Why is my marine AIS transponder receiving targets but not transmitting?
I fitted a Raymarine AIS700 last season and it worked perfectly. Now, I can see other ships on my plotter, but my friends say I'm not showing up on their screens or on MarineTraffic. The unit has a solid green light, but I'm definitely not transmitting. What should I check?
Answer
If your AIS transponder receives targets but fails to transmit, the most common culprit is a high VSWR fault caused by a degraded VHF antenna or a corroded PL259 connector. The unit automatically shuts down its transmitter to protect internal circuitry against high antenna resistance, leaving the receiver to pick up incoming signals perfectly well.
Antenna and VSWR Faults
Your AIS receiver is highly sensitive. It will pick up nearby ships even if you stick a wire coat hanger into the antenna socket. Transmitting requires a perfectly tuned route to the atmosphere. A Class B transponder pushes out 2 Watts, while SOTDMA units like the Vesper Cortex or Raymarine AIS700 output 5 Watts.
Corrosion in the RG58 or RG213 coaxial cable, water ingress at the antenna base, or a poorly soldered PL259 connector will reflect transmitted power back down the cable. This reflection is measured as VSWR (Voltage Standing Wave Ratio). If the VSWR exceeds 3.0:1, the transponder cuts the transmission to stop the final stage amplifier from burning out. Borrow an SWR meter or use the diagnostic software provided with the unit to isolate the fault. A healthy marine VHF antenna shows a VSWR of 1.5:1 or lower.
GPS Fix and Timing Issues
AIS relies entirely on the GPS network for timing. The transponder uses precise GPS clocks to slot your transmission between other vessels on a crowded VHF frequency. The unit will instantly stop transmitting if it loses the GPS fix.
Common causes for GPS failure include:
- Blocked signal: The GPS patch antenna sits under a steel deck or next to a radar scanner emitting interference.
- Cable damage: The thin RG316 cable used for external GPS antennas is easily crushed behind bulkheads.
- Connector corrosion: The TNC or SMA connector at the back of the AIS unit has vibrated loose or oxidised.
Your AIS must have a dedicated GPS antenna. It cannot legally or technically use GPS data from a chartplotter via NMEA2000 for transmission timing.
Silent Mode and MMSI Settings
Every marine AIS unit features Silent Mode to stop transmissions while leaving the receiver active. A physical switch wired into the auxiliary loom or a software toggle on the multi-function display (MFD) triggers this state. Check the MFD settings to ensure nobody accidentally tapped the TX disable button. If you wired a physical toggle switch, test the contacts with a multimeter. A short circuit forces the unit to stay quiet.