This set of Microwave Engineering Multiple Choice Questions & Answers (MCQs) focuses on “Reflections”.

a) True

b) False

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Explanation: Discontinuities in the matching network cause reflections which result in considerable attenuation of the transmitted signal. Hence, discontinuities in transformers are not negligible.

2. The overall reflection coefficient of a matching quarter wave transformer cannot be calculated because of physical constraints.

a) True

b) False

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Explanation: Though the computation of total reflection is complex, the total reflection can be computed in two ways. They are the impedance method and the multiple reflection method.

3. In the multiple reflections analysis method, the total reflection is:

a) An infinite sum of partial reflections

b) An infinite sum of partial reflection and transmissions

c) Constant value

d) Finite sum of partial reflections

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Explanation: The number of discontinuities in the matching circuit (quarter wave transmission line) is theoretically infinite since the exact number cannot be practically determined. Hence, the total reflection is an infinite sum of partial reflections and transmission.

4. The expression for total reflection in the simplified form is given by:

a) Г=Г_{1}+ Г_{3}e^{-2jθ}

b) Г=Г1_{1}+Г_{3}

c) Г=Г_{12}+ Г_{3}e^{-2jθ}

d) Г= Г_{1}+ Г_{2}e^{-2jθ}

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Explanation: This expression dictates that the total reflection is dominated by the reflection from the initial discontinuity between Z1 and Z2 (Г

_{1}), and the first reflection from the discontinuity between Z2 and ZL (Г

_{3}e

^{-2jθ}).

^{-2jθ}term in the expression for total reflection in a single section quarter wave transformer impedance matching network Г=Г

_{1}+ Г

_{3}e

^{-2jθ}signifies:

a) Phase delay

b) Frequency change

c) Narrowing bandwidth

d) None of the mentioned

View Answer

Explanation: The term e

^{-2jθ}in Г=Г

_{1}+ Г

_{3}e

^{-2jθ}accounts for phase delay when the incident wave travels up and down the line. This factor is a result of multiple reflections.

6. If the first and the third reflection coefficients of a matched line is 0.2 and 0.01, then the total reflection coefficient if quarter wave transformer is used for impedance matching is:

a) 0.2

b) 0.01

c) 0.21

d) 0.19

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Explanation: The total reflection co-efficient of a matched line due to discontinuities is given by Г=Г

_{1}+ Г

_{3}e

^{-2jθ}. Given that Г

_{1}=0.2 and Г

_{3}=0.01, β=2π/λ, l=λ/4. θ=βl, Substituting the given values in the above 2 given equations, the total reflection coefficient is 0.19.

7. If a λ/4 transmission line is used for impedance matching, then always Г_{1}> Г_{3}.

a) True

b) False

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Explanation: Since the load is matched to the transmission line the reflection from the load towards the source will be very less (Г

_{3}). Г

_{1}is the reflection from the junction of the transmission line and the λ/4 matching section. Since this end will have some improper matching and discontinuities, Г

_{1}is always greater than Г

_{3}.

8. To compute the total reflection of a multi-section transmission line, the lengths of the transmission lines considered are all unequal.

a) True

b) False

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Explanation: The computation of total reflection of a matched line due to discontinuities is theoretically complex. In order to obtain an approximated simple expression, the lengths of the multi section matching transformers is a constant or all of them are equal.

_{L}< Z

_{0}, then the reflection coefficient at that junction is:

a) Г

_{N}<0

b) Г

_{N}>0

c) Г

_{N}>1

d) None of the mentioned

View Answer

Explanation: When there is no proper matching between load impedance and the characteristic impedance of a transmission line and given the condition that Z

_{L}< Z

_{0}, then the reflection coefficient at that junction is always negative. That is, Г

_{N}<0.

10. The total approximate reflection coefficient is a finite sum of reflection co-efficient of individual matching section of the matching network.

a) True

b) False

View Answer

Explanation: In a multi section transformer there are N sections, if the reflection from each section is Г

_{N}, then the total reflection is the sum of reflections that occur due to individual sections. There is an exponential component associated with each reflection coefficient that decays exponentially.

11. Using the relation for total reflection co-efficient certain designs of matching networks can be made as per practical requirements.

a) True

b) False

View Answer

Explanation: We can synthesize any desired reflection coefficient response as a function of frequency by properly choosing the Г

_{N}and using enough sections (N).

**Sanfoundry Global Education & Learning Series – Microwave Engineering.**

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