AIS..,....

[jssailem]

AIS and VHF signals are very close in wavelength.
Vespers idea:

Finally an antenna designed for both VHF and AIS!
This antenna is specially designed by Vesper Marine to be used with our Antenna Splitter. Up until now you've had to choose an AIS-tuned antenna that is optimized for the AIS frequencies but has lower performance for your VHF radio. Or you could choose an antenna that is designed for VHF radio communications but isn't optimal for AIS. Either of those works but we've got a better solution.

We've created an antenna that solves this dilemma. It has two key features that combine to make it ideal for use with our antenna splitter:

• Wide bandwidth of 6 MHz means it will cover 3 MHz above and below the center point
• 159 MHz center frequency gives excellent performance for VHF radio (156 Mhz) and AIS (162 MHz)

@Bob S I have no experience connecting the Vesper and Garmin products. Vesper is NMEA 2000 compliant. It would depend on Garmin. You should post the question on the Vesper website. They are great at getting back to you. I believe there is now a Vesper distributor in NJ. You might give them a shout.
Pre-sales or other questions: info@vespermarine.com
New Zealand Headquarters
Vesper Marine Ltd.
45 Sale Street, Freemans Bay
Auckland, New Zealand
Phone: +64 9 950 4848


United States Sales and Service
Vesper Marine, Inc.
128 Bartlett Ave
West Creek, NJ 08092

Phone: +1 609 246-5067

 

[Gunni]

Has anyone come to a definitive answer over masthead vs stern rail antenna?

My AIS antenna is on my stern-mounted radar pole. I see AIS targets out to at least 20 miles. More than enough distance to have full situational awareness. The pole is a KATO product with provision to level the radar dome and attached AIS antenna. That enhances radar reception, not sure that it does much for VHF. Short co-ax run to my chart plotter where the AIS overlays the chart.

 

[Bob S]

My AIS antenna is on my stern-mounted radar pole.

Wonder how well it would work on top of a dinghy davit. Might not be high enough.

 

[jssailem]

Here you go Bob. The higher you go the further you will transmit and receive.
https://www.everythingrf.com/rf-calculators/line-of-sight-calculator

 

[dlochner]

Does AIS share the same antenna as the VHF? I did a little searching here and on the Cruisers forum and it seems a lot of people are very happy with the Vesper XB8000s and less money than Garmin's AIS 800. My boat has the Garmin 7612 at the helm but it's not integrated with my Raymarine ST6001 which I learned when troubleshooting it. The Garmin chartplotter has WiFi as does the Vesper XB8000 would thy share or would there be conflict?

They can share antennas with a splitter. Vesper sells one that allows the VHF, AIS, and FM stereo to share the same antenna. It is not possible to simply connect the 3 antennas together without a splitter, if you could, then when transmitting on the VHF or AIS the other devices would fry.

 

[Hello Below]

Here is another calculator that I find very useful particularly for AIS:

http://cepd.com/calculators/propagation.htm

 

[JohnShannon]

Hello Hello what a handy calculator. So looking at the GX2200 data sheet the AIS sensitivity is 0.5uV (that works out to -113dbM I think) then plunking that # into the calculator with a 2W transmit power it looks like AIS line of sight is 1000 miles or more! I guess I swag that antenna gain compensates for cable loss. In any case even at 2W transmit power it looks like range in any practical sense is completely determined by the antenna heights and not by loss of signal due to distance. Makes me wonder of course why the big ship are blasting away at 25W?

 

[jssailem]

why the big ship are blasting away at 25W

Maybe, because they can.

 

[JohnShannon]

Well maybe but too much range jams the channels. Still marine traffic dot com shows receiver stats. Receiver 112 is at 925 meters high and is showing reception out to 1000 nm while line of sight should stop the party at 75mi or so. So that is some weird rf propagation thing I suppose.

 

[jssailem]

@JohnShannon This link might be helpful to understand how AIS works.

https://www.navcen.uscg.gov/?pageName=AISworks
The explanation of how it works helps to address the question of "high power" transmission.

This paragraph helps identify the importance of height of antenna.

The system coverage range is similar to other VHF applications, essentially depending on the height of the antenna. Its propagation is slightly better than that of radar, due to the longer wavelength, so it’s possible to “see” around bends and behind islands if the land masses are not too high. A typical value to be expected at sea is nominally 20 nautical miles. With the help of repeater stations, the coverage for both ship and VTS stations can be improved considerably.​

 

[SG]

Does AIS share the same antenna as the VHF?

Depends on the AIS unit. Some do some don't. Sharing the antenna is a big plus to me, as I try to keep my "wiring" to a minimum to make tidy installations.

You NEED a SPLITTER if you try to multiplex on the same antenna because otherwise you'll likely damage the receive function of the your AIS unit. The "sentences" of the AIS unit are allowed in periodically (they broadcast on a different frequency within the VHF range).

Some splitters work better than others.

 

[Richard19068]

Makes me wonder of course why the big ship are blasting away at 25W?

I think so they can step on all the people transmitting at lower wattage. (cynical I know).

Edit: Addressed toward VHF voice communications, not AIS.

 

[Gunni]

AIS was built out for commercials, rec boaters just get to play in their world. And nobody is transmitting at 25 watts. The commercials broadcast at slightly higher power than Class B does but at 2 watts on your Class B your signal won’t reliably travel far unless it gets picked up by a repeater.

 

[Brian D]

AIS uses CSTDMA which is Carrier Sense Time Division Multiple Access. Maybe, just maybe someone will interfere with another but I am sure that is far and few between. TDMA are slot assignments. Your AIS just does not transmit when it wants to. It listens to what is going on and finds an empty time slot. With permission from a control station, you transmit on that time slot. Now, a master station controls all these time slots. Probably the USCG. However, since this is Carrier Sense it is possible that there is someone sending either used time slots or empty time slots. Regardless, your AIS just does not transmit when it wants to.

TDMA is just not an open transmit when you want protocol. That protocol is called ALOHA. ALOHA uses time slots but does not have anyone controlling those slots. Your ALOHA system transmits when it thinks there is no one using that slot. Lots of data collisions using ALOHA.

 

[jssailem]

@Brian D has got the idea... From the Link above (message #50)... The system is self limiting. Not based on RF signal strength.

The required ship reporting capacity according to the IMO performance standard amounts to a minimum of 2000 time slots per minute, though the system provides 4500 time slots per minute. The SOTDMA broadcast mode allows the system to be overloaded by 400 to 500% through sharing of slots, and still provide nearly 100% throughput for ships closer than 8 to 10 NM to each other in a ship to ship mode. In the event of system overload, only targets further away will be subject to drop-out, in order to give preference to nearer targets that are a primary concern to ship operators. In practice, the capacity of the system is nearly unlimited, allowing for a great number of ships to be accommodated at the same time.​

 

[Brian D]

SOTDMA has priority over CSTDMA. Just so you know. LOL (trivia question at the next club outing)

One more tiny note. If there is no control station then TIME is the control based on the protocol's standard, or Interface Control. This means that GPS is the controlling factor. Everyone MUST be on the same time click in order for this to work, else we will be interfering with everyone else. The protocol probably has certain time slots for priority data, certain time slots for maintenance messages, certain time slots for SOTDMA users, etc... See how this works?

 

[jssailem]

The protocol .... has certain time slots

For all data. Yes it does.

 

[Monterey385]

Greg: I’m sitting in Great Salt Pond right now and agree completely with your assessment. Just concluded the 20 hr transit from Sandy Hook, NJ and my AIS was the most valuable piece of equipment I used. I have the Raymarine Class B unit. Highly recommend transpond feature to “be seen” and positively identified by ship’s bridge. Ambrose Channel crossing coupled with fishing vessels running the 3 mile line were but two examples where the AIS picked up returns long before my radar. Numerous NavAids in NY Harbor are now sqwaking AIS as well.

 

[SG]

More from: https://www.navcen.uscg.gov/?pageName=AISBroadcasts
If you seek information on the subject, you might peruse this ths site.

Note: There are now navigational aids (including those floating and fixed) which are reported (i.e., located) on AIS displays. If you don't use a chart plotter, and instead use the dedicated display of an standalone AIS unit, you might become alarmed that the place you heading is co-located with something :^)))