half wave resonant wire radiators are placed at 90° to each other in the same plane and are excited |
| Answer» Since the two wire radiators are at 90° to each other, they have to be excited 90° out of phase with each other. | |