AIS Antenna VSWR exceeds Limit

[s/v Jedi]

Originally Posted by wsconner

Thanks-- very helpful for me as I formulate my own hypothesis. Agree I likely have two issues occurring. The unit was working properly for two weeks, and was transmitting AIS target data, plus instrument data from the B924 AIS to the E80 chart plotter. So it's likely I developed an antenna connection issue (existing cables are 20+ years old), and maybe that is the root of all my issues. I did troubleshoot by isolating the antenna connection (new cable, antenna and fittings) connected directly to the AIS and the VSWR reading on AIS ProAIS software was only 3:1. I thought this would have solved it, but the E80 was still getting the same error. Anyways, appreciate the time folks take to comment.. it does help my problem solving approach to hear the different ideas. thx.

A VSWR of 3:1 is bad. The transmitter is likely to either stop working to protect itself plus when it is right on the maximum allowable value, when it does transmit, the output stage will experience high temperatures which translates to a premature failure. VSWR needs to go down to better than 2:1

My suggestions above are really at a minimum of what must be done to fix this. The tools I suggested are of a level where a pro will roll their eyes but they are a a price level within reach for DIY and they are way good enough to get it right. They also enable one to find the exact fault, i.e. determine if it’s the cable or the antenna or the transponder

[Brian.D]

So as to help show what the NanoVNA will do, I have taken a photo of my NanoVNA H4 (4" display version) attached to my home made 2 meter (144~148 MHz) vertical dipole antenna (this is just a dipole with the elements placed in an up/down orientation).

The antenna was tuned for the 2 meter band at 146.3 MHz (Marker 1) and the resulting SWR is 1.169:1. If we round this it would be 1.17:1. The other markers are Marine Frequencies:
M2 = Ch16 (156.8 MHz) SWR 1.482:1
M3 = AIS (161.975 MHz) SWR 1.757:1
M4 = AIS (162.025 MHz) SWR 1.771:1 (marker is located at 162.050 MHz)

The NanoVNA H4 was calibrated for a frequency range of 140MHz to 170MHz. Cable RG-8X length in the order of 60 feet and there are two adapters between three cables. Antenna is outside.

So we can see the benefits of having a NanoVNA available. Granted, it is not a professional instrument, but it does serve a purpose.