# Microwave Engineering Questions and Answers – Double Stub Tuning

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

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

Explanation: 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

Explanation: 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

Explanation: 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

Explanation: 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

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

Explanation: 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

Explanation: 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

Explanation: 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

Explanation: 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