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# Multiple choice question for engineering

## Set 1

1. The major disadvantage of single stub tuning is:
a) it requires a variable length of line between the load and the stub
b) it involves 2 variable parameters
c) complex calculation
d) none of the mentioned

Answer: a [Reason:] Single stub matching requires a variable length line between the stub and the load for matching which is a major disadvantage since the length of the stub plays a crucial role in matching.

2. The major advantage of double stub tuning is:
a) it uses 2 tuning stubs in fixed positions
b) it involves 2 stubs
c) length of the stub is variable
d) none of the mentioned

Answer: a [Reason:] The disadvantage of single stub tuning is overcome in double stub tuning. It uses 2 tuning stubs in fixed positions so that the length between the first stub and the load is not variable.

3. In a double stub tuner circuit, the load is of _______ length from the first stub.
a) fixed length
b) arbitrary length
c) depends on the load impedance to be matched
d) depends on the characteristic impedance of the transmission line

Answer: b [Reason:] The position of the first stub in a double stub tuner is variable from the load end. But the distance between the 2 stubs is fixed based on the value to which impedance is matched.

4. Double stub tuners are fabricated in coaxial line are connected in shunt with the main co-axial line.
a) true
b) false

Answer: a [Reason:] Most of the transmission lines used in most of the practical applications use coaxial cables, for which impedance matching of the load are done using double stub tuners which are made of coaxial cables for their best suited properties.

5. Impedance matching with a double stub tuner using a smith chart yields 2 solutions.
a) true
b) false

Answer: a [Reason:] Both single stub tuning and double stub tuning give two solutions. The intersection of the admittance and the 1+jb circle drew on the smith chart yields 2 points from which 2 solutions can be generated.

6. All load impedances can be matched to a transmission line using double stub matching.
a) true
b) false

Answer: a [Reason:] When a smith chart is used for impedance matching, if the normalized load admittance yL were inside the g+jb circle, no value of stub susceptance b1 could ever bring the load point to intersect with the 1+jb circle; this forms a forbidden range of admittance that cannot be matched.

7. The simplest method of reducing the forbidden range of impedances is:
a) increase the distances between the stubs
b) reduce the distance between the stubs
c) increase the length of the stubs
d) reduce the length of the stubs

Answer: b [Reason:] Reducing the distances between the stubs reduces the forbidden area in the smith chart which involves the load impedances that cannot be matched. Thus, more number of load impedances (range) can be matched to the transmission line.

8. Stub spacing that are near 0 and λ/2 lead to more frequency sensitive matching networks.
a) true
b) false

Answer: a [Reason:] Though theoretically the stub spacing must be small enough to reduce the forbidden area, for practical considerations, the stubs have to be placed sufficiently far enough for fabrication ease and reduce frequency sensitivity.

9. The standard stub spacing usually used is:
a) 0, λ/2
b) λ/4, λ/8
c) λ/8, 3λ/8
d) none of the mentioned

Answer: c [Reason:] While stub spacing of 0, λ/2 lead to frequency sensitive matching circuits, an optimum value of spacing is chosen taking into consideration, the various design constraints. This optimum spacing usually used is λ/8, 3λ/8.

10. If the length of the line between the first stub and the load can be adjusted, the admittance can be moved from the forbidden region.
a) true
b) false

Answer: a [Reason:] If the design requirements for impedance matching are more flexible, then the length of the line between the load and the first stub can be varied. This would result in moving the load admittance point out of forbidden region in the smith chart thus enabling impedance matching.

## Set 2

1. The level of coupling required between a resonator and its attached circuitry is a standard and independent of the application where coupling is required.
a) true
b) false

Answer: b [Reason:] The level of coupling required between a resonator and its attached circuitry depends on the application. A waveguide cavity to a frequency meter is loosely coupled to maintain high Q and good accuracy.

2. A measure of the level of coupling between a resonator and a feed is given by:
a) coupling coefficient
b) power transfer coefficient
c) voltage coefficient
d) reflection coefficient

Answer: a [Reason:] Coupling coefficient tells how the resonator is coupled to the external circuitry. A resonator can be coupled in three ways. They can be under coupled, critically coupled or over coupled.

3. To obtain maximum power transfer between a resonator and feed line, the resonator should be matched to the load at:
a) resonant frequency
b) cutoff frequency
c) zero frequency
d) none of the mentioned

Answer: a [Reason:] In order to obtain a maximum power transfer between a resonator and a feed line, the resonator should be matched to the feed line at the resonant frequency of the resonator which is coupled.

4. When impedance matching is done between a resonator and a feed line, the condition for impedance matching is:
a) R=Z0
b) R=Z0/2
c) R=2Z0
d) R=√Z0

Answer: a [Reason:] If R is the resistance of the series RLC circuit and Z0 is the characteristic impedance of the feed line, for proper coupling their impedance has to be matched. This is the condition for impedance matching between a feed line and a resonator.

5. Coupling coefficient Q can be defined as the ratio of unloaded Q to external Q.
a) true
b) false

Answer: a [Reason:] Coupling coefficient Q defined as the ratio of unloaded Q to external Q can be applied to both series resonance and parallel resonance circuits. For series resonant circuits, coupling coefficient is defined as the ratio of characteristic impedance of feed line to the resistance in the resonant circuit.

6. When the coupling coefficient is lesser than one, the resonator is over coupled to the feed line.
a) true
b) false

Answer: b [Reason:] When the coupling coefficient is less than one, the resonator is under coupled to the feed line. For a series resonant circuit, resonator is under coupled implies that the resistance in the resonator is greater than the characteristic impedance of the transmission line.

7. Direct measurement of the unloaded Q of a resonator is not possible.
a) true
b) false

Answer: a [Reason:] Direct measurement of the unloaded Q of a resonator is not possible because of the loading effect of the measurement system, but it is possible to determine unloaded Q from measurements of the frequency response of the loaded resonator when it is connected to a transmission line.

8. For practical applications cavity resonators can be modified as per the requirement of the application where it is used.
a) true
b) false

Answer: a [Reason:] Small changes in the cavity resonator can be made by changing their shape, or by introducing small pieces of dielectric or metallic materials. The resonant frequency of a cavity resonator can be easily tuned with a small screw that enters the cavity volume or by changing the size of the cavity by a movable wall.

9. When coupling coefficient is 1, the resonator is ________ to the feed line.
a) under coupled
b) over coupled
c) critically coupled
d) none of the mentioned

Answer: c [Reason:] The resonator is critically coupled to the feed line when the coupling coefficient is 1. Maximum power is transferred between the resonator and the feed line since the resistance of the resonator is equal to the characteristic impedance of the transmission line.

10. In aperture coupling, a small aperture in the transverse wall of the waveguide acts as:
a) shunt inductance
b) shunt capacitance
c) series inductance
d) series capacitance

Answer: a [Reason:] In aperture coupling, a small aperture is made in the transverse wall of the cavity which is to be coupled to an external microwave circuit. This aperture made in the transverse wall of the cavity acts as a shunt inductance.

## Set 3

1. The major disadvantage of binomial multi section transformer is higher bandwidth cannot be achieved.
a) true
b) false

Answer: a [Reason:] In some applications, a flat curve in the operating frequency is a major requirement. This requirement can be satisfied using a binomial transformer. But the disadvantage is that a higher bandwidth can be achieved.

2. Advantage of chebyshev matching transformers over binomial transformers is:
a) higher gain
b) low power losses
c) higher roll-off in the characteristic curve
d) higher bandwidth

Answer: d [Reason:] Chebyshev transformers when designed to operate at a certain frequency called center frequency, the reflection co-efficient is low for a large frequency range implying that they have a higher operating range. This is the major advantage of chebyshev filters.

3. There are passband ripples present in the chebyshev characteristic curve.
a) true
b) false

Answer: a [Reason:] This is a major difference between chebyshev and binomial transformer. Binomial transformers have a flat curve in the passband while chebyshev transformers have ripples in the transformer passband.

4. Chebyshev matching transformers can be universally used for impedance matching in any of the microwave networks.
a) true
b) false

Answer: b [Reason:] Chebyshev transformers have passband ripples in the characteristic curve. In some critical applications, these ripples are not tolerable in the operating bandwidth. Hence, chebyshev transformers cannot be used for all the microwave networks for impedance matching.

5. The 4th order chebyshev polynomial is:
a) 8x4-8x2+1
b) 4x3-4x2+1
c) 4x3-3x
d) none of the mentioned

Answer: a [Reason:] nth order polynomial for a chebyshev polynomial is generated using lower polynomials by the expression Tn (x) = 2xTn-1(x) – Tn-2(x). T2(x) = 2x2-1, T3(x)= 4x3-3x. Substituting the lower level polynomials in the given expression, T4(x) = 8x4-8x2+1.

6. Chebyshev polynomials do not obey the equal-ripple property.
a) true
b) false

Answer: b [Reason:] For -1≤x≤1,│T(x)│≤ 1. In this range, the chebyshev polynomials oscillate between±1. This is the equal ripple property. Chebyshev polynomials obey the equal-ripple property.

7. Chebyshev polynomial can be expressed in trigonometric functions as:
a) Tn(cos θ)=cos nθ
b) Tn(sin θ)= sin nθ
c) Tn(cos θ)=cos nθ.sin nθ
d) none of the mentioned

Answer: a [Reason:] If the chebyshev polynomial variable x is equated to a trigonometric variable cos θ, then the higher order chebyshev polynomials can be defined in terms of the same function with multiples of θ. This can be theoretically proved and function generation becomes simpler.

8. For values of x greater than 1, the chebyshev polynomial in its trigonometric form cannot be determined.
a) true
b) false

Answer: b [Reason:] Since cosine function is defined for values of x between -1 and +1, for x values greater than 1, hyperbolic function is used to define the chebyshev polynomial. Tn(x)=cosh (n cosh-1x).

9. Reflection co-efficient Гn in terms of Zn and Zn+1, successive impedances of successive sections in the matching network are:
a) 0.5 ln (Zn+1/Zn)
b) 0.5 ln (Zn/Zn+1)
c) ln (Zn+1/Zn)
d) ln (Zn/Zn+1)

Answer: a [Reason:] When multiple sections are used in the chebyshev matching network, the reflection co-efficient of the nth matching section, given the impedances at the ends of the section, reflection co-efficient can be obtained using the expression 0.5 ln (Zn+1/Zn).

10. In a 3 section multisection chebyshev matching network, if Z3 = 100Ω, and Z2=50Ω, then the reflection co-efficient Г2 is:
a) 0.154
b) 0.3465
c) 0.564
d) none of the mentioned

Answer: b [Reason:] Гn for ‘n’ section matching chebyshev network is given by Гn=0.5 ln (Zn+1/Zn). substituting the given values in the expression, Г2 is 0.3465.

11. If Г3=0.2 and Z3=50Ω, then the impedance of the next stage in the multi-section transformer is:
a) 100Ω
b) 50Ω
c) 74.6Ω
d) 22.3Ω

Answer: c [Reason:] Гn for ‘n’ section matching chebyshev network is given by Гn=0.5 ln (Zn+1/Zn). Substituting the given values in the expression, the impedance of the next stage is Z4=74.6Ω.

## Set 4

1. ________ is a three-port microwave device that can be lossless and matched at all spots.
a) Hybrid junction
b) Magic Tee
c) Circulator
d) Isolator

Answer: c [Reason:] A circulator is a three-port microwave device that can be lossless and matched at all ports; by using the unitary properties of scattering matrix it is proved that such a device must be non-reciprocal.

2. The total number of ones in the scattering matrix of an ideal circulator is:
a) 4
b) 3
c) 2
d) 5

Answer: b [Reason:] Since the circulator is matched at all the ports, the diagonal elements are zero. As the circulator allows power flow in only one direction, only one of the elements in each row has a 1 in the matrix. With three rows, there are three ones.

3. There is no method in which the scattering matrix of the opposite circularity can be obtained from the matrix we have.
a) True
b) False

Answer: b [Reason:] By transposing the port indices of the existing matrix, the opposite circulatory can be obtained. For example, if S13 is a 1 in the given circulator, then S31 is automatically one in the opposite circulator.

4. Practically, opposite circulatory in a ferrite circulator can be obtained by:
a) Changing the order of port operation
b) Impedance matching the input ports
c) Changing the polarity of the magnetic bias field
d) None of the mentioned

Answer: c [Reason:] For a ferrite circulator, opposite circulatory can be produced by changing the polarity of the magnetic bias field. This change in polarity causes power to flow in opposite direction but only in one direction.

5. A circulator device can also used as an isolator with a few modifications.
a) True
b) False

Answer: a [Reason:] A circulator can be used as an isolator by terminating one of the circulator ports with known impedance so that the remaining two ports are used for operation. As power flow occurs only in one direction in these two ports, they can be used as isolators.

6. In the scattering matrix representation of a non-ideal circulator, the diagonal elements of the matrix are:
a) Zero
b) One
c) Reflection coefficient Г
d) None of the mentioned

Answer: c [Reason:] In a non-ideal circulator, the three ports of the circulator are not properly matched and hence there will be some reflection back to the same ports. This impedance mismatch can be represented by the reflection co-efficient Г.

7. In a stripline junction circulator, the ferrite material is present in the form of a:
a) Slab
b) Ferrite disk
c) Ferrite material is not used in a microstrip circulator
d) Ferrite cubes

Answer: b [Reason:] In a stripline junction circulator, two ferrite disks fill the space between the center metallic disk and the ground planes of the stripline. Three striplines are attached to the periphery of the center disk and the ground plane on the stripline.

8. The dielectric resonator in the circulator has a single highest order resonant mode.
a) True
b) False

Answer: b [Reason:] In operation of a microstrip circulator, the ferrite disks form a dielectric resonator; in the absence of the bias field this resonator has a single lowest order resonant mode with a cos φ dependence.

9. In the plot of the magnitude of electric field around the periphery of the junction circulator, the curve has:
a) Three peaks
b) Two peaks
c) Four peaks
d) None of the mentioned

Answer: a [Reason:] In the plot of the magnitude of electric field around the periphery of the junction circulator, the curve has three peaks. These three peaks are due to the three ports of the circulator where the field measured is maximum.

10. In the non-ideal scattering matrix representation of the circulator, the attenuation constant and phase constant α, β respectively are approximated as 1.
a) True
b) False

Answer: b [Reason:] In the non-ideal scattering matrix representation of the circulator, the attenuation constant and phase constant α, β respectively are approximated in terms of the reflection coefficient which represents the impedance mismatch in the network. Α is approximated as 1-Г2 and β is approximated as Г.

## Set 5

1. Ferrite isolators are ____ port microwave devices.
a) Two
b) Three
c) Four
d) None of the mentioned

Answer: a [Reason:] Ferrite isolators are two port devices having unidirectional transmission characteristics. Isolator provides isolation between the two ports and the power flow occurs only in one direction.

2. The matrix of an ideal isolator is not ______
a) Unitary
b) Symmetric
c) Lossless
d) None of the mentioned

Answer: a [Reason:] Ferrite isolator allows wave propagation in only one direction and attenuates propagation in the other direction. So the isolator is lossy. Since the isolator is lossy, the scattering matrix of isolator is not unity.

3. A very common application of isolator is to provide isolation between a low power source and the load.
a) True
b) False

Answer: b [Reason:] Isolators have a wide variety of applications. The most common among them is the use of an isolator between a high-power source and a load to prevent the possible reflections from damaging the source. An isolator can be used in place of a matching network, but it should be realized that any power reflected from the source is absorbed by the isolator.

4. The attenuation of a ________ is very large near the gyro magnetic resonance of the ferrite.
a) Linearly polarized wave
b) Circularly polarized wave
c) Left polarized wave
d) Right polarized wave

Answer: b [Reason:] The attenuation of the circularly polarized wave is very large near the gyro magnetic resonance of the ferrite, while the attenuation of the wave propagating in the opposite direction is very small.

5. The isolators constructed using ferrite materials must operate at:
a) Gyro magnetic resonance
b) Magnetic resonance
c) Isolator resonance
d) None of the mentioned

Answer: a [Reason:] The attenuation of the circularly polarized wave is very large near the gyro magnetic resonance of the ferrite, while the attenuation of the wave propagating in the opposite direction is very small. Isolator constructed using ferrite must hence operate at gyro magnetic resonance.

6. Forward attenuation provided by a resonance ferrite isolator is:
a) Zero
b) Low
c) High
d) None of the mentioned

Answer: b [Reason:] Zero forward attenuation cannot be obtained in resonance isolators because the internal magnetic field is not truly circularly polarized. Because of this, there is some amount of forward attenuation in the isolator.

7. An isolator has a very large operating bandwidth and independent of any isolator parameter.
a) True
b) False

Answer: b [Reason:] The bandwidth of an isolator is relatively narrow, dictated essentially by the line width ∆H of the ferrite material.

8. The length of a ferrite slab required operating with a minimum forward insertion loss and 30 dB reverse attenuation and the reverse attenuation at this point is:
a) 3 cm
b) 2.4 cm
c) 4 cm
d) 3.6 cm

Answer: b [Reason:] Length of the ferrite slab required is equal to the ratio of the minimum forward insertion loss to the reverse attenuation at the point. Substituting the given values in the above equation, length of the ferrite slab is 2.4 cm.

9. The electric field distribution of the forward and reverse waves in a ferrite slab-loaded waveguide is quite different. This property is used in:
a) Field displacement resonator
b) Resonance isolator
c) Waveguide isolator
d) None of the mentioned