AIS / strange range behavior

[doschhar]

Hello fellow sailors,

Boat Beneteau Sense 43
Raymarine AIS 650
Banten Diplexor Splitter Code 00260
Standart VHF Antenna top of mast
External GPS Antenna

Finaly started to investigate a problem with receiving AIS targets hundreds of miles away but only transmiting my AIS target about 1 mile or even less.

Started to realize that there is a problem when family could not track my location via the ususal AIS tracking websites anymore. First thing that came to my mind was a VHF Antenna problem so I sailed close to a huge cargo ship - like 10 miles and called them via VHF for a radio check. They replied "loud and clear" and so I did the cargoship. Asking if they can see me on AIS the answer was no even though I could see them on my screen. So I asked if they can call me back once they can see me and took heading to intercept. About 1 mile or even less they said I finaly appeared.

So maybe its not the VHF Antenna since at least VHF voice works further then one mile both ways, I checked the AIS 650 via the proAIS2 application, all systems green - no error messages. No strange data except for VSWR which is around 1.8 and does not change even if I remove the antenna cable from the splitter. Doing so the number of received AIS targets which is normaly hundreds in the harbor go down to a few dozen and my VHF quality is next to none - so the antenna does something - still VSHR stays 1.8. Splitter shows no abnormal lights and got power and switches the relay once I press the TX button on my VHF radio.

So next I would connect a spare VHF antenna and do some more tests with range etc ... but I wonder maybe something else is wrong and you guys can point me into another direction.

Yours,
Harald

[wholybee]

My first guess is that it is highly likely an issue with the splitter/switch. Test this by connecting the AIS directly to the antenna and checking the transmitted range. Obviously communicating with the other ship via handheld for the test.

What I expect is happening, is that the switch in the splitter is not switching to the AIS while the AIS is transmitting. This is supported by the VSWR remaining at 1.8 even with the antenna disconnected. The switch has an internal resistive load that whatever device isn't active is connected to. So, the AIS will always have a good VSWR.

Also, understand that your reception of cargo ships will be much, much better than your transmitted distance. During normal conditions, I expect Class B AIS to go 6-10 miles. However, you should receive Class A from 20-50 miles. During exceptional conditions that reception can go up to 1000 miles, but your transmitted range won't extend as much.

[wholybee]

Also, note that I call it a switch, because that is what it is. It has 3 positions. Position one, it acts as a splitter, connected the antenna to both the AIS and VHF for receptions. Position 2, The AIS is transmitting, and the VHF is disconnected (rather, connected to a resistor instead of the antenna) and position 3, the VHF is transmitting, and the AIS is disconnected.

Understanding how that works helps in the diagnosis.

[smac999]

the swr in the app will only change after it transmits, and it will only transmit every ~5 mins. so you'd want to leave the ant off for 10 mins to see the diff. let it transmit at least twice. I bet after 10 mins it'll change.

otherwise 1.8 is pretty normal if using a vhf antenna

[doschhar]

Originally Posted by wholybee

My first guess is that it is highly likely an issue with the splitter/switch. Test this by connecting the AIS directly to the antenna and checking the transmitted range.

Will do, will need an extension cable however or a backup antenna connected only to the AIS.

[doschhar]

Originally Posted by smac999

the swr in the app will only change after it transmits, and it will only transmit every ~5 mins. so you'd want to leave the ant off for 10 mins to see the diff. let it transmit at least twice. I bet after 10 mins it'll change.

I was aware of that and did wait for a few transmissions 10~15 minutes but the value did not change.

[Scorpius]

I had a similar problem. It turned out to be the antenna cable from the AIS to the splitter. There was some bad soldering in one of the connectors. The AIS was receiving just fine but the transmission was weak and spotty. Putting a new connector on that end of the cable solve the problem.

[ItDepends]

Originally Posted by doschhar

No strange data except for VSWR which is around 1.8 and does not change even if I remove the antenna cable from the splitter. Doing so the number of received AIS targets which is normaly hundreds in the harbor go down to a few dozen and my VHF quality is next to none - so the antenna does something - still VSHR stays 1.8. Splitter shows no abnormal lights and got power and switches the relay once I press the TX button on my VHF radio.

This would get my attention... I have a very similar setup (minus the splitter) and the reported SWR has always been 1.0 I'd consider 1.8 a problem, or at the very least a change of significance in a bad direction.. I am not a radio geek, but that's my experience. If the AIS actually tries to transmit without an antenna connected (it normally would not), I'll put money (Not a lot of money, mind you!) on a short in your antenna cable or splitter..

[wholybee]

Originally Posted by ItDepends

This would get my attention... I have a very similar setup (minus the splitter) and the reported SWR has always been 1.0 I'd consider 1.8 a problem, or at the very least a change of significance in a bad direction.. I am not a radio geek, but that's my experience. If the AIS actually tries to transmit without an antenna connected (it normally would not), I'll put money (Not a lot of money, mind you!) on a short in your antenna cable or splitter..

1.0 is perfect, and isn't achievable in real world scenarios on a boat. Not with PL-259 connectors, or even N connectors. If you are reading 1.0, something isn't right, probably the meter. If you are reading that from your AIS device, that is a pretty inaccurate and not calibrated measurement method.

1.8 is normal. Maybe a bit on the high side, but still ok. I would consider 1.2 or less very good. Over 2.0 is considered a problem, and is about a 10% loss of power. A completely disconnected antenna or a short in the line would read close to infinity. (or as high as the meter were capable of measuring.)

[smac999]

1.8 on an vhf ant would be pretty normal.

I'd expect 1.2-1.5 on an AIS antenna

you will never get 1.0 unless you are in a lab...

[doschhar]

Thanks for all your replies.

I ordered a spare cable to connect a backup antenna and to be able to go directly into the AIS and also a new splitter to replace a maybe damaged one or keep as a spare.

Lets see if anything changes.