AIS Antenna FaultCritical Findings Aboard Yacht Bijou
Follow the onboard AIS antenna fault investigation as F&C Marine traces a recurring em-trak B923 transmission error, a 5.5:1 SWR reading, Raymarine network concerns and missing GPS position data at the vessel’s DSC VHF radio.
AIS antenna fault investigations often begin with a symptom seen on a display. Aboard Yacht Bijou, the owner reported that AIS targets had disappeared from the Raymarine display, while a previous loss of depth information had also been mentioned.
Part 1 covered the information available before attending the vessel. The next stage was to inspect the installation onboard, reproduce the fault and establish whether the problem originated in the AIS transceiver, antenna system, Raymarine network or more than one connected system.
AIS targets had disappeared
The Raymarine display was no longer showing the expected AIS traffic, while a previous depth issue had also been reported.
TX error and 5.5:1 SWR
The B923 transmission warning was reproduced and testing revealed a significant AIS antenna fault within the connected RF system.
Repair and commissioning
The antenna path, network, VHF position feed and MMSI details now require corrective work and final testing.
The physical repairs have not yet been carried out. Part 2 records the evidence found onboard without claiming that the final failed component has already been isolated or repaired.
AIS Antenna Fault Investigation: Reported Symptoms Versus Live Evidence
The owner reported that AIS targets were no longer visible on the Raymarine multifunction display. It had also been suggested that depth information had disappeared on an earlier occasion.
Once onboard, the first useful finding was that depth information was present. The display was showing depth normally and the historical depth fault could not be reproduced during the visit.
This did not prove that an intermittent depth issue had never existed. It simply meant there was no active depth failure available to trace at that moment.
Reported symptoms are the starting point
What appears on a display may be the final result of several possible failures. Effective fault finding means reproducing the problem and following the connected systems rather than replacing the first item that appears suspicious.
With depth operating during the inspection, attention moved to the em-trak B923 AIS transceiver and the reason AIS information was no longer appearing as expected.
The AIS Appeared Normal — Then the Red TX Error Returned
The em-trak B923 was showing a red transmission error indication. To establish whether the warning was permanent, intermittent or left over from an earlier condition, the AIS was restarted.
For approximately two minutes, everything appeared normal. No warning light was visible and the B923 initially looked as though it had recovered.
The red TX error then returned.
The restart did not cure the problem. It produced a short period of normal operation before the B923 detected the same fault again.Confirmed onboard finding
Reproducing the warning was an important step because the fault was now visible under test rather than being based only on the owner’s previous observations.
What the AIS Antenna Fault Investigation Revealed in ProAIS
The B923 connected successfully to em-trak’s ProAIS diagnostic software. A valid GPS position was available at the AIS transceiver.
This ruled out one possible explanation for the transmission warning. The B923 was not simply failing because it had no vessel position.
ProAIS also reported antenna-related warnings. Combined with the red TX indication, this shifted the investigation toward the antenna, coaxial feeder, connectors and the wider RF path.
AIS Antenna Fault Confirmed by a 5.5:1 SWR Reading
The connected AIS antenna system was tested with a RigExpert AA-1000 antenna analyser. The measured standing wave ratio was approximately 5.5:1.
This was far too high for a healthy AIS installation and showed that a significant proportion of the transmitter’s RF energy was being reflected back toward the transceiver instead of being radiated efficiently through the antenna.
High reflected power can reduce transmission performance and trigger an AIS antenna or TX warning. Possible causes include antenna damage, water ingress, deteriorated cable, poor connectors or an unsuitable component in the RF path.
The test confirmed an AIS antenna fault within the overall antenna system, but it did not yet prove that the antenna element itself was the failed component.
Antenna
Internal failure, physical damage or moisture at the antenna connection.
Feeder cable
Deterioration, water ingress, physical damage or excessive loss within the coaxial run.
Connectors
Poorly fitted UHF connectors, corrosion, adaptors or exposed joints.
The confirmed problem was within the AIS antenna system
Until the antenna and feeder are isolated and tested separately, it would be misleading to blame one component. Part 3 will record the isolation tests and final repair result.
The AIS Antenna Fault Did Not Fully Explain the Missing Targets
An AIS transmission fault and missing AIS targets can be related, but they are not automatically the same problem.
An AIS transceiver may still receive targets while suffering a transmission fault. A serious antenna-system defect can affect both transmission and reception, but targets can also disappear from an MFD because of a separate data-network or display configuration issue.
A red AIS TX light and a blank AIS display may be connected, but reception, transmission and display must still be commissioned separately.Diagnostic principle
Receive
Confirm that real AIS targets are being received and are visible within the diagnostic software.
Transmit
Confirm that the B923 transmits normally without returning a red TX error.
Display
Confirm that received AIS targets pass through the network and appear on the Raymarine display.
The Investigation Moved to SeaTalkNG and NMEA 2000
The Raymarine equipment, iTC-5 and connected electronics were installed together around the port side of the vessel. Once the RF fault had been established, attention moved to the data path between the AIS and the Raymarine display.
Inspection identified a network terminator requiring replacement. Correct termination is essential for stable communication on a CAN-based NMEA 2000 or SeaTalkNG backbone.
This introduced a second potential fault path. The AIS could have an RF problem while the Raymarine display also had an independent network issue affecting the passage of AIS information.
The iTC-5 converts compatible instrument transducer information onto SeaTalkNG. It can handle data such as depth, wind, speed, temperature or rudder information, but vessel position must come from another GPS source.
The Icom VHF Was Not Receiving a Valid GPS Position
The owner had not reported a basic operating fault with the Icom IC-M411 VHF radio. However, while checking the connected equipment, another issue became apparent.
The radio was not receiving valid GPS position information.
This did not mean the radio itself had failed. The problem was that valid position data was not reaching it.
The em-trak B923 can provide NMEA 0183 GPS position output at 4,800 baud. That output can be configured and connected to the IC-M411 so that the VHF receives automatic position and time data.
A DSC distress alert may still be transmitted without an automatic GPS input, but the alert may not include the vessel’s current position unless it has been manually entered and remains up to date.
The MMSI Appeared to Relate to the Previous Owner
The programmed MMSI was not simply unknown or unreadable. The information available indicated that it related to the yacht’s previous owner.
AIS Antenna Fault Investigation Findings
By the end of the visit, the original complaint had developed into a much clearer picture involving several connected but separate issues.
The AIS antenna fault was confirmed by the abnormal SWR measurement, while the network and VHF checks revealed additional issues that also require corrective work.
Repairing the AIS Antenna Fault and Connected System Issues
Diagnosis must lead to controlled repairs and measurable commissioning results. The return visit will work through the following sequence.
Replace the network terminator
Fit the correct SeaTalkNG or NMEA 2000 terminator and inspect the complete backbone.
Replace the AIS feeder cable
Install a suitable low-loss marine coaxial feeder selected for the cable length, route and AIS frequencies.
Replace the UHF connectors
Fit new RF connectors and inspect the cable entry, deck gland, adaptors and exposed joints for corrosion or moisture.
Isolate the antenna and feeder
Test the antenna and cable separately so the failed part can be identified rather than assumed.
Restore GPS data to the VHF
Configure and connect the B923 NMEA 0183 output to the IC-M411 using the correct baud rate, polarity and data sentences.
Verify the MMSI and licence
Check the current Ship Radio Licence before making permanent AIS or VHF programming changes.
Commission each system separately
Confirm AIS transmission, reception, target display, network stability, VHF position input and DSC operation.
Additional Offshore Safety Observation
The original visit concerned AIS, instrument data and VHF DSC. It was not a statutory safety-equipment survey. However, no EPIRB or SART was observed onboard during the investigation.
The owner indicated that the yacht may operate more than 10 nautical miles from the coast. At that distance, it is sensible to consider whether the vessel has an independent method of raising an alert if VHF communication is unavailable or no receiving vessel or coast station is within effective range.
A compact satellite beacon registered to an individual and carried on the person. This may be a practical starting point for the sailing described.
A vessel-based satellite distress beacon designed to identify the vessel and alert search-and-rescue authorities.
Local locating devices intended to help nearby AIS-equipped vessels find a person, survival craft or distress position.
A PLB may provide a sensible personal satellite-alerting option, while an EPIRB is a vessel-focused device. AIS MOB and AIS-SART equipment mainly assist local location by nearby AIS-equipped vessels.
This is not a declaration that one particular beacon is legally mandatory for Yacht Bijou. It is a recommendation that the owner considers an additional independent distress-alerting method before undertaking longer offshore passages.
AIS Antenna Fault FAQs
What does a red TX error light on an AIS transceiver mean?
It means the AIS has detected a problem affecting transmission. Possible causes include excessive SWR, a damaged antenna, deteriorated coaxial cable, poor connectors, low supply voltage or an internal equipment fault.
Is an AIS SWR reading of 5.5:1 acceptable?
No. A reading around 5.5:1 is abnormally high and indicates a significant mismatch or defect within the antenna system.
Does high SWR prove that the AIS antenna has failed?
No. High SWR confirms an RF-path problem, but the cause could be the antenna, coaxial feeder, connector, deck gland, adaptor or water ingress.
Can an AIS receive targets while showing a TX fault?
In some fault conditions, yes. AIS reception and transmission are separate functions.
Why might AIS targets disappear from a Raymarine display?
Possible causes include poor AIS reception, a network termination problem, incorrect data-source selection, disabled display settings or a fault within the AIS transceiver.
Why does an NMEA 2000 network require two terminators?
NMEA 2000 and SeaTalkNG backbones use termination at both physical ends to maintain correct electrical characteristics.
Will a DSC VHF radio still work without a GPS position?
Normal voice operation may continue, but the distress message may not automatically include the vessel’s current position.
Can the em-trak B923 provide GPS data to an older VHF radio?
The B923 provides NMEA 0183 connectivity and can output suitable position information when correctly configured.
Does an MMSI change when a yacht is sold?
The MMSI programmed into the AIS and DSC radio should be checked against the current Ship Radio Licence.
Is a PLB suitable for a yacht operating more than 10 miles offshore?
A registered PLB can provide an independent personal satellite distress alert and may be a sensible option.
What will Part 3 of the Yacht Bijou case study cover?
Part 3 will document the physical repairs, antenna and feeder isolation tests, network work, restoration of GPS data to the VHF, MMSI verification and final commissioning.
The Repair and Commissioning Visit
The AIS antenna fault investigation has established what requires attention, but the story is not finished. The return visit will reveal whether the high SWR originates in the antenna, feeder or connectors and whether the network and VHF faults are fully resolved.
Part 3 will only be published after the physical work has been completed and every function has been tested onboard.
Do You Have an AIS Antenna Fault?
F&C Marine provides practical onboard marine electronics fault finding for yachts, leisure craft and commercial vessels. We diagnose AIS transmission and reception faults, marine antenna systems, NMEA 0183, NMEA 2000, Raymarine SeaTalkNG and DSC position problems.
This case study records observations made during an individual onboard investigation. It is not a statutory survey or a declaration of legal carriage requirements.