VHF radio and AIS transceiver proximity

[Marc S]

I’m planning to add a Sitex AIS transceiver to my nav system. I’ll make use of its antenna splitter to simplify things. Nothing I’ve read seems to indicate that there’s any problem with installing the 2 units near each other. In my case they’ll be adjacent. Does anyone know of a problem with installing the VHF and AIS adjacent to each other?

[s/v Jedi]

Originally Posted by Marc S

I’m planning to add a Sitex AIS transceiver to my nav system. I’ll make use of its antenna splitter to simplify things. Nothing I’ve read seems to indicate that there’s any problem with installing the 2 units near each other. In my case they’ll be adjacent. Does anyone know of a problem with installing the VHF and AIS adjacent to each other?

Installing them beside each other isn’t a problem. That said, I recommend you do not buy an AIS transceiver with built-in antenna splitter.

It is much better to use a separate antenna. The splitter is the first to fail so eliminating it prevents the failure and stand alone splitters are expensive.

[markxengineerin]

I have a Sitex transceiver and Vesper splitter nearly touching each other in my installation, no issues. I prefer to use one antenna for both VHF and AIS because it is mounted on the mast head for best range. I don't think it's possible to install 2 antennas on the mast head (interference?), so you end up with a less effective (and ugly) antenna installation somewhere else, which is more of an issue the smaller the boat. If the single antenna or splitter fails, I have the spare parts on hand to add a backup antenna to the stern railing, plus the usual handheld VHF for a real emergency.

[rls8r]

The manual for my Em-Trak B954 AIS Transceiver says:

"For optimum performance the transceiver should be mounted at least one metre away from other high power transmitting equipment like VHF radios."

My VHF radio (Standard Horizon GX2200) is located about 0.5 m from the AIS transceiver and I haven't seen any problems. Of course, that suggestion may be only for Em-Trak AIS transceivers ... don't know.

[s/v Jedi]

Originally Posted by markxengineerin

I have a Sitex transceiver and Vesper splitter nearly touching each other in my installation, no issues. I prefer to use one antenna for both VHF and AIS because it is mounted on the mast head for best range. I don't think it's possible to install 2 antennas on the mast head (interference?), so you end up with a less effective (and ugly) antenna installation somewhere else, which is more of an issue the smaller the boat. If the single antenna or splitter fails, I have the spare parts on hand to add a backup antenna to the stern railing, plus the usual handheld VHF for a real emergency.

For a ketch it’s simple as you have two masts. For one mast you can put the second antenna on a top spreader.

Some mount two antennas at the masthead and claim it works just fine but I’m not a fan.

[Marc S]

Thanks to all for the replies. Given my sloop rig and the need to put the antenna(s) high, it’s sounding like relying on the splitter is my best option. Separating 2 antennas with only one mast available and considering the requirement that AIS antennas be kept away from the rigging and other antennas seems to point to sharing the existing VHF antenna. In a pinch both radio and AIS transceiver can be connected to emergency antennas.

Good to hear the lack of problems mounting the two units in proximity.

[s/v Jedi]

Originally Posted by Marc S

Thanks to all for the replies. Given my sloop rig and the need to put the antenna(s) high, it’s sounding like relying on the splitter is my best option. Separating 2 antennas with only one mast available and considering the requirement that AIS antennas be kept away from the rigging and other antennas seems to point to sharing the existing VHF antenna. In a pinch both radio and AIS transceiver can be connected to emergency antennas.

Good to hear the lack of problems mounting the two units in proximity.

Well, no. One antenna at the masthead and a second on the top spreader conforms to all the regulations and recommendations and is a proven setup.

Also, there is no requirement to put the AIS antenna at the masthead. The kind of AIS range achieved with a masthead antenna is not practical and never used. Commercial shipping will not consider possible collision warnings before the target is within a couple of miles away. For this reason, many AIS antennas are mounted on a radar arch or pole, something like 15’ above water.

Of course you can opt for a transceiver with built in splitter. The advantages are an easier install and a cheaper solution. Keep in mind though that this requires a broadband antenna that can do both AIS and VHF. So it may not be as easy as one would think.

[Marc S]

Originally Posted by s/v Jedi

Well, no. One antenna at the masthead and a second on the top spreader conforms to all the regulations and recommendations and is a proven setup.

Also, there is no requirement to put the AIS antenna at the masthead. The kind of AIS range achieved with a masthead antenna is not practical and never used. Commercial shipping will not consider possible collision warnings before the target is within a couple of miles away. For this reason, many AIS antennas are mounted on a radar arch or pole, something like 15’ above water.

Of course you can opt for a transceiver with built in splitter. The advantages are an easier install and a cheaper solution. Keep in mind though that this requires a broadband antenna that can do both AIS and VHF. So it may not be as easy as one would think.

Right. My original post did say that I'll have a transponder with built-in splitter. Given that my current AIS-enabled VHF receiver routinely picks up AIS broadcasts from 8 or more miles away and that I'll be at least initially using the existing antenna, it sounds like I should be OK.
I'm surprised by your nonchalance about only being seen by commercial traffic "a couple of miles away". I'd like to pop up on a freighter's plotter sooner than that especially if the watch is busy making coffee. Realistically, as with visual collision avoidance, it's more important to see them than the other way around.
Thanks again for the posts!

[mojoman2]

I have the two antennas mounted within a foot of each other and they have been working fine for 6 years now. I know that this is closer than recommended, but it works for me.

[SailingHarmonie]

Originally Posted by Marc S

Right. My original post did say that I'll have a transponder with built-in splitter. Given that my current AIS-enabled VHF receiver routinely picks up AIS broadcasts from 8 or more miles away and that I'll be at least initially using the existing antenna, it sounds like I should be OK.
I'm surprised by your nonchalance about only being seen by commercial traffic "a couple of miles away". I'd like to pop up on a freighter's plotter sooner than that especially if the watch is busy making coffee. Realistically, as with visual collision avoidance, it's more important to see them than the other way around.
Thanks again for the posts!

8 mile range for a masthead AIS is quite poor. You should be seeing large Type A AIS targets with a significant antennae height at 25 miles, and properly functioning Type B systems at 10 miles.

Granted, the number of poorly functioning Type B targets is large. It’s not unusual for some of them to pop up 1 or 2 miles way. Kind of useless, and they haven’t a clue.

[s/v Jedi]

Originally Posted by Marc S

Right. My original post did say that I'll have a transponder with built-in splitter. Given that my current AIS-enabled VHF receiver routinely picks up AIS broadcasts from 8 or more miles away and that I'll be at least initially using the existing antenna, it sounds like I should be OK.
I'm surprised by your nonchalance about only being seen by commercial traffic "a couple of miles away". I'd like to pop up on a freighter's plotter sooner than that especially if the watch is busy making coffee. Realistically, as with visual collision avoidance, it's more important to see them than the other way around.
Thanks again for the posts!

No, it will not be okay. Receiving is very different from transmitting. A VHF antenna does not work well on an AIS transceiver and many will simply refuse to transmit due to bad SWR.

Here is an example of a combination antenna. Read the description to understand why this is important (frequencies are different)
https://www.milltechmarine.com/gam-s...s-antenna.html

Also, it does not matter if you are received by ships 20 miles away because their settings completely ignore you until you are close. They have better things to do. In my experience the only ships with collision warnings for targets more than 2nm away are cruise ships. All other commercial ships we see them react at less than 2nm… sometimes 1.7nm but other times only 1nm (which is close!)

So it simply doesn’t matter if you are being received beyond 5nm. It’s fun for friends etc. but the commercial shipping will not even look at you until you are closer.