# Microwave Engineering Questions and Answers – Impedance Matching Using Slotted Lines

This set of Microwave Engineering Interview Questions and Answers for freshers focuses on “Impedance Matching Using Slotted Lines”.

1. Slotted line is a transmission line configuration that allows the sampling of:
a) electric field amplitude of a standing wave on a terminated line
b) magnetic field amplitude of a standing wave on a terminated line
c) voltage used for excitation
d) current that is generated by the source

Explanation: Slotted line allows the sampling of the electric field amplitude of a standing wave on a terminated line. With this device, SWR and the distance of the first voltage minimum from the load can be measured, from this data, load impedance can be found.

2. A slotted line can be used to measure _____ and the distance of _____________ from the load.
a) SWR, first voltage minimum
b) SWR, first voltage maximum
c) characteristic impedance, first voltage minimum
d) characteristic impedance, first voltage maximum

Explanation: With a slotted line, SWR and the distance of the first voltage minimum from the load can be measured, from this data, load impedance can be found.

3. A modern device that replaces a slotted line is:
a) Digital CRO
b) generators
c) network analyzers
d) computers

Explanation: Although slotted lines used to be the principal way of measuring unknown impedance at microwave frequencies, they have largely been superseded by the modern network analyzer in terms of accuracy, versatility and convenience.

4. If the standing wave ratio for a transmission line is 1.4, then the reflection coefficient for the line is:
a) 0.16667
b) 1.6667
c) 0.01667
d) 0.96

Explanation: ┌= (SWR-1)/ (SWR+1). Substituting for SWR in the above equation for reflection co-efficient, given SWR is 1.4, reflection co-efficient is 0.16667.

5. If the reflection coefficient of a transmission line is 0.4, then the standing wave ratio is:
a) 1.3333
b) 2.3333
c) 0.4
d) 0.6

Explanation: SWR= (1+┌)/ (1-┌). Where ┌ is the reflection co-efficient. Substituting for the reflection co-efficient in the equation, SWR is 2.3333.
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6. Expression for ϴ means phase angle of the reflection co efficient r=|r|-e^jθ, the phase of the reflection co-efficient is:
a) θ=2π+2βLmin
b) θ=π+2βLmin
c) θ=π/2+2βLmin
d) θ=π+βLmin

Explanation: here, θ is the phase of the reflection co-efficient. Lmin is the distance from the load to the first minimum. Since voltage minima repeat every λ/2, any multiple of λ/2 can be added to Lmin .

7. In the expression for phase of the reflection coefficient, Lmin stands for :
a) distance between load and first voltage minimum
b) distance between load and first voltage maximum
c) distance between consecutive minimas
d) distance between a minima and immediate maxima

Explanation: Lmin is defined as the distance between the terminating load of a transmission line and the first voltage minimum that occurs in the transmission line due to reflection of waves from the load end due to mismatched termination.

8. If SWR=1.5 with a wavelength of 4 cm and the distance between load and first minima is 1.48cm, then the reflection coefficient is:
a) 0.0126+j0.1996
b) 0.0128
c) 0.26+j0.16
d) none of the mentioned

Explanation: ┌= (SWR-1)/ (SWR+1). Substituting for SWR in the above equation for reflection co-efficient, magnitude of the reflection co-efficient is 0.2. To find θ, θ=π+2βLmin, substituting Lmin as 1.48cm, θ=86.4⁰. Hence converting the polar form of the reflection co-efficient into rectangular co-ordinates, reflection co-efficient is 0.0126+j0.1996.

9. If the characteristic impedance of a transmission line 50 Ω and reflection coefficient is 0.0126+j0.1996, then load impedance is:
a) 47.3+j19.7Ω
b) 4.7+j1.97Ω
c) 0.26+j0.16
d) data insufficient

Explanation: ZL=Z0 (1+┌)/ (1-┌). Substituting the given values of reflection co-efficient and characteristic impedance, ZL is 47.3+j19.7Ω .

10. If the normalized load impedance of a transmission line is 2, then the reflection co-efficient is:
a) 0.33334
b) 1.33334
c) 0
d) 1

Explanation: ZL=Z0 (1+┌)/ (1-┌), this is the expression for load impedance. Normalized load impedance is the ratio of load impedance to the characteristic impedance, taking ZLL/Z0 as 2, the reflection co-efficient is equal to 0.33334.

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