# Microwave Engineering MCQ || Microwave Engineering Questions and Answers

1. The bit rate of the digital communication system is M kbps. The modulation used is 16 QAM. The minimum bandwidth required for ideal transmission is _________

1. M/2 kHz
2. M/16 kHz
3. M kHz
4. M/8 kHz

Explanation

In the M-Array modulation scheme, the minimum bandwidth required for ideal transmission is given by:

${\left( {BW} \right)_{min}} = \frac{{{2R_b}}}{{{{\log }_2}N}}Hz$

Where,

Rb = bit rate in bps

N = number of levels in M-Array scheme

Calculation:

Given that,

Bit rate = M kbps

Number of levels = N = 16

$\therefore {\left( {BW} \right)_{{\rm{min}}}} = \frac{{{2R_b}}}{{{{\log }_2}N}}Hz = \frac{2M}{{{{\log }_2}16}}kHz$

$= \frac{2M}{{{{\log }_2}{2^4}}}kHz$

$= \frac{2M}{{4\; \times\; {{\log }_2}2}}kHz$

$(BW)_{min}= \frac{M}{{2}}kHz$

So. The minimum bandwidth will be M/2 kHz, for ideal transmission.

2. Assertion (A): In microwave communication links, intensive fading at the 18 GHz band occurs due to rain-drop attenuation.

Reason (R): Collective scattering from water droplets in the atmosphere will result in the diminution of energy in the forward path and this is maximum at the 18 GHz band because the criterion for scattering are more satisfied by the wavelength dimensions at these frequencies.

1. Both A and R individually true and R is the correct explanation of A
2. Both A and R are individually true but R is not the correct explanation of A
3. A is true but R is false
4. A is false but R is true

Answer.1. Both A and R individually true and R is the correct explanation of A

Explanation

Rain fade refers primarily to the absorption of a microwave radio frequency (RF) signal by atmospheric rain, snow, or ice, and losses which are especially prevalent at frequencies above 11 GHz.

It also refers to the degradation of a signal caused by the electromagnetic interference of the leading edge of a storm front. Rain fade can be caused by precipitation at the uplink or downlink location.

It is practically observed at 18 GHz it is maximum. So both statements A and R are true and R is the correct explanation of A.

3. Which of the following devices cannot be used as microwave oscillators?

1. IMPATT diode
2. PN Junction diode
3. TRAPATT diode
4. BARRITT diode

Explanation

1. Microwave oscillators are semiconductor devices that generator microwave radiation at a specific frequency only. Example: ring oscillator, I/Q oscillator, etc.

2. Microwave resonators are widely used in microwave oscillators, microwave narrowband amplifiers, and microwave frequency meters because resonators have very narrow frequency bands around the resonant frequency and the duality factor value is also high in the resonator circuit.

PN junction diodes cannot be used as microwave oscillators as they have a very high reverse recovery time compared to IMPATT, TRPATT, and BARRITT diodes.

4. The ‘Double minimum’ or the “Width of minimum power” method is used in microwave measurements for the measurement of:

1. Velocity modulation
2. Frequency distortion
3. High V.S.W.R.
4. Low V.S.W.R.

Explanation

Double minimum method- The measurement of high VSWR  whose value is greater than 10 can be measured by a method called the double minimum method. In this method, the reading at the minimum value is taken, and the readings at the half point of the minimum value in the crest before and the crest after are also taken.

YSWR can be calculated by using following formula:

YSWR = $\frac{{{\lambda _g}}}{{\pi \left( {{d_2} – {d_1}} \right)}}$

λ = c/f

λc = cutoff wavelength

5. The measurement of low VSWR can be done by adjusting the _______

1. Capacitor
2. Inductor
3. Resistor
4. Attenuator

Explanation

The measurement of low VSWR can be done by adjusting the attenuator to get a reading on a DC millivoltmeter which is a VSWR meter.

6. Microwave is a form of _______

4. None of the above

Explanation

A microwave is a form of electromagnetic radiation with wavelengths ranging from about one meter to one millimeter corresponding to frequencies between 300 MHz and 300 GHz respectively.

Radio waves are EM (Electromagnetic)waves that have wavelengths between 1 millimeter and 100 kilometers (or 300 GHz and 3 kHz in frequency). So Microwaves are electromagnetic waves with wavelengths longer than those of terahertz (THz) wavelengths, but relatively short for radio waves.

7. A microwave tube amplifier uses an axial magnetic field and a radial electric field. This is a

1. Reflex klystron
2. Co-axial magnetron
3. Traveling wave magnetron
4. CFA

Explanation

• The Crossed Field Amplifier (CFA) is a broadband microwave amplifier that can also be used as an oscillator.
• It is a microwave tube amplifier that uses an axial magnetic field and radial electric field
• Its operation is similar in operation to the magnetron and can provide relatively large amounts of power with high efficiency.

Travelling Wave Tube:

• A specialized vacuum tube is used in electronics to amplify radio frequency (RF) signals in the microwave range.
• The TWT belongs to a category of “linear beam” tubes, such as the klystron, in which the radio wave is amplified by absorbing power from a beam of electrons as it passes down the tube.
• A major advantage of the TWT over some other microwave tubes is its ability to amplify a wide range of frequencies i.e. a large bandwidth.
• The bandwidth of the helix TWT can be as high as two octaves, while the cavity versions have bandwidths of 10% – 20%.
• Operating frequencies range from 300 MHz to 50 GHz.
• The power gain of the tube is on the order of 40 to 70 decibels, and output power ranges from a few watts to megawatts.

8. The frequency range of the Ku band is:

1. 4-8 GHz
2. 18-27 GHz
3. 2-4 GHz
4. 12-18 GHz

Explanation

• The microwave spectrum is usually defined as a range of frequencies ranging from 1 GHz to over 300 GHz.
• The Ku band represents the frequency range of 12 to 18 GHz
• This range has been divided into a number of frequency bands, each represented by a letter.

The commonly used microwave frequency spectrum is as shown:

 BAND FREQUENCY RANGE (GHz) L-Band 1 to 2 GHz S-Band 2 to 4 GHz C-Band 4 to 8 GHz X-Band 8 to 12 GHz Ku-Band 12 to 18 GHz K-Band 18 to 26.5 GHz Ka-Band 26.5 to 40 GHz

9. ________ is the most widely used microwave amplifier.

1. Coaxial magnetron
2. TWT
3. Klystron
4. None of the above

Explanation

A Two cavity klystron is the most widely used microwave amplifier operated by the principle of “velocity and current modulation”.

Working:

• A high-speed electron beam travel through the first cavity and the electrons are accelerated or deaccelerated depending on their time of entry.
• These electrons of unequal velocity form groups or bunches, and travel with various thicknesses of electron beams.
• This bunch of electrons feeds the second cavity and oscillation occurs.

Klystrons are O-type microwave tubes.

Transfer electron device (TEDs):

• TEDs are negative resistance two-terminal solid-state devices.
• TEDs are bulk devices with no junction.
• Operates with hot electrons.
• Fabricated from Gas, GTe, etc.

Metal-semi conductor field-effect transistor (MESFET):

• It is FET with a metal-semiconductor Schottky diode.
• The mainly used material is GaAs because it has higher electron mobility, higher electron saturation drift velocity, than Si so output power is greater and the noise figure is low.
• It is based on pinch-off conditions.

Traveling – wave tube (TWT):

• It is a slow structure non-resonant microwave circuit, in which waves propagate with the same speed as the e beam.
• In TWT interaction of ebeam and Rf field is continuous over the entire length of the circuit.

• Klystron is based on bunching effects.
• TEDs are not electron
• MESFE is based on the pinch-off effect.
• TWT is a slow-wave structure.

10. Which of the following microwave tube amplifiers uses an axial magnetic field and radial electric field?

1. CFA
2. Coaxial magnetron
3. Traveling wave magnetron
4. Reflex klystron