Channel 16 transmission blacks out the bridge
At 10:20 local time, the 22 m motor yacht Maestrale is underway 6 nm off Civitavecchia with three crew. The bridge has a VHF DSC set connected to a GPS, a chartplotter with an external GPS antenna, a Class B AIS transceiver, and a second handheld VHF. Whenever the main VHF transmits on channel 16 at 25 W, the chartplotter loses its GPS fix for 5–10 seconds, the AIS target list clears, and a loud buzz is heard on the intercom. At 5 W the fault does not appear. The main VHF antenna is mounted 1.2 m from the GPS antenna and 0.8 m from the AIS VHF antenna; the antenna coax and the GPS cable run together for about 3 m. An inline VSWR meter shows 2.8:1 on channel 16 and 1.5:1 on channel 72. The battery voltage at the VHF terminals drops from 13.6 V to 12.9 V during transmission. Traffic is moderate, and the yacht is approaching a traffic separation scheme.
1. What immediate actions should the skipper take to preserve radio watch and navigation safety?
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The skipper should stop using 25 W on the main VHF until the cause is understood. Switch the main set to 5 W if that keeps the GPS and AIS stable, and use the handheld VHF as a backup for channel 16. Reduce speed, post an extra lookout, and navigate by paper chart, radar and visual bearings rather than relying on the affected chartplotter. The AIS target list may be lost intermittently, so the skipper should not assume that AIS is giving a complete picture and should make appropriate sound or light signals in restricted visibility. A short SECURITE call is not automatically required for an internal equipment fault, but nearby traffic should be avoided and the situation monitored. The immediate priority is to keep a continuous radio watch and a safe navigational position while avoiding a transmission mode that causes safety equipment to fail.
2. How should the fault be diagnosed? Distinguish between antenna/coax problems and radio-frequency interference.
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The VSWR of 2.8:1 on channel 16 indicates an antenna or coax mismatch, but a mismatch alone would not normally erase a GPS fix and AIS targets. The fact that the fault appears only at 25 W points strongly to radio-frequency interference or a power/ground problem. The skipper should test the VHF into a suitable dummy load: if the GPS and AIS symptoms disappear, the antenna, coax or connector is radiating or coupling RF badly; if the symptoms remain, the RF is entering through power leads, grounding or the intercom wiring. Check the antenna coax for damaged jacket, poor connectors, water ingress and sharp bends, and check that the GPS and AIS antennas are separated from the VHF antenna. Re-route the GPS cable away from the VHF coax and power cables, improve bonding and check the 0.7 V drop at the VHF terminals. Do not transmit at 25 W without an antenna or dummy load.
3. What repairs and operating restrictions should apply before the yacht resumes normal operation, and what should be reported?
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The yacht should not resume 25 W operation until the antenna system has been repaired or replaced and the interference has been retested. A VSWR of 2.8:1 should be corrected; connectors should be replaced if corroded, and the coax should be inspected or renewed. The GPS and AIS antenna separation should be increased if practical, and cable routing should be changed so that the GPS cable does not run parallel to the VHF coax. If the fault persists with a dummy load, the power and ground wiring and the intercom installation need attention, including screening and ferrite suppression where appropriate. Until then, operate on 5 W or the handheld, keep a paper plot and extra lookout, and avoid the traffic separation scheme if navigation confidence is reduced. Record the symptoms, tests, repairs and restrictions in the radio log and maintenance record, and report the defect to the service agent and, where required, the flag or port state authority.