AIS Antenna Location / Splitter

[daddle]

Originally Posted by Jon Hacking

I see [some] ships reliably at 20-25nm.

The number of ships I've seen and the AIS has not is astonishing. So antenna height is quite useless.

[Jon Hacking]

Our visible horizon (& effective radar range) are both much less than our AIS horizon. There might be some distant ships that are outside our visual range that we don't see on AIS for some reason (how would we know?) but they're also too far away for us to worry about. Some boats don't carry AIS so we still need to keep a good watch, but they're small.

But virtually all ships that we see visually also show up on our AIS.

We just re-crossed the shipping lanes coming into Singapore - probably the busiest shipping lanes in the world, often exceeding a ship every minute. AIS was MAGIC. The only ships that didn't show up were 2 military vessels, which is understandable.

Some tugs don't have AIS, which I think is really poor, as their tows are often more than the 300 tons requiring AIS. And those tows can be really dangerous, especially at night (& the tugs out here often don't show correct lights...) But some tugs (& even some small, tramp freighters) are now carrying AIS, so it's getting better.

[fairbank56]

Jon,

Yes, you will see significant improvement in range with an antenna that is 50' higher. AIS generally has somewhat better range than voice because AIS is digital. You will also see better range over open water than I do with my shore station because signals I'm receiving are going partially over land which causes some signal loss. Which splitter was your friend using? I have not heard of any problems with them. I'm talking about active splitter's used with transponders, not trying to use a passive one for AIS receive only. The one from Vesper Marine has much less insertion loss than most others. Other's can have 3-4db loss on the receive side but that sounds much worse than it actually is. You could use a splitter to share the masthead antenna and still have your stern mounted antenna for emergency purposes. It's not advisable to place two VHF antenna's next to each other anyway.

Eric

[skipmac]

Originally Posted by btrayfors

I agree with Capt. Douglas (who, by the by, is also an Extra Class ham). Do not use a splitter. Why risk losing the most important communications device you have aboard, when there are other inexpensive and better alternatives which also offer some redundancy?

Bill
WA6CCA

Hi Bill,

Does that apply to all splitters? My previous boat I used a manually switched coax splitter from the masthead antennae to feed the boat's VHF and the FM stereo. I was told by the shop that the splitter was rated for the full transmit output of the VHF.

Kept it for several years and had no problems but the splitter was right at the nav station where I could remove it and connect the antennae directly to the VHF if necessary. Never did any specific tests to compare range but subjectively didn't notice a significant loss after adding the splitter. It certainly improved the FM stereo reception.

[donradcliffe]

First of all, a pushpit mounted antenna will give you more than adequate range for the AIS as a collision avoidance tool. I use a separate pushpit antenna and usually pick up ships at about 12 miles and other AIS B transponders at 3-5 miles (probably depending on whether they have masthead antennas or not). The ships typically see me at 6-8 miles.

My transponder and VHF radio sit side-by-side, and I can easily swap antennas. When offshore I sometimes use the masthead antenna for the AIS, and start to pick up ships at 15-20 miles. I set the Vesper alarm for CPA's of less than one mile and TCPA of less than 45 minutes, and am usually tracking the ship for some time before the TCPA alarm goes off.

On the flip side, I ALWAYS use the pushpit antenna when I am inshore, to cut down on the targets which the system has to monitor. When I get to congested areas like New York harbor or Fort Lauderdale, the AIS picks up over 100 targets with the pushpit antenna, and I don't want to deal with additional targets which are outside my immediate range of concern.

[donradcliffe]

Forgot to mention that the neighbor's 6 month-old VHF splitter has already crapped out, giving him no radio connections whatsoever until he bypassed it.

[Jon Hacking]

Originally Posted by fairbank56

Jon,

... Which splitter was your friend using? I have not heard of any problems with them. I'm talking about active splitter's used with transponders, not trying to use a passive one for AIS receive only... Eric

Hi Eric - This was 4 years ago so I'm afraid I don't remember the splitter brand. I think it was an active splitter, but probably only on the one side (cutting off the AIS receiver when the mic was keyed). We were trying to get it to work with an AIS receiver & were eventually successful, but it was a trial. Sounds like splitters have progressed since then so maybe I should give them another chance, but they still seem $$ for what they are.

[goboatingnow]

On boats it is a good engineering philosophy to set up systems so that the failure of one device minimizes the chance of failure of other systems.

Well that rules out networking, gas ranges, steering pedestals, etc. The best approach is failure mode analysis and backups rather then irrelevant redundancy.

Fact AIS , especially transponders benefits by having antenna height . Fact your masthead antenna is a completely under-utilised resource .

Hence use a good quality active splitter and carry a seperate emergency antenna that can be used for either AIS or VHF. In my opinion an emergency antenna should never be permentantly mounted ( the I can use my ais antenna for my VHF idea) as often the failure mode takes out all deck based gear.

Dave

[Jon Hacking]

So, why is everyone calling AIS devices "transponders"? I've always thought a transponder is a contraction of transmitter/responder. That is, devices that planes (& boats, with C-U-C-Me gear) carry that responds to a received radar signal by sending out a pulse of their own.

Certainly VHF gear can be made to work the same way, but that's not the way AIS works. AIS doesn't respond to an external signal. I'd call active AIS gear transceivers rather than transponders. Am I missing something?

[goboatingnow]

Technically correct but many people use the term transponder to mean a device that specifically identifies itself. Is it's sends an ID AIS does respond to shore based control also.

Dave

[daddle]

AIS Class A units are considered transponders because they reply at specific times (slots) depending on a protocol established in cooperation with other Class A units. I.e. they are responding to external protocol signals, if any can be heard. Class B units may not really be transponders as they aren't quite as cooperative, as I understand it, they simply blurt out their data when there is a quiet moment on the channel.

[Jon Hacking]

Cooperating with other units to fit in with transmission slots seems a bit thin to be a transponder. Maybe...

Originally Posted by goboatingnow

Technically correct but many people use the term transponder to mean a device that specifically identifies itself... Dave

You're absolutely right, Dave, & many folks use the term "Amps per hour" as well. That doesn't make it correct usage.

My EPIRB identifies itself. Does that make it a transponder?

I guess I'm wondering if we've got an opportunity to correct common (but perhaps incorrect) usage here.

[daddle]

AIS units are more like a transponder than a transceiver, or beacon. They do not respond to a specific request but instead listen and learn when to transmit based on the other units they hear. It almost as if the interrogation signal is coming from the GPS.

Transponder is the best term of those available.

If Amp-hour can be misused for energy then transponder is close enough for AIS.

[Paul Elliott]

The term "Transponder" is used by the standards organizations ITU, IEC, and IALA in their published documents that define AIS systems (I just checked). It may not be 100% technically correct terminology, but I think it's going to stick...

[fairbank56]

Originally Posted by daddle

They do not respond to a specific request

Yes, they do. Shore based AIS stations have the ability to interrogate a vessels AIS unit requesting specific information that it will respond directly to. Combine this with the fact that the unit listens for signals before transmitting and you have enough to classify it as a transponder. That's what they are called in all the official technical documentation, that's what the manufacturer's call them and that's what we will call them.

Eric