There’s some ongoing discussion and trouble-shooting in the comments of recent posts in this series…take a look here, especially. Good stuff.
If you’re a gadget-obsessed boating geek, I mean! 
Meanwhile, I wanted to report on some antenna testing I conducted. I’ve got two antennas (or antennae, if you prefer) mounted on the cabin top of my C-Dory. I was able to change them back and forth on my AIS receiver, and then wait several minutes to see how many targets appeared. Here are the results of that test, conducted while tied up inside the breakwater at Edmonds Marina.
A cheap, bottom-of-the-line eight-foot antenna with 6db gain picks up target up to about 15-20 miles away.
A somewhat more expensive four-foot antenna with 3db gain picks up targets up to about 10 miles away. That’s the image above.
Adding the Shakespeare AM/FM/VHF antenna splitter causes maximum range to decrease by about a third. In other words, with the splitter connected, the long antenna receives AIS transmissions out to twelve miles or so and the short antenna out to seven miles or so. Seven miles ain’t much on the "dual carriageway" we call Puget Sound!
By the way, this antenna splitter only improves FM reception by about 5-10%, in terms of subjective sound quality. The Edmonds Marina location is good for testing this, because it’s at the base of a bluff that obstructs signals from Seattle stations. Nothing comes in very well. What’s surprising is that adding the VHF antenna is only marginally better than no antenna at all!
Moral of the story: Get the tallest antenna you can, or mount it as high as you can, and leave the antenna splitter out of the equation.
Addendum: Here’s a picture of Two Lucky Fish with the two antennae visible. You can see how low they are…and why I’m not going to get them much higher without a radar arch!
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I was about to forward this to a friend headlined “objective testing of AIS splitter” … but then realized my friend couldn’t make use of this. There is a better test that would help him and others to make a decision to use a splitter or install a 2nd antenna. For most of us sailors that don’t have the option of adding a 2nd antenna to our masts, that 2nd antenna needs to go on our stern rail.
In a new test … could you compare using a splitter on our only masthead VHF antenna, vs. a reasonably inexpensive option of installing a 2nd antenna on our rail. E.g.
(CASE 1) Measure with a mast mounted antenna at 40 feet (typical sailboat mast height) and 30 feet (typical power boat with flying bridge mast height) both thru the splitter box (how high is a c-dory mast ??)
(CASE 2) vs. Option-1 A 4 foot antenna on a typical height stern rail.
(CASE 3) vs. Option-2 An 8 foot antenna on a typical height stern rail.
Any chance you could do that and publish the result ? Then we can see the trade off or benefit to splitting our mast head VHF’s.
Thanks !
Just installed an AIS transceiver (won’t say which class) here in Sidney BC. I went with the AIS antenna (tuned to the AIS frequency) and am picking up a target, MMSI NO. 003669976, 9.2kn at a distance of 32.5nm. This target is on the far side of the San Juans from Sidney. Sounds like antenna type is important
Tim, did you consider an alternative to the Shakespeare splitter mentioned, namely the clever “active splitter” from Miltech that electrically “moves” the antenna from AIS to VHF via a relay actuated from receipt of a transmit signal from the radio? My own limited testing shows very little actual loss (well less than 1db) due to antenna switching using this device.
Drew
Drew: I considered it, but I was trying to stay CHEAP. I already had the passive AM/FM/VHF splitter on board.
My informal testing really needs to be repeated outside the breakwater. Luckily, I’ve got an ideal testing location just off Edmonds, with line-of-sight right up Admiralty Inlet and down Puget Sound. There are plenty of targets present, so it should be pretty clear what sort of range you can expect.
To clarify, my C-Dory is a 22-foot cabin cruiser…no mast at all. Well, the radar is mounted on a “mast” about eight inches tall!
In some ways, the C-Dory is the OPPOSITE of an ideal boat for radio reception: It’s very short. The cabin floor is actually below the waterline, so the rails to which I’m mounting these antennae are only about six feet above the surface of the water. Maybe less!
And while I would love to conduct the test Jeff suggested, I don’t really have the resources (sailboat, money, time, equipment) to do it right. If somebody in my region would like to try something similar, though, I would be happy to help coordinate the effort, establish more rigorous testing methodologies, and publish the results here. Sounds like fun!
Another interesting test … does an antenna designed for AIS use (such as Shine Micro’s) actually have better performance that is measurable or a smaller form factor that can get the same performance as a 4 foot antenna ?
Maybe someone can send Tim such an antenna for his C-Dory ?
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