# Antenna Measurements Questions and Answers – Standing Wave Ratio Method

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This set of Antennas Multiple Choice Questions & Answers (MCQs) focuses on “Standing Wave Ratio Method”.

1. Which of the following statement is true?
a) Standing waves occur when mismatches occurs
b) Standing waves occur when no mismatches occurs
c) Minimum value of SWR is 0
d) Maximum value of SWR is 1

Explanation: When there is mismatch, reflections occur and this causes for the occurrence of the standing waves. The standing wave ratio is defined as the maximum voltage to minimum voltage.
$$S=\frac{V_{max}}{V_{min}} =\frac{1+ρ}{1-ρ}$$ And reflection coefficient $$ρ=\frac{P_{ref}}{P_{inc}}.$$ Therefore S ranges from 1 to infinity.

2. Relation between SWR ‘S’ and reflection coefficient ‘ρ’ is given by _________
a) $$S=\frac{1+ρ}{1-ρ}$$
b) $$S=\frac{1-ρ}{1+ρ}$$
c) $$S=\frac{1+ρ}{ρ}$$
d) $$S=\frac{ρ}{1-ρ}$$

Explanation: Standing waves occur when there is mismatch or reflection due to uneven surfaces. The standing wave ratio is defined as the maximum voltage to minimum voltage and its relation with the reflection coefficient is given as
$$S=\frac{V_{max}}{V_{min}} =\frac{1+ρ}{1-ρ}.$$

3. If there are no any reflections, then the value of the SWR will be ________________
a) 1
b) 0
c) ∞
d) 2

Explanation: The standing wave ratio is defined as the maximum voltage to minimum voltage.
$$S=\frac{V_{max}}{V_{min}} =\frac{1+ρ}{1-ρ}$$
And reflection coefficient is $$ρ=\frac{P_{ref}}{P_{inc}} =0$$ (Since there are no reflections).
$$S=\frac{1+ρ}{1-ρ}=1$$
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4. The value of the SWR when the reflection power is equal to the incident power is ____________
a) 1
b) 0
c) ∞
d) 2

Explanation: The Reflection coefficient is $$ρ=\frac{P_{ref}}{P_{inc}} =1$$ (Since there are reflections power and incident power are equal). The SWR S is given by
$$S=\frac{1+ρ}{1-ρ}=\frac{1}{0}=∞$$

5. Which of the following level can be measurable by the Low VSWR?
a) 15
b) 11
c) 9
d) 20

Explanation: The low VSWR measurement is able to measure the readings of the VSWR not exceeding 10. For High VSWR measurements (S>10) there is a special method called double minimum method.

6. The empirical formula for the measurement of the high VSWR is give by ____
a) $$VSWR =\frac{λ_g}{π(d_2-d_1)}$$
b) $$VSWR =\frac{λ_c}{π(d_2-d_1)}$$
c) $$VSWR =\frac{λ_g}{2(d_2-d_1)}$$
d) $$VSWR =\frac{2λ_g}{(d_2-d_1)}$$

Explanation: High VSWR measurements (S>10) there is a special method called double minimum method. $$VSWR =\frac{λ_g}{π(d_2-d_1)}$$ where d2, d1 are the readings noted on the slotted line where dips (3-db power) is observed.

7. Which of the following is not used in the VSWR measurement?
a) Reflective Klystron
b) Slotted line
c) Frequency meter
d) Spectrum analyzer

Explanation: Spectrum analyzer frequency domain instrument and is contrast to the oscilloscope which displays in time domain. The Klystron acts as a microwave source and the by moving the slotted line the maximum reading on the meter is observed. For high VSWR the frequency of operation is noted using the frequency meter.

8. The relation between the λg, λc, λ0 is given by ____
a) $$\frac{1}{λ_0} =\sqrt{\frac{1}{λ_g^2} +\frac{1}{λ_c^2}}$$
b) $$\frac{1}{λ_g}=\frac{1}{λ_0^2} -\frac{1}{λ_c^2}$$
c) $$\frac{1}{λ_0} =\sqrt{\frac{1}{λ_g^2} -\frac{1}{λ_c^2}}$$
d) $$\frac{1}{λ_g}=\frac{1}{λ_0^2} +\frac{1}{λ_c^2}$$

Explanation: The relation between the λg, λc, λ0 is given by $$\frac{1}{λ_g^2} =\frac{1}{λ_0^2} -\frac{1}{λ_c^2}$$
⇨ $$\frac{1}{λ_0} =\sqrt{\frac{1}{λ_g^2} +\frac{1}{λ_c^2}}$$
and $$λ_0=\frac{c}{f}.$$

Sanfoundry Global Education & Learning Series – Antennas.