Chapter 8 - Moving Target Indication

Question 1

Explain Klystron

Answer 1

o Electron Gun: Emits a beam of electrons.
o Input Cavity (‘Buncher’): The input RF signal is fed to this cavity, causing velocity modulation of the electron beam. The electrons are accelerated or decelerated based on the input signal’s phase.
o Drift Space: The modulated electrons drift down the tube, where faster electrons catch up with slower ones, forming bunches.
o Output Cavity (‘Catcher’): The electron bunches pass through this cavity, inducing a strong RF signal, which is extracted and sent to the output.
o Collector: The remaining kinetic energy in the electron beam is absorbed as heat and dissipated.

Question 2

Explain TWT

Answer 2

o Electron Gun: Emits a beam of electrons.
o Slow Wave Structure (SWS): A helix or coupled cavities slow down the RF signal so it can interact continuously with the electron beam.
o Microwave Input: A low-power microwave signal is introduced at the input end.
o Velocity Modulation: The continuous interaction causes velocity modulation of the electron beam, amplifying the signal.
o Microwave Output: The amplified signal is extracted at the output.
o Collector: Absorbs remaining kinetic energy in the electron beam as heat, which is dissipated.

Question 3

compare magnetron, klystron and TWT

Answer 3

Magnetron:
o Advantages: High efficiency, small size, low cost.
o Disadvantages: Frequency stability issues, unsuitable for coherent applications, short pulse transmission only.
Klystron:
o Advantages: Good frequency stability, high power output, coherent operation.
o Disadvantages: Large size, high cost, complex cooling requirements, limited bandwidth due to resonant cavities.
TWT:
o Advantages: Wide bandwidth, high gain, continuous interaction for signal amplification.
o Disadvantages: Complex cooling requirements, larger size compared to solid-state devices, higher cost.

Question 4

Draw MTI

Answer 4

Question 5

35. Draw the filter response (frequency vs amplitude) of a single delay and double delay line cancellers including the clutter response in order to explain why we require multiple pulse cancellers in MTI radar.

Answer 5

Question 6

36. What are the two main problems with MTI radar and how are they overcome?

Answer 6

Blind Speeds:
Problem: This occurs when the target Doppler frequency is equal to or a multiple of the PRF, making moving targets appear stationary.
Solution: Use staggered PRF to shift the blind speeds, ensuring that the target’s Doppler frequency does not match the PRF.

Blind Phase:
Problem: This occurs when successive pulses from a moving target have the same amplitude, causing cancellation in the delay line canceller and creating nulls.
Solution: Vector Processing, which uses quadrature channels (I and Q channels) with a 90° phase shift to ensure that nulls do not occur simultaneously, improving target detection.