The WatchMate 850 Transponder will report the error you are seeing if it senses the VSWR of the antenna connection is too high. This is done to ensure that most of the power that is being generated by the WatchMate 850 on a transmission is delivered to the antenna, and not just reflected back and lost.
A high VSWR typically indicates poor power transfer to the antenna. Practically speaking, this will mean your effective transmission range is reduced significantly, so it is important to get it right.
A high VSWR can be caused by a bad cable, connector or a poorly tuned antenna. The VSWR response of an antenna is frequency specific, so although your VHF antenna may work well at 156MHz, it may not be well tuned to the AIS channels at the upper end of the VHF band (162MHz).
Things to check:
- Are the cables and connectors connecting your antenna to the AISWatchMate in good working order, connected tightly and corrosion free?
- What is the specified VSWR of your antenna at 162MHz (information generally available from the antenna manufacturer)? A VSWR of 3:1 or more at 162MHz is not recommended for use with the WatchMate 850.
- Antenna mounting arrangement. Is it mounted close to a metalic object? Are there any obstructions around the antenna pole? Does the antenna require a grounding connection to stay tuned? Is the antenna positioned at least 3 meters from the AISWatchMate?
- Does your antenna present a DC open or short (normally stated in the datasheet)? Can you verify this with a multimeter at the AISWatchMate VHF connector end?