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***Tom - Raymarine - Moderator***

Hello Antonio,

What does ProAIS report is the antenna's VSWR value? That sounds like a high VSWR. VSWR is a measurement of how well-matched the antenna's impedance is with the AIS, and a poor (high) VSWR means poor power transfer between the antenna and the AIS.

It is worth being aware that VSWR depends on frequency, and VHF radio does not operate on quite the same frequency as AIS. Most VHF antennas are tuned to give their best performance at around the Channel 16 frequency (approx. 156MHz), whereas AIS operates at 162MHz. Many VHF antennas have a good VSWR at 156MHz but a poor VSWR at 162MHz. For good AIS performance it's important to either use a broadly-tuned antenna that has a good VSWR from 156 all the way to 162MHz, or (only if you are using this antenna just for the AIS) use an AIS-tuned antenna. Don't use an AIS-tuned antenna for VHF using the AIS700's internal splitter because the VHF RF power can be reflected back into the AIS causing damage.

If the antenna VSWR reported by ProAIS is good (e.g. under 2) then it's possible you have a source of RF interference which is drowning out the VHF reception. This sort of thing is increasingly common, and LED lighting drivers are a common culprit.

Regards,
Tom