Antenna and Microwave Engineering MCQs with answers Page - 13

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Q. In a oscillator, the RF wave travels along the helix from the collector towards the electron gun.

• (A) interaction oscillator
• (B) backward wave oscillator
• (C) magnetrons
• (D) none o the mentioned

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Correct answer - Option (B)

Explanation:

in a backward wave oscillator, the rf wave travels along the helix from the collector towards the electron gun. thus the signal for oscillation is provided by the bunched electron beam itself and oscillation occurs.

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Q. Magnetrons are microwave devices that offer very high efficiencies of about 80%.

• (A) true
• (B) false
• (C) ---
• (D) ---

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Correct answer - Option (A)

Explanation:

magnetrons are capable of very high power outputs, on the order of several kilowatts, and with efficiencies of 80% or more. but disadvantage of magnetron is that they are very noisy and cannot maintain frequency or phase coherence when operated in pulse mode.

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Q. Klystron amplifiers have high noise output as compared to crossed field amplifiers.

• (A) true
• (B) false
• (C) ---
• (D) ---

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Correct answer - Option (B)

Explanation:

crossed filed amplifiers have very good efficiencies – up to 80%, but the gain is limited to 10-15 db) in addition, the cfa has a noisier output than either a klystron amplifier or twt. its bandwidth can be up to 40%.

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Q. is a microwave device in which the frequency of operation is determined by the biasing field strength.

• (A) vtm
• (B) gyratron
• (C) helix bwo
• (D) none of the mentioned

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Correct answer - Option (B)

Explanation:

gyratron is a microwave device in which the frequency of operation is determined by the biasing field strength and the electron velocity, as opposed to the dimensions of the tube itself. this makes the gyrator especially useful for microwave frequencies.

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Q. 2 IMPEDANCE MATCHING

• (A) swr, first voltage minimum
• (B) swr, first voltage maximum
• (C) characteristic impedance, first voltage minimum
• (D) characteristic impedance, first voltage maximum

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Correct answer - Option (A)

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.

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Q. A modern device that replaces a slotted line is:

• (A) digital cro
• (B) generators
• (C) network analyzers
• (D) computers

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Correct answer - Option (C)

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.

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Q. 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

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Correct answer - Option (A)

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.

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Q. High gain is not achievable at microwave frequencies using BJT amplifiers because:

• (A) device construction
• (B) complex architecture
• (C) ports are not matched at high frequencies
• (D) none of the mentioned

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Correct answer - Option (C)

Explanation:

at higher frequencies, if higher bandwidth is desired, a compromise on maximum achievable gain is made. but at these higher frequencies, the ports of the amplifier are not matched to 50 Ω.

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Q. To flatten the gain response of a transistor:

• (A) biasing current has to be increased
• (B) input signal level has to increased
• (C) increase the operational bandwidth
• (D) give negative feedback to the amplifier

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Correct answer - Option (D)

Explanation:

negative feedback can be used to increase the gain response of the transistor, improve the input and output match, and increase the stability of the device.

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Q. In conventional amplifiers, a flat gain response is achieved at the cost of reduced gain. But this drawback can be overcome by using:

• (A) balanced amplifiers
• (B) distributed amplifiers
• (C) differential amplifiers
• (D) none of the mentioned

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Correct answer - Option (A)

Explanation:

in conventional amplifiers, a flat gain response is achieved at the cost of reduced gain. but this drawback can be overcome by using balanced amplifiers. this is overcome by using two 900 couplers to cancel input and output reflections from two identical amplifiers.

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