AIS: Standalone or Overlay

[jim_thomsen]

Paul,
Our antenna is not that high, just mounted on the radar pole on the stern, probably 2 meters above the deck. At first I used a splitter and our VHF antenna on the top of the mast (20 meters), but our VHF had a major blowout and the repairman blamed it on the splitter (Raymarine did replace the VHF) and I added another VHF antenna to the pole.
At night we see their lights first, then Radar, then AIS. Daytime usually AIS will see them before we do.
Maybe I need to move it higher (also it's just a normal VHF antenna, should I use a "AIS" antenna?).
Thanks for the information. Maybe I need to look into NavMonPc.

[jim_thomsen]

Jon,
Thanks for the info about the transceiver for $495. I would like to add something like this to our current setup.
Also, your website has a lot of greeat info on it...thanks!

[jeffrobbins]

Just so you know... I'm from Vesper Marine....

I agree on the clutter problem. I've been experimenting for quite some time with different filters for reducing the number of targets displayed so there isn't a confusing mass of vessels. The same thing applies to alarms. Nuisance alarms are annoying and just cause people to switch them off. For that reason, on my boat I don't use proximity alarms very much. I prefer to use a CPA/TCPA alarm but suppress it with various things like not triggering for stationary targets, etc.

I think the comment about not caring about targets moving away or not posing a collision danger is really important to reducing clutter on the screen and allowing me to focus on the targets that are most important.

Awhile ago someone suggested to me to show a graphical representation of whether a target will pass ahead/astern or port/stbd. I liked that idea and it's worked out well.

If you go with a dedicated display that also outputs the AIS data you can also do overlays on other devices if/when you want to. You can run them at different scales and I often find that the scale I use for navigating isn't the same scale I want for collision avoidance. But I like the redundancy of a dedicated display and the ability to see what's around me at a quick glance. Often I prefer to switch off power-hungry devices like plotters and computers. It's personal preference I guess, but I find that AIS alerts me to targets I don't see initially, so I don't want to rely on switching things back on when I see lights.

Also, someone mentioned an anchor alarm... I've been thinking about this for awhile. It's a bit off-topic but it's related to AIS because an AIS transponder "knows" where your GPS antenna is located on your boat and that's one of the missing bits of info an anchor alarm needs to know.

Regarding antenna height... I've experimented a lot recently with different antenna heights. I find I get adequate range for collision avoidance with the antenna on a stern rail, but I get much greater range by raising the antenna to a spreader or better yet the masthead. But I definitely wouldn't recommend mounting two antennas at the masthead.

I don't mean for this to be a product plug but just wanted to answer the questions and share my experience. I hope that's ok.

Jeff, Nordic 40 "Vesper"

[Paul Elliott]

Originally Posted by jim_thomsen

Paul,
Our antenna is not that high, just mounted on the radar pole on the stern, probably 2 meters above the deck. At first I used a splitter and our VHF antenna on the top of the mast (20 meters), but our VHF had a major blowout and the repairman blamed it on the splitter (Raymarine did replace the VHF) and I added another VHF antenna to the pole.
At night we see their lights first, then Radar, then AIS. Daytime usually AIS will see them before we do.
Maybe I need to move it higher (also it's just a normal VHF antenna, should I use a "AIS" antenna?).
Thanks for the information. Maybe I need to look into NavMonPc.

Jim,

I would have expected slightly better range with a 2 meter antenna height, but as long as the AIS detects the ships reliably in enough time to respond, it's probably not a big deal. An advantage of *not* having the antenna on the mast/spreader is that you have a decent chance that you will have a working spare antenna should the mast come down. With luck and care this is a very low-probability issue!

An antenna specifically cut for the AIS frequencies should perform a little better, but probably not dramatically so.

NavMonPc does some stuff very well, and other stuff not at all. It won't replace a charting program, but it might provide some other useful functions. I always appreciate feedback, so if you do try it, please let me know what you think.

[donradcliffe]

The AIS frequency is a marine VHF channel, so AIS-specific antennas are not going to improve things. It must be my eyes, but we always see ships on our AIS with a pulpit-mounted (3' VHF) antenna long before they are visible.

[dacust]

Originally Posted by donradcliffe

The AIS frequency is a marine VHF channel, so AIS-specific antennas are not going to improve things. It must be my eyes, but we always see ships on our AIS with a pulpit-mounted (3' VHF) antenna long before they are visible.

A marine VHF antenna is a compromise. It is tuned somewhere in the middle of the band. Theoretically, you'd get the best performance by having a separate antenna for each frequency you use.

Marine VHF covers 156-174 Mhz. AIS is at 161.975 MHz and 162.025 MHz, which is about 3 Mhz below the middle (165) of the band.

Theoretically you'd get better AIS performance with an antenna specifically tuned for that frequency. It would take someone with more knowledge than me to know if 3 Mhz is enough difference where a specifically tuned antenna would produce a noticeable difference.

-dan

[duncan_ellison]

Don,

Not exactly correct, but probably close enough for all practical purposes. There ARE specific AIS antenna which claim to be tuned to the AIS channels whereas a normal VHF antenna is tuned to the center of the VHF band (hopefully).

Having said that, height is king in VHF land, my AIS antenna is my VHF antenna at the masthead 62ft above sea level and I reliably see targets 25 nm or more away.

Seems like there are a lot of so called marine electronics installation 'experts' out there putting the supplied AIS antenna on the pushpit for customers because they want a nice simple installation.

Duncan

[CAELESTIS]

Originally Posted by JDGreenlee

Am very near procuring Vesper Marine Ltd. (New Zealand) AIS WatchMate 850 - Display with built in AIS Transponder; see AIS WatchMate Transponder - Display with built-in GPS antenna
as soon as FCC approval is obtained.
Comments?

That will be the one I buy. You can use it as a stand alone with very low power draw, or send the info to your chartplotter as an overlay. The best of both worlds.

[sinbad7]

Using a chartplotter with all navigation information available on the same chart is definately preferrable. I can thoroughly recommend the FREE OpenCPN chartplotter which has it all. Have a look at the attached sample.

Tore

[jeffrobbins]

A "tuned" antenna affects transmit much more than receive. Even still, in practical terms the difference in this case is fairly small.

We did some range testing out on the water and couldn't see any significant difference. Interestingly we did notice differences based on antenna/vessel orientation and antenna location on the vessel which I suspect is due to radiation patterns and interference sources. However, as Duncan said height is the most important factor.

However, how much range do you need? Also, keep in mind the decision to make Class B 2W was based on achieving a usable range of about 7nm-10nm with an average height antenna.