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# Multiple choice question for engineering

## Set 1

1. The value of ‘α’ for a lossless line is:
a) 0
b) 1
c) Infinity
d) Data insufficient

Answer: a [Reason:] α-for a transmission line signifies the attenuation constant. For a lossless transmission line attenuation constant is zero and the propagation occurs without losses.

2. If propagation constant is 12:60°, then the value of phase constant and attenuation constant is:
a) α=6, β=10.39
b) α=61, β=78
c) α=12, β=20.6
d) none of the mentioned

Answer: a [Reason:] The given propagation constant is in polar form .converting from polar form to rectangular form and equating the real and imaginary parts, we get α=6 and β=10.39.

3. If a transmission line with inductive reactance of 41.97 Ω and capacitive reactance of 1132.5Ω is operated at 1 GHz , then its phase constant is:
a) 0.0305
b) 0.3
c) 30.3
d) 0.6

Answer: a [Reason:] From the given inductive reactance and capacitive reactance, L and C are calculated using XL =2πfL and Xc = 1/2πfC. β=ω√LC, substituting the calculated L and C, we get β=0.0305.

4. The expression for a phase velocity of a transmission line is:
a) √LC
b) 1/√LC
c) XL+Xc
d) XL/Xc

Answer: b [Reason:] The expression for phase velocity is derived from known basic transmission line equations and the derived equation comes out to be 1/√LC .

5. If the admittance and the impedance of a transmission line are 100 Ω and 50 Ω of a respectively, then value of phase constant β is:
a) 0
b) 20
c) 132
d) 50

Answer: a [Reason:] β=ω√LC. Since both the line impedance and line admittance are both real, there is no phase difference caused and hence substituting in the above equation, we get β=0.

6. For a lossless line, which of the following is true?
a) γ=jβ
b) γ=α
c) γ=α+jβ
d) γ=α*jβ

Answer: a [Reason:] For a lossless line, attenuation constant α is 0. Hence substituting α=0 in γ=α+jβ, we get γ= jβ.

7. Expression for phase constant β is:
a) √LC
b) ω √LC
c)1/ (ω √LC)
d) None of the mentioned

Answer: b [Reason:] From the equation of γ in terms of Z and Y(impedance and admittance of the transmission line respectively), expanding the equation and making certain approximations, β= ω √LC.

8. A microwave generator at 1.2 GHz supplies power to a microwave transmission line having the parameters R=0.8Ω/m, G=O.8millisiemen/m, L=0.01µH/m and C=0.4PF/m. Propagation constant of the transmission line is:
a) 0.0654 +j0.48
b) 0.064+j4.8
c) 6.4+j4.8
d) none of the mentioned

Answer: a [Reason:] Z=R+jωL and Y=G+jωC, hence finding out Z and Y from these equations, substituting in γ=√ZY, value of γ is found out to be 0.0654+j0.48.

9. In a certain microwave transmission line, the characteristic impedance was found to be 210 10°Ω and propagation constant 0.2 78°.What is the impedance Z of the line, if the frequency of operation is 1 GHz?
a) 0.035+j41.97
b) 0.35+j4.97
c) 35.6+j4.28
d) 9.254+j4.6

Answer: a [Reason:] Impedance Z of a transmission line is given by the product of propagation constant γ and characteristic Zₒ, Z= γZₒ , we get Z=0.035+j41.97.

10. For a transmission line, L=1.8mh/m C=0.01pF/m, then the phase constant of the line when operated at a frequency of 1 GHz is:
a) 4.2426
b) 2.2
c) 0.3
d) 1

Answer: a [Reason:] Formula to calculate the phase constant β is β=ω√LC.substituting the given values of L,C and f, the value of β is 4.2426.

## Set 2

1. For a low loss line when both conductor and di-electric loss is small, the assumption that could be made is:
a) R < < ωL and G < < ωC
b) R > > ωL and G > >ωC
c) R < <ωC and G < < ωL
d) R > >ωC and G > >ωL

Answer: A [Reason:] For a low loss line, the real part of impedance and admittance, that is resistance and conductance must be very small compared to the complex part of admittance and impedance for maximum power transfer. Hence R < <ωL and G < < ωC.

2. Expression for α(attenuation constant) in terms of R , G, L and C of a transmission line is:
a) (R√(C/L)+G√(L/C))0.5
b) (R√(C/L)+G√(L/C))
c) (R√(L/C)+G√(C/L))
d) (R√(L/C)+G√(C/L))0.5

Answer: A [Reason:] For a low loss line, the real part of impedance and admittance, that is resistance and conductance must be very small compared to the complex part of admittance and impedance for maximum power transfer. Hence R < <ωL and G < < ωC, with this assumption, modifying the expression for propagation constant, the simplified expression for attenuation constant α is (R√(C/L)+G√(L/C))0.5.

3. Expression for characteristic impedance Zₒ of a transmission line in terms of L and C the transmission line is:
a) √(C/L)
b) √(CL)
c) √(L/C)
d) 1/√(LC)

Answer: C [Reason:] For a low loss line, the real part of impedance and admittance, that is resistance and conductance must be very small compared to the complex part of admittance and impedance for maximum power transfer. HenceR < <ωL and G < < ωC, with this assumption, modifying the expression for characteristic impedance√(((R+jωL))/√(G+jωC)), the ratio reduces to √ (L/c).

4. If the inductance and capacitance of a loss line transmission line are 45 mH/m and10 µF/m, the characteristic impedance of the transmission line is:
a) 50Ω
b) 67.08Ω
c) 100Ω
d) none of the mentioned

Answer: B [Reason:] The expression for characteristic impedance of a transmission line in terms of inductance and capacitance of a transmission line is√((L)/C). Substituting the given values in this equation, the characteristic impedance of the transmission line is 67.08Ω.

5. If the characteristic impedance of a transmission line is 50 Ω, and the inductance of the transmission line being 25 mH/m, the capacitance of the lossy transmission line is:
a) 1µF
b) 10 µF
c) 0.1 µF
d) 50 µF

Answer: B [Reason:] The expression for characteristic impedance of a transmission line in terms of inductance and capacitance of a transmission line is√((L)/C). Substituting the given values in this equation, and solving for C, value of C is 10µF.

6. If R = 1.5Ω/m, G = 0.2 mseimens/m, L = 2.5 nH/m, C = 0.1 pF/m for a low loss transmission line, then the attenuation constant of the transmission line is:
a) 0.0.158
b) 0.0523
c) 0.0216
d) 0.0745

Answer: A [Reason:] The expression for attenuation constant of a low loss transmission line is (R√(C/L)+G√(L/C))0.5. Substituting the given values in the above expression, the value of attenuation constant is 0.0158.

7. A lossy line that has a linear phase factor as a function of frequency is called:
a) distortion less line
b) terminated lossy line
c) loss less line
d) lossy line

Answer: A [Reason:] A distortion less transmission line is a type of a lossy transmission line that has a linear phase factor as a function of frequency. That is, as the frequency of operation changes, the phase variation is linearly dependent.

8. The condition for a distortion less line is:
a) R/L=G/C
b) R/C=G/L
c) R=G
d) C=L

Answer: A [Reason:] The special case of a lossy transmission line that has a linear phase factor as a function of frequency is called distortion less line. The relation between the transmission line constants for such a distortion less line R/L=G/C.

9. For a distortion less line, R= 0.8Ω/m, G= 0.8 msiemens/m, L= 0.01µH/m then C is:
a) 10 pF
b) 1pF
c) 1nF
d) 10nF

Answer: A [Reason:] The special case of a lossy transmission line that has a linear phase factor as a function of frequency is called distortion less line. The relation between the transmission line constants for such a distortion less line R/L=G/C. substituting the given values in the equation, we get 10pF.

10. For a lossy transmission line, γ=0.02+j0.15 and is 20m long. The line is terminated with an impedance of a 400Ω. Then the input impedance of the transmission line given that the characteristic impedance of the transmission line is 156.13+j11.38Ω is:
a) 100+j50 Ω
b) 228+j36.8 Ω
c) 50+36.8j Ω
d) none of the mentioned

Answer: B [Reason:] The relation between source impedance, propagation constant and characteristic impedance is given by ZS= Z0 (ZLcosh(γl) + Z0 sinh(γl))/( Z0cosh(γl) + ZL sinh(γl)). Substituting the given values in the above equation, input impedance of the transmission line is 228+j36.8 Ω.

## Set 3

1. Micro strip can be fabricated using:
a) Photo lithographic process
b) Electrochemical process
c) Mechanical methods
d) None of the mentioned

Answer: a [Reason:] Microstrip lines are planar transmission lines primarily because it can be fabricated by photolithographic processes and is easily miniaturized and integrated with both passive and active microwave devices.

2. The mode of propagation in a microstrip line is:
a) Quasi TEM mode
b) TEM mode
c) TM mode
d) TE mode

Answer: a [Reason:] The exact fields of a microstrip line constitute a hybrid TM-TE wave. In most practical applications, the dielectric substrate is very thin and so the fields are generally quasi-TEM in nature.

3. Microstrip line can support a pure TEM wave.
a) True
b) False
c) Microstrip supports only TM mode
d) Microstrip supports only TE mode

Answer: b [Reason:] The modeling of electric and magnetic fields of a microstrip line constitute a hybrid TM-TE model. Because of the presence of the very thin dielectric substrate, fields are quasi-TEM in nature. They do not support a pure TEM wave.

4. The effective di electric constant of a microstrip line is:
a) Equal to one
b) Equal to the permittivity of the material
c) Cannot be predicted
d) Lies between 1 and the relative permittivity of the micro strip line

Answer: d [Reason:] The effective dielectric constant of a microstrip line is given by (∈r + 1)/2 + (∈r-1)/2 * 1/ (√1+12d/w). Along with the relative permittivity, the effective permittivity also depends on the effective width and thickness of the microstrip line.

5. Effective dielectric constant of a microstrip is given by:
a) (∈r + 1)/2 + (∈r-1)/2 * 1/ (√1+12d/w)
b) (∈r+1)/2 + (∈r-1)/2
c) (∈r+1)/2 (1/√1+12d/w)
d) (∈r + 1)/2-(∈r-1)/2

Answer: a [Reason:] The effective dielectric constant of a microstrip line is (∈r + 1)/2 + (∈r-1)/2 * 1/ (√1+12d/w). This relation clearly shows that the effective permittivity is a function of various parameters of a microstrip line, the relative permittivity, effective width and the thickness of the substrate.

6. The effective dielectric constant of a micro strip line is 2.4, then the phase velocity in the micro strip line is given by:
a) 1.5*108 m/s
b) 1.936*108 m/s
c) 3*108 m/s
d) None of the mentioned

Answer: b [Reason:] The phase velocity in a microstrip line is given by C/√∈r. substituting the value of relative permittivity and the speed of light in vacuum, the phase velocity is 1.936*108 m/s.

7. The effective di electric constant of a micro strip line with relative permittivity being equal to 2.6, with a width of 5mm and thickness equal to 8mm is given by:
a) 2.6
b) 1.97
c) 1
d) 2.43

Answer: b [Reason:] The effective dielectric constant of a microstrip line is given by (∈r + 1)/2 + (∈r-1)/2 * 1/ (√1+12d/w). Substituting the given values of relative permittivity, effective width, and thickness, the effective dielectric constant is 1.97.

8. If the wave number of an EM wave is 301/m in air , then the propagation constant β on a micro strip line with effective di electric constant 2.8 is:
a) 602
b) 503.669
c) 150
d) 200

Answer: b [Reason:] The propagation constant β of a microstrip line is given by k0√∈e. ∈e is the effective dielectric constant. Substituting the relevant values, the effective dielectric constant is 503.669.

9. For most of the micro strip substrates:
a) Conductor loss is more significant than di electric loss
b) Di electric loss is more significant than conductor loss
c) Conductor loss is not significant
d) Di-electric loss is less significant

Answer: a [Reason:] Surface resistivity of the conductor (microstrip line) contributes to the conductor loss of a microstrip line. Hence, conductor loss is more significant in a microstrip line than dielectric loss.

10. The wave number in air for EM wave propagating on a micro strip line operating at 10GHz is given by:
a) 200
b) 211
c) 312
d) 209

Answer: d [Reason:] The wave number in air is given by the relation 2πf/C. Substituting the given value of frequency and ‘C’, the wave number obtained is 209.

11. The effective dielectric constant ∈r for a microstrip line:
a) Varies with frequency
b) Independent of frequency
c) It is a constant for a certain material
d) Depends on the material used to make microstrip

Answer: b [Reason:] The effective dielectric constant of a microstrip line is given by (∈r + 1)/2 + (∈r-1)/2 * 1/ (√1+12d/w). The equation clearly indicates that the effective dielectric constant is independent of the frequency of operation, but depends only on the design parameters of a microstrip line.

12. With an increase in the operating frequency of a micro strip line, the effective di electric constant of a micro strip line:
a) Increases
b) Decreases
c) Independent of frequency
d) Depends on the material of the substrate used as the microstrip line

Answer: c [Reason:] As the relation between effective permittivity and the other parameters of a microstrip line indicate, effective dielectric constant is not a frequency dependent parameter and hence remains constant irrespective of the operation of frequency.

## Set 4

1. In microwave oscillators, negative resistance transistors and diodes are used in order to generate oscillations in the circuit.
a) True
b) False

Answer: a [Reason:] In microwave oscillator, for a current to flow in the circuit the negative impedance of the device must be matched with positive impedance. This results in current being non-zero and generates oscillation.

2. Any device with negative impedance as its characteristic property can be called:
a) Energy source
b) Energy sink
c) Oscillator
d) None of the mentioned

Answer: a [Reason:] A positive resistance implies energy dissipation while a negative resistance implies an energy source. The negative resistance device used in the microwave oscillator, thus acts as a source. The condition Xin+ XL=0 controls the frequency of oscillation. Xin is the impedance of the negative resistance device.

3. In a microwave oscillator, a load of 50+50j is connected across a negative resistance device of impedance -50-50j. Steady state oscillation is not achieved in the oscillator.
a) True
b) False

Answer: b [Reason:] The condition for steady state oscillation in a microwave oscillator is Zin=-ZL. Since this condition is satisfied in the above case, steady state oscillation is achieved.

4. For achieving steady state oscillation, the condition to be satisfied in terms of reflection coefficients is:
a) ГinL
b) Гin=-ГL
c) Гin=1/ГL
d) None of the mentioned

Answer: c [Reason:] The condition for steady state oscillation to be achieved in terms of reflection coefficient is Гin=1/ГL. Here Гin is the reflection coefficient towards the reflection coefficient device and ГL is the reflection coefficient towards the load.

5. A one port oscillator uses a negative resistance diode having Гin=0.9575+j0.8034 (Z0=50Ω) at its desired frequency point. Then the input impedance of the diode is:
a) -44+j123
b) 50+j100
c) -44+j145
d) None of the mentioned

Answer: a [Reason:] The input impedance of the diode given reflection coefficient and characteristic impedance is Z0 (1+Гin)/ (1-Гin). Substituting in the given equation, the input impedance is -44 +j123 Ω.

6. If the input impedance of a diode used in the microwave oscillator is 45-j23 Ω, then the load impedance is to achieve stable oscillation is:
a) 45-j23 Ω
b) -45+j23 Ω
c) 50 Ω
d) 23-j45 Ω

Answer: b [Reason:] The condition for stabilized oscillation is Zin=-ZL. According to this equation, the load impedance required for stabilized oscillation is – (45-j23) Ω. The load impedance is thus -45+j23 Ω.

7. To achieve stable oscillation, Zin + ZL=0 is the only necessary and sufficient condition to be satisfied by the microwave oscillator.
a) True
b) False

Answer: b [Reason:] The condition Zin + ZL=0 is only a necessary condition for stable oscillation and not sufficient. Stability requires that any perturbation in current or frequency is damped out, allowing the oscillator to return to its original state.

8. In transistor oscillators, the requirement of a negative resistance device is satisfied using a varactor diode.
a) True
b) False

Answer: b [Reason:] In a transistor oscillator, a negative resistance one port network is created by terminating a potentially unstable transistor with impedance designed to drive the device in an unstable region.

9. In transistor oscillators, FET and BJT are used. Instability is achieved by:
a) Giving a negative feedback
b) Giving a positive feedback
c) Using a tank circuit
d) None of the mentioned

Answer: b [Reason:] Oscillators require a device that has high instability. To achieve this condition, transistors are used with a positive feedback to increase instability.

10. In a transistor amplifier, if the input impedance is -84-j1.9 Ω, then the terminating impedance required to create enough instability is:
a) -84-j1.9 Ω
b) 28+j1.9 Ω
c) – (28+j1.9) Ω
d) None of the mentioned

Answer: b [Reason:] Relation between terminating impedance and input impedance is Zs=-Rin/3. Zs is the terminating impedance. Substituting in the given equation, the terminated impedance is 28+j1.9 Ω.

## Set 5

1. The production of power at higher frequencies is much simpler than production of power at low frequencies.
a) True
b) False

Answer: b [Reason:] As frequency increases to the millimeter and sub millimeter ranges, it becomes increasingly more difficult to produce even moderate power with solid state devices, so microwave tubes become more useful at these higher frequencies.

2. Microwave tubes are power sources themselves at higher frequencies and can be used independently without any other devices.
a) True
b) False

Answer: b [Reason:] Microwave tubes are not actually sources by themselves, but are high power amplifiers. These tubes are in conjunction with low power sources and this combination is referred to as microwave power module.

3. Microwave tubes are grouped into two categories depending on the type of:
a) Electron beam field interaction
b) Amplification method
c) Power gain achieved
d) Construction methods
d) None of the mentioned

Answer: a [Reason:] Microwave tubes are grouped into two categories depending on the type of electron beam field interaction. They are linear or ‘O’ beam and crossed field or the m type tube. Microwave tubes can also be classified as oscillators and amplifiers.

4. The klystron tube used in a klystron amplifier is a _________ type beam amplifier.
a) Linear beam
b) Crossed field
c) Parallel field
d) None of the mentioned

Answer: a [Reason:] In klystron amplifier, the electron beam passes through two or more resonant cavities. The first cavity accepts an RF input and modulates the electron beam by bunching it into high density and low density regions.

5. In crossed field tubes, the electron beam traverses the length of the tube and is parallel to the electric field.
a) True
b) False

Answer: b [Reason:] In a crossed field or ‘m’ type tubes, the focusing field is perpendicular to the accelerating electric field. Since the focusing field and accelerating fields are perpendicular to each other, they are called crossed field tubes.

6. ________ is a single cavity klystron tube that operates as on oscillator by using a reflector electrode after the cavity.
a) Backward wave oscillator
b) Reflex klystron
c) Travelling wave tube
d) Magnetrons

Answer: b [Reason:] Reflex klystron is a single cavity klystron tube that operates as on oscillator by using a reflector electrode after the cavity to provide positive feedback via the electron beam. It can be tuned by mechanically adjusting the cavity size.

7. A major disadvantage of klystron amplifier is:
a) Low power gain
b) Low bandwidth
c) High source power
d) Design complexity

Answer: b [Reason:] Klystron amplifier offers a very narrow operating bandwidth. This is overcome in travelling wave tube (TWT). TWT is a linear beam amplifier that uses an electron gun and a focusing magnet to accelerate beam of electrons through an interaction region.

8. In a _________ oscillator, the RF wave travels along the helix from the collector towards the electron gun.
a) Interaction oscillator
b) Backward wave oscillator
c) Magnetrons
d) None o the mentioned

Answer: b [Reason:] In a backward wave oscillator, the RF wave travels along the helix from the collector towards the electron gun. Thus the signal for oscillation is provided by the bunched electron beam itself and oscillation occurs.

9. Extended interaction oscillator is a ________ beam oscillator that is similar to klystron.
a) Linear beam
b) Crossed beam
c) Parallel beam
d) M beam

Answer: a [Reason:] Extended interaction oscillator is a linear beam oscillator that uses an interaction region consisting of several cavities coupled together, with positive feedback to support oscillation.

10. Magnetrons are microwave devices that offer very high efficiencies of about 80%.
a) True
b) False

Answer: a [Reason:] Magnetrons are capable of very high power outputs, on the order of several kilowatts, and with efficiencies of 80% or more. But disadvantage of magnetron is that they are very noisy and cannot maintain frequency or phase coherence when operated in pulse mode.

11. Klystron amplifiers have high noise output as compared to crossed field amplifiers.
a) True
b) False