CMRT Installation & Maintenance Procedures 3 — Questions and Answers
Question 1: A radar installation requires a rotary joint between the stationary waveguide and the rotating antenna. The key performance parameter checked for a rotary joint is:
- Pulse repetition frequency
- Insertion loss and VSWR across rotation (Correct answer)
- Magnetron duty cycle
- Receiver noise figure
Correct answer: Insertion loss and VSWR across rotation
A rotary joint is verified for consistent insertion loss and acceptable VSWR throughout its full 360° rotation to ensure uniform signal transfer.
Question 2: During a post-installation performance test, the radar's minimum range is found to be excessive. The most likely cause is:
- High magnetron output power
- Excessive transmit pulse length or recovery time (Correct answer)
- Low PRF setting
- Antenna tilt angle set downward
Correct answer: Excessive transmit pulse length or recovery time
Minimum range is limited by the transmit pulse length and receiver recovery (dead) time; longer pulses or slow recovery blind the receiver at close range.
Question 3: When installing radar on a vessel with an aluminum superstructure, bonding straps between the antenna mount and the vessel's ground plane should be:
- Made of copper braid with low impedance (Correct answer)
- Omitted to prevent galvanic corrosion
- Made of stainless steel wire rope
- Connected only at the display end
Correct answer: Made of copper braid with low impedance
Low-impedance copper braid bonding straps provide effective RF ground and also reduce galvanic corrosion risk by equalizing potentials.
Question 4: A technician notices the radar antenna rotation rate has slowed from 24 RPM to 18 RPM. The FIRST component to inspect is:
- The magnetron's output power level
- The antenna drive motor and gearbox (Correct answer)
- The modulator capacitor bank
- The IF amplifier gain control
Correct answer: The antenna drive motor and gearbox
Reduced rotation rate points directly to the antenna drive motor or gearbox — worn gears, low lubricant, or motor winding failure are common causes.
Question 5: What is the correct procedure when installing a new magnetron to ensure proper break-in?
- Immediately run at full power for 30 minutes
- Gradually increase power over several hours per manufacturer schedule (Correct answer)
- Replace the modulator simultaneously
- Test at full power only in port
Correct answer: Gradually increase power over several hours per manufacturer schedule
New magnetrons require a manufacturer-specified conditioning (burn-in) procedure, gradually increasing power to stabilize emission characteristics and extend tube life.
Question 6: Radar cables passing through below-deck spaces must be routed to avoid proximity to:
- Navigation lights
- Engine exhaust lines and high-heat sources (Correct answer)
- VHF antenna coax
- Compass repeater wiring
Correct answer: Engine exhaust lines and high-heat sources
Heat from exhaust lines degrades cable insulation and connectors, causing intermittent faults and potential fire hazard in radar cable runs.
Question 7: When performing an ARPA radar commissioning test, which check specifically validates target tracking accuracy?
- Adjusting sea clutter rejection level
- Tracking a target at a known course and speed and comparing to ARPA output (Correct answer)
- Testing the heading marker alignment
- Verifying the magnetron duty cycle
Correct answer: Tracking a target at a known course and speed and comparing to ARPA output
ARPA accuracy is validated by acquiring a target with a known course and speed (e.g., a buoy or vessel) and confirming the ARPA displayed values match.
CMRT Installation & Maintenance Procedures 3
A radar installation requires a rotary joint between the stationary waveguide and the rotating antenna.
The key performance parameter checked for a rotary joint is: