Question 1

If antenna diameter is increased four times, the maximum range is increased by a factor of

Accepted Answer

Range of radar is proportional to antenna diameter.

Question 2

Assertion (A): In the interaction region of magnetron an electron is subjected to three forces: force due to electric field, force due to magnetic field and centrifugal force. Reason (R): The mechanism of generation of microwaves, in a magnetron, in…

Accepted 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.

Question 3

If VSWR is infinite, the transmission line is terminated in

Accepted Answer

and . Since VSWR = ∞, |rv| = 1. This can happen if ZL = 0 or ZL = ∞, i.e., line is o.c or s.c.

Question 4

Impedance inversion may be obtained with

Accepted Answer

Impedance inversion occurs when load impedance is not matched with characteristic impedance of line.

Question 5

In a circular waveguide TE21 mode has lowest cutoff frequency

Accepted 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.

Question 6

Assertion (A): For high frequency lines inductive reactance is very high as compared to ac resistance. Reason (R): Due to skin effect ac resistance of line is higher than dc resistance.

Accepted Answer

XL = ωL. As frequency increases line resistance increases slightly but XL increases directly as per frequency.

Question 7

For matching over a range of frequencies in a transmission line it is best to use

Accepted Answer

Double stub is useful for matching over a range of frequencies.

Question 8

Read the following statements about TWT It uses thermionic emission. It uses an attenuater. It is inherently a resonant device. It has broad bandwidth. Which of the above are correct?

Accepted 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.

Question 9

Assertion (A): Gunn diode is a transferred electron device. Reason (R): A Gunn oscillator uses the phenomenon of transferred electron effect.

Accepted Answer

A Gunn diode uses GaAs which has a negative differential mobility, i.e., a decrease in carrier velocity with increase in electric field. This effects is called transferred electron effect. The impedance of a Gunn diode is tens of ohms. A Gunn diode oscillator has a resonant cavity, an arrangement to couple Gunn diode to cavity, biasing arrangement for Gunn diode and arrangement to couple RF power to load. Applications of Gunn diode oscillator include continuous wave radar, pulsed radar and microwave receivers.

Question 10

A reflex klystron oscillator is a

Accepted 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.

Question 11

Assertion (A): Impatt diode can be used in both amplifiers and oscillators. Reason (R): Impatt diode has a low resistance.

Accepted Answer

An Impatt diode has n+ - p - i - p + structure and is used with reverse bias. It exhibits negative resistance and operates on the principle of avalanche breakdown. Impatt diode circuits are classified as broadly tunable circuit, low Q circuit and high Q circuit. The impedance of Impatt diode is a few ohms. The word Impatt stands for Impact Avalanche Transit Time diode. The features of Impatt diode oscillator are : frequency 1 to 300 GHz, Power output (0.5 W to 5 W for single diode circuit and upto 40 W for combination of several diodes), efficiency about 20%. Its applications include police radar systems, low power microwave transmitter etc.

Question 12

Assertion (A): Microstrip is very commonly used in microwave integrated circuits. Reason (R): Microstrip has an easy access to the top-surface so that active and passive discrete components can be easily mounted.

Accepted 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.

Microwave Communication Questions