AIS: How much antenna do you really need?

[Here's our MadMariner feature from a couple weeks back. —Tim]

About a year ago, when I first installed an AIS receiver aboard my boat, I made an effort to see how little I could spend to display AIS vessels on my chartplotter. The result: $267.46 (here is the breakdown).

The reason I bring this up is that I’m still using the same antenna with the Shine Micro RadarPlus AIS-BX Class B AIS transceiver I installed last fall: a four-foot Shakespeare Galaxy 5400-XT, which is an antenna intended for conventional VHF use. On my boat, a C-Dory 22, this antenna is mounted about six feet above the surface of the water.

Virtually every radio manufacturer and installer will advise you to select an antenna tuned for the frequencies you will use most. A quick look at any antenna manufacturer’s product catalog reveals that there are “VHF” antennas and there are “AIS” antennas. AIS antennas are optimized for the two specific frequencies AIS uses, while VHF antennas are optimized to perform over a wider range of frequencies encompassing all the marine VHF channels.

Further complicating matters is the availability of antenna “splitters” that allow you to connect an AIS transceiver to the same antenna you use for your primary VHF radio. Easy! However, most authorities suggest that a separate, dedicated AIS antenna is preferable, especially if you have a Class B AIS unit that transmits your vessel’s data.

So, what is the story on antennas?

I don’t claim to be the ultimate authority on the intricacies of radio technology. Far from it. It’s practically black magic, as far as I’m concerned. Nevertheless, my experience suggests that some of the AIS antenna guidance discussed above is important for the average boater, but some is not. Furthermore, you can save a bit of money if you understand the distinction.

ANTENNA VS. SPLITTER

To wit: I think a separate antenna, dedicated to AIS, is far preferable to any splitter. Common sense and anecdotal evidence suggest that this should improve the performance of both your AIS and your VHF gear. If two radios share an antenna, particularly if both are transmitters, not just receivers, it’s inevitable that conflicts will arise.

The splitter is designed to prevent conflicts from damaging either component, but to accomplish this, the splitter essentially ignores one of the devices, isolating it from the antenna for the duration of the conflict. No matter how fast or efficient the splitter is, some signal has been lost. Also, if the splitter fails, there is a tiny possibility that either or both of the radio units could be damaged. Why take these risks?

Since a splitter costs about the same as an antenna, I think this is an easy choice to make. To be fair, I’ll admit that it isn’t so much the financial savings that make antenna splitters attractive, it’s the time and labor savings: A splitter eliminates the need to mount and run cable for a new antenna. Also, on some vessels it can be difficult to find a spot to mount another antenna.

VHF VS. AIS ANTENNA

Despite those perks, I would encourage you to bite the bullet and install a separate antenna for your AIS equipment.

But I want to suggest that once you’ve decided to mount a separate antenna for AIS, it doesn’t really matter much whether it’s an AIS antenna or a plain old marine VHF antenna.

If you’re buying a new antenna, then by all means select one designed specifically for AIS. But if you’ve got an old VHF antenna sitting around, you can use it, and I’ll bet you never notice the difference.

Here’s why: A general-purpose marine VHF antenna is designed to work reasonably well over the whole range of frequencies that comprise the marine VHF spectrum, 156.000 through about 162.000 MHz.

AIS uses two channels way up at the top of this range; AIS channel 1 is 161.975 MHz, and AIS channel 2 is 162.025 MHz. So while a general-purpose marine VHF antenna isn’t optimized for these specific frequencies, it is designed to work reasonably well over the entire range of marine VHF channels, including these frequencies. For most recreational users, and almost certainly for those equipped with receive-only AIS units, this is probably good enough.

How can I make such a pronouncement? I’ve got some evidence to support my position.

Remember the antenna I use for Class B AIS on my boat? It’s a plain old VHF antenna, four feet tall, mounted six feet above the surface of the water. The top of the antenna is therefore about 10 feet above the surface of the water—awfully low. It’s also mounted within one meter (about three feet) of the eight-foot antenna attached to my VHF radio, closer than is strictly advisable.

In short, this antenna, mounted in this location, is not ideal for AIS. Yet I consistently observe AIS targets much farther away than I care about.

In the screenshot above from Rose Point Coastal Explorer 2009, for example, my boat is about to enter Union Bay from Lake Washington, and I’m receiving AIS data from vessels across Puget Sound on Bainbridge Island, through the range of low hills that constitute Seattle. The ferry boat Kitsap off Point Vashon (in the upper-left-hand portion of the chart) is nine nautical miles away, with two hills in between (Capitol Hill and West Seattle).

Pretty good for an antenna that’s sub-optimal in at least three ways.

But wait. How does my sub-optimal antenna work on the broadcast side of the equation? How far away can my boat be seen by other AIS-equipped vessels? I suspect that this is where having a tuned AIS antenna would really make a difference, especially when you consider the low-power (2-watt) transmission specification for Class B AIS units.

I’ve been discussing this with the folks at Shine Micro, and we’re currently making plans to take some test measurements on my boat using a couple different antennas, so stay tuned.

Meanwhile, I’m going to stand by my current advice: If you’re just starting to consider adding AIS to your navigation system, and especially if you’re on a budget, see if you can re-purpose an old VHF antenna instead of buying either a splitter or a new AIS antenna. If you don’t have an old VHF antenna, buy a new AIS antenna rather than a splitter.