# Radar Questions and Answers – Magnetron & Crossed Field Amplifiers

This set of Radar Multiple Choice Questions & Answers (MCQs) focuses on “Magnetron & Crossed Field Amplifiers”.

1. Magnetron has been the only high-power RF power source used for radar that is a power oscillator rather than a power amplifier.
a) True
b) False

Explanation: It is true since it is a crossed field device where its electric and magnetic field are perpendicular to one another. The compact size and efficient operation of the magnetron at microwave frequencies allowed radars to be small enough to fly in military aircraft, be mobile for ground warfare, and even be used on submarines.

2. Which device can be used to change the frequency of a coaxial magnetron by moving one of the end plates?
a) Coaxial cavity
b) Tuning piston
c) Coupling slots
d) Vanes

Explanation: Tuning piston can be used to change the frequency of a coaxial magnetron by mechanically moving one of the end plates. The tuning piston can be positioned mechanically from outside the vacuum by means of a vacuum bellows.

3. Magnetron can shift, almost unpredictably, from one mode to another because of change in ____________
a) power
b) frequency
c) voltage
d) coupling

Explanation: Magnetron can shift, almost unpredictably, from one mode to another because of change in voltage or as the input impedance that the magnetron sees changes. The shift from one mode to another is especially bad since it can occur when the radar antenna scans and views different environments. It is important to avoid moding.

4. What is the preferred magnetron mode of operation?
a) Circular mode
b) Rectangular mode
c) σ mode
d) π mode

Explanation: The preferred magnetron mode of operation is π mode that occurs when the RF field configuration is such that the RF phase alternates 180° (π radians) between adjacent cavities. The advantage of the π mode is that its frequency can be more readily separated from the frequencies of the other possible modes.

5. The power that can be produced by a magnetron depends on its ________
a) size
b) efficiency
c) output
d) speed

Explanation: The power that can be produced by a magnetron depends on its size. A larger size means more resonators, which then makes it more difficult to separate the various modes of oscillation in a conventional magnetron. The coaxial magnetron permits stable operation with a larger number of resonant cavities, and thus with greater power.

6. The anode and cathode structures of a coaxial magnetron can be smaller than those of a conventional magnetron, which further allows operation at larger powers.
a) True
b) False

Explanation: The anode and cathode structures of a coaxial magnetron can be bigger than those of a conventional magnetron, which further allows operation at larger powers. The larger structure permits more conservative design, with the result that the coaxial magnetron exhibits longer life and better reliability than conventional magnetrons.

7. Although the magnetron can produce a peak power of several megawatts, its average power is limited to about _______
a) 4 – 5 kW
b) 5 – 8 kW
c) 1 – 2 kW
d) 5 – 10 kW

Explanation: Although the magnetron can produce a peak power of several megawatts, its average power is limited to about 1 – 2 kW. This may be sufficient for some medium range radars and for civilian air-traffic control radars that use large antennas, but it is not large enough for many military radar applications.

8. In cross field amplifier (CFA), electrons are emitted from the cylindrical cathode, which is coaxial to the RF slow-wave circuit that acts as the ___________
a) collector
b) amplifier
c) stabilizer
d) anode

Explanation: The electrons are emitted from the cylindrical cathode, which is coaxial to the RF slow-wave circuit that acts as the anode. The electrons, under the action of the crossed electric and magnetic fields, form into rotating electron bunches, or spokes. These bunches drift along the slow-wave circuit in phase with the RF signal.

9. In high power cross field amplifier, rather than thermionic emission with a heated cathode, the electrons can be generated by ____________
a) cold cathode field emission
b) hot cathode field emission
c) warm cathode field emission
d) neutral cathode field emission

Explanation: In high power cross field amplifier, rather than thermionic emission with a heated cathode, the electrons can be generated by cold cathode field emission. Cold cathode field emission requires the presence of both the d-c voltage between cathode and anode as well as the RF drive signal applied to the tube.

10. The insertion loss in cross field amplifier is less than ____________
a) 0.2 dB
b) 0.5 dB
c) 0.8 dB
d) 0.1 dB

Explanation: The insertion loss in cross field amplifier is less than 0.5 dB. Sometimes this can be an advantage in a multistage transmitter. By omitting the application of d-c- voltage to the final stage of a multistage transmitter, the lower level RF drive can be fed directly through the final stage with little attenuation.

Sanfoundry Global Education & Learning Series – Radar.

To practice all areas of Radar, here is complete set of Multiple Choice Questions and Answers.

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