Radial Lines and an Open Waveguide
A 90-foot offshore supply vessel is alongside in a busy harbor. Its 25 kW X-band radar display shows bright dotted radial lines from the center on every range scale. The lines rotate with the antenna and disappear when the radar is switched to standby. Another vessel nearby is operating its radar. A technician wants to disconnect the waveguide at the antenna and watch the magnetron through the open flange to confirm that the transmitter is firing. Crew members are working on deck near the scanner. The vessel is not underway, but the radar is needed for a departure later that day. The display otherwise shows normal echoes from docks and moored boats. The radar has a 6-foot slotted waveguide antenna mounted about 8 m above the deck.
1. Identify the likely cause of the radial lines and explain why switching to standby makes them disappear.
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The radial dotted lines are characteristic of radar-to-radar interference. The other vessel’s radar transmits pulses that this radar receives directly or by scattering. Because the interfering pulses arrive at different ranges as this antenna scans, they paint radial spokes or dotted lines rather than a normal target echo. When this radar is switched to standby, its receiver is no longer processing the incoming signals, so the lines disappear.
The normal echoes from docks and boats show that the receiver and display are basically working. The fault is not a failed magnetron or a broken waveguide; it is external interference. The operator should try the interference rejection or FTC control, reduce gain slightly, and if the radar allows it, change the pulse repetition frequency or pulse width to reduce the effect. The interference is an operational nuisance, not by itself evidence of a defective radar.
2. Evaluate the technician’s plan to disconnect the waveguide and look into the open flange while transmitting.
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The plan is unsafe and should be refused. A 25 kW X-band radar produces a highly directional RF beam that can cause serious eye and tissue injury at close range. Looking into an open waveguide or flange while the transmitter is firing exposes the technician to the full radiated field. The scanner is also mounted about 8 m above the deck, and crew members are working nearby; an open waveguide changes the radiation pattern and can create hazardous exposure in unexpected directions.
A waveguide should never be opened while the transmitter is on. The correct approach is to turn off the radar, remove power, lock out the supply, wait for the high-voltage capacitors to discharge, and ground the high-voltage point. RF output should be checked with a calibrated power meter, a dummy load, or the manufacturer’s test procedure. The technician should also observe the scanner’s exclusion area and keep personnel clear during any brief test.
3. What should the supervisor do before the vessel departs, and how should the interference and the testing issue be documented?
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The supervisor should first verify that the radial lines are external interference by switching to standby and by comparing with another radar or by moving away from the interfering vessel if possible. If the radar performs normally otherwise, the supervisor can release the vessel with a note that interference rejection may be needed in harbor. The supervisor should not allow the open-waveguide test. If RF output must be verified, an endorsed technician should use a proper power meter or dummy load with the transmitter controlled and the area cleared.
The documentation should record the symptom, the standby test, the suspected interfering source, the controls tried, and the fact that no transmitter fault was found. It should also record that the open-waveguide test was rejected as an RF safety hazard. If the radar later shows weak echoes or no tuning peak after departure, that is a separate transmitter problem and should be troubleshoot with the correct test equipment.