CMRT Power Systems & Interference Mitigation 1 — Questions and Answers
Question 1: What is the typical magnetron anode voltage used in a commercial marine radar transmitter?
- 12–24 VDC
- 110–230 VAC only
- Several kilovolts DC (typically 2–25 kV depending on power class) (Correct answer)
- 48 VDC as used in PoE systems
Correct answer: Several kilovolts DC (typically 2–25 kV depending on power class)
Marine radar magnetrons require several kilovolts of DC anode voltage (commonly 2–25 kV) to produce the high-power microwave pulses needed for long-range target detection.
Question 2: What is the purpose of STC (Sensitivity Time Control) in a marine radar receiver?
- To increase radar power during heavy rain
- To reduce receiver gain for close-range returns immediately after each transmitted pulse, suppressing sea clutter near the vessel (Correct answer)
- To stabilize the display brightness under varying ambient light
- To synchronize the antenna rotation with the display sweep
Correct answer: To reduce receiver gain for close-range returns immediately after each transmitted pulse, suppressing sea clutter near the vessel
STC automatically reduces receiver sensitivity for a brief period after each pulse transmission, attenuating the strong close-range sea clutter that would otherwise obscure nearby targets.
Question 3: What component in a radar power supply converts the AC mains supply to the high-voltage DC required by the magnetron?
- The TR limiter
- A high-voltage transformer and rectifier assembly (Correct answer)
- The AFC discriminator circuit
- The bearing encoder
Correct answer: A high-voltage transformer and rectifier assembly
A high-voltage transformer steps up the AC mains voltage, which is then rectified and filtered to produce the stable high-voltage DC needed to drive the magnetron.
Question 4: What effect do significant voltage fluctuations on the ship's supply bus have on radar performance?
- They increase antenna rotation speed
- They can cause erratic magnetron frequency, reduced output power, and false triggering of the modulator (Correct answer)
- They improve receiver sensitivity
- They have no effect if the radar has an internal fuse
Correct answer: They can cause erratic magnetron frequency, reduced output power, and false triggering of the modulator
Supply voltage variations can destabilize the magnetron's operating point, leading to frequency drift, reduced transmitted power, and unreliable pulse timing.
Question 5: What is the typical duty cycle of a marine radar transmitter operating in standard pulse mode?
- 50% — equal on and off time
- Nearly 100% continuous wave
- Less than 0.1% — very short pulses relative to the pulse repetition interval (Correct answer)
- Exactly 10% as mandated by IMO
Correct answer: Less than 0.1% — very short pulses relative to the pulse repetition interval
Marine radars transmit extremely short pulses with long listening periods between them, resulting in a duty cycle well below 0.1%, which limits average radiated power while maintaining high peak power.
Question 6: What is the function of a modulator in a marine radar transmitter?
- It demodulates the received echo signals
- It generates the high-voltage pulse that triggers the magnetron to emit a microwave burst (Correct answer)
- It rotates the antenna at the correct speed
- It converts radar video signals to NMEA sentences
Correct answer: It generates the high-voltage pulse that triggers the magnetron to emit a microwave burst
The modulator produces a precisely timed, high-voltage pulse that switches the magnetron on and off, controlling the transmitted pulse width and repetition rate.
CMRT Power Systems & Interference Mitigation 1
What is the typical magnetron anode voltage used in a commercial marine radar transmitter?