# Microwave Engineering Questions and Answers – Single Stub Matching

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

1. The major advantage of single stub tuning over other impedance matching techniques is:
a) Lumped elements are avoided
b) It can be fabricated as a part of transmission line media
c) It involves two adjustable parameters
d) All of the mentioned

Explanation: Single stub matching does not involve any lumped elements, it can be fabricated as a part of transmission media and it also involves to adjustable parameters namely length and distance from load giving more flexibility.

2. Shunt stubs are preferred for:
a) Strip and microstrip lines
b) Coplanar waveguides
c) Circular waveguide
d) Circulators

Explanation: Since microstrip and strip lines are simple structures, impedance matching using shunt stubs do not increase the complexity and structure of the transmission line. Hence, shunt stubs are preferred for strip and microstrip lines.

3. The two adjustable parameters in single stub matching are distance‘d’ from the load to the stub position, and _________
a) Susceptance or reactance provided by the stub
b) Length of the stub
c) Distance of the stub from the generator
d) None of the mentioned

Explanation: Reactance or susceptance of the matching stub must be known before it used for matching, since it is the most important parameter for impedance matching between the load and the source.
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4. In shunt stub matching, the key parameter used for matching is:
a) Admittance of the line at a point
c) Impedance of the stub

Explanation: In shunt stub tuning, the idea is to select d so that the admittance Y, seen looking into the line at distance d from the load is of the form Yₒ+jb) Then the stub susceptance is chosen as –jB, resulting in a matched condition.

5. For series stub matching, the parameter used for matching is:
a) Impedance of the transmission line at a point
b) Voltage at a point on the transmission line
c) Admittance at a point on the transmission line

Explanation: In series sub matching, the distance‘d’ is selected so that the impedance, Z seen looking into the line at a distance‘d’ from the load is of the form Zₒ+jX. Then the stub reactance is chosen as –jX resulting in a matched condition.

6. For co-axial lines and waveguides, ________ is more preferred.
a) Open circuited stub
b) Short circuited stub
c) Slotted section
d) Co-axial lines cannot be impedance matched

Explanation: For co-axial cables and waveguides, short-circuited stub is usually preferred because the cross-sectional area of such an open-circuited line may be large enough to radiate, in which case the stub is no longer purely reactive.

7. For a load impedance of ZL=60-j80. Design of 2 single-stub shunt tuning networks to match this load to a 50Ω line is to be done. What is the normalized admittance obtained so as to plot it on smith chart?
a) 1+j
b) 0.3+j0.4
c) 0.4+j0.3
d) 0.3-j0.4

Explanation: To impedance match a load to a characteristic impedance of the transmission line, first the load has to be normalized. That is, zL=ZL/Z0. For impedance matching using shunt stubs, admittance is used. Taking the reciprocal of impedance, normalized load admittance is 0.3+j0.4.

8. If the normalized admittance at a point on a transmission line to be matched is 1+j1.47. Then the normalized susceptance of the stub used for shunt stub matching is:
a) 1Ω
b) 1.47 Ω
c) -1.47 Ω
d) -1 Ω

Explanation: When shunt stubs are used for impedance matching between a load and transmission line, the susceptance of the shunt stub must be negative of the line’s susceptance at that point for impedance matching.

9. After impedance matching, if a graph is plot with frequency v/s reflection co-efficient of the transmission line is done, then at the frequency point for which the design is done, which of the following is true?
a) There is a peak at this point of the curve
b) There is a dip at this point of the curve
c) The curve is a straight line
d) Such a plot cannot be obtained

Explanation: Since the plot is frequency v/s reflection co-efficient, after impedance matching the reflection co-efficient will be zero or minimum. Hence, there is a dip at that point of the curve.

10. In series stub matching, if the normalized impedance at a point on the transmission line to be matched is 1+j1.33. Then the reactance of the series stub used for matching is:
a) 1 Ω
b) -1.33 Ω
c) -1 Ω
d) 1.33 Ω

Explanation: The reactance of the series stub is negative of the reactance of the line at the point at which it has to be matched. That is, if the line reactance is inductive, the series stub’s reactance is capacitive.

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