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Old 04-12-2021, 02:45 PM
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I'm in the process of a complete electronics replacement on my 30ft fishing boat (old setup was mid 90's vintage). The boat has a fly bridge and then a main helm in the cabin. I'm planning to put a VHF at each station for redundancy. My plan was to use a dual VHF / AIS antenna for each radio, and wire them with main helm connected to the NEMA network and then the other would be wired separately and not on the network (but same radio so could switch out in a pinch if needed). My question - if I go this route, when you use the main helm VHF will it stop transmitting AIS? I've got two antennas on the radar arch (both are getting replaced), would I be better served by making one AIS only and the other VHF only? Thank you!
Old 04-12-2021, 02:55 PM
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I can't tell if you want to receive AIS or transmit it. If receive you could use a SH 2400 with NMEA2000 wired to your MFD to receive it and use a remote mike at the 2nd station with a smaller MFD, maybe GIL will chime in here.
Old 04-12-2021, 03:23 PM
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Originally Posted by cmb1998 View Post
I'm in the process of a complete electronics replacement on my 30ft fishing boat (old setup was mid 90's vintage). The boat has a fly bridge and then a main helm in the cabin. I'm planning to put a VHF at each station for redundancy. My plan was to use a dual VHF / AIS antenna for each radio, and wire them with main helm connected to the NEMA network and then the other would be wired separately and not on the network (but same radio so could switch out in a pinch if needed). My question - if I go this route, when you use the main helm VHF will it stop transmitting AIS? I've got two antennas on the radar arch (both are getting replaced), would I be better served by making one AIS only and the other VHF only? Thank you!
If you want to use a single antenna for VHF and an AIS transponder, you need an active antenna splitter or an AIS such as the Digital Yacht AIT5000 that has a built-in splitter. Then what happens is that the AIS will briefly stop transmitting while you have the microphone keyed to transmit on the VHF. It works seamlessly and you can't really tell that there is a pause in the AIS transmission.

Normally you would not have two AIS transponders on board because it would create lots of confusion to have two units sending out the data for a single MMSI. Two VHFs and two VHF antennas is a very important form of redundancy, however.

Maybe it will help you to describe how my boat is set up, which also has two helms. I have a VHF mounted at the main helm that has a wired remote mike at the tower helm. I have second VHF mounted down below that has a wired remote mike at the main helm. There are two 17 foot VHF antennas. One is connected only to the VHF at the main helm. The other is connected to the built-in splitter on my AIS transponder and shared between the AIS and the VHF mounted down below in the salon.

I don't think it is a good idea to try and set things up so you can "mix and match" the VHF radios and the antennas. You introduce a lot more connectors into the antenna wiring as well as needing a switch. All of that can lead to impairing the signal quality and increase the risk of failure. Modern VHF radios are extremely reliable if you get a quality unit such as Icom or Standard Horizon. They just don't fail very often.
Old 04-12-2021, 03:56 PM
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I recommend a standalone AIS transponder. AIS transponders integrated in VHF's are CSTDMA, standalone transponders are available both as CSTDMA and SOTDMA. SOTDMA is the same transmission scheme as used in AIS Class A. SOTDMA Class B has 5W transmit power, is transmitting every 5s and has guaranteed slots in congested areas. CSTDMA has 2W transmit power, transmits every 30s and don't have guaranteed slots, it's just checking if the air is not used by other AIS transponders before transmitting.

The differences between CSTDMA and SOTDMA are explained here:
https://www.yachtbits.com/blog/sotdma-v-cstdma

Old 04-12-2021, 04:30 PM
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Originally Posted by abbor View Post
I recommend a standalone AIS transponder. AIS transponders integrated in VHF's are CSTDMA, standalone transponders are available both as CSTDMA and SOTDMA. SOTDMA is the same transmission scheme as used in AIS Class A. SOTDMA Class B has 5W transmit power, is transmitting every 5s and has guaranteed slots in congested areas. CSTDMA has 2W transmit power, transmits every 30s and don't have guaranteed slots, it's just checking if the air is not used by other AIS transponders before transmitting.

The differences between CSTDMA and SOTDMA are explained here:
https://www.yachtbits.com/blog/sotdma-v-cstdma
^^X2

While it's a little more expensive, I plan on adding a Cortex SOTDMA combo VHF/AIS transponder on my new to me 43' Sport Fish. I had the Vesper XB8000 on my last boat and was very impressed with it, but wanted the higher transmit power in SOTDMA. I need to add another VHF anyway (for redundancy) and the boat already has a 2nd, unused antenna.
Old 04-12-2021, 04:46 PM
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Simrad V3100, Raymarine AIS700, Em-Trak B951, Garmin AIS800, Amec Widelink B600 and Si-Tex MDA-5 are some of the SOTDMA Class B transponders available. The internals of these are all made by Amec or SRT.
Old 04-12-2021, 05:07 PM
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Originally Posted by Schmaltz~Herring View Post
I can't tell if you want to receive AIS or transmit it. If receive you could use a SH 2400 with NMEA2000 wired to your MFD to receive it and use a remote mike at the 2nd station with a smaller MFD, maybe GIL will chime in here.
I would like to send and receive.
Old 04-12-2021, 05:09 PM
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Originally Posted by Nomans View Post
If you want to use a single antenna for VHF and an AIS transponder, you need an active antenna splitter or an AIS such as the Digital Yacht AIT5000 that has a built-in splitter. Then what happens is that the AIS will briefly stop transmitting while you have the microphone keyed to transmit on the VHF. It works seamlessly and you can't really tell that there is a pause in the AIS transmission.

Normally you would not have two AIS transponders on board because it would create lots of confusion to have two units sending out the data for a single MMSI. Two VHFs and two VHF antennas is a very important form of redundancy, however.

Maybe it will help you to describe how my boat is set up, which also has two helms. I have a VHF mounted at the main helm that has a wired remote mike at the tower helm. I have second VHF mounted down below that has a wired remote mike at the main helm. There are two 17 foot VHF antennas. One is connected only to the VHF at the main helm. The other is connected to the built-in splitter on my AIS transponder and shared between the AIS and the VHF mounted down below in the salon.

I don't think it is a good idea to try and set things up so you can "mix and match" the VHF radios and the antennas. You introduce a lot more connectors into the antenna wiring as well as needing a switch. All of that can lead to impairing the signal quality and increase the risk of failure. Modern VHF radios are extremely reliable if you get a quality unit such as Icom or Standard Horizon. They just don't fail very often.
Sorry I wasn't clear, I was only intended to use AIS on one radio (one transponder), but have them both be enabled so if I needed to I could swap the radio's out.

Very helpful to have the description provided. Thank you.
Old 04-12-2021, 05:10 PM
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Originally Posted by abbor View Post
I recommend a standalone AIS transponder. AIS transponders integrated in VHF's are CSTDMA, standalone transponders are available both as CSTDMA and SOTDMA. SOTDMA is the same transmission scheme as used in AIS Class A. SOTDMA Class B has 5W transmit power, is transmitting every 5s and has guaranteed slots in congested areas. CSTDMA has 2W transmit power, transmits every 30s and don't have guaranteed slots, it's just checking if the air is not used by other AIS transponders before transmitting.

The differences between CSTDMA and SOTDMA are explained here:
https://www.yachtbits.com/blog/sotdma-v-cstdma
Very helpful. Thank you.
Old 04-12-2021, 05:27 PM
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Originally Posted by cmb1998 View Post
Sorry I wasn't clear, I was only intended to use AIS on one radio (one transponder), but have them both be enabled so if I needed to I could swap the radio's out.

Very helpful to have the description provided. Thank you.
The AIT5000 with the splitter that I have and mentioned above is a Class B+ SOTDMA AIS by the way.
Old 04-12-2021, 07:17 PM
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+1 on the Vesper Cortex. I have that installed at the helm. In the cabin I have a cortex remote handset. For redundancy I have a SH handheld. The cortex has a built-in switch which also has a secondary antenna connection to which I have my Fusion AM/FM connected through an adapter. With this configuration I run AIS, VHF, and Stereo off my single VHF antenna.
Old 04-12-2021, 09:38 PM
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Originally Posted by cmb1998 View Post
I would like to send and receive.

you can not send from a vhf ( well I think there is one weird model now). Generally all vhf ais are receive only.

Buy 2 non ais vhf radios and vhf antennas. And buy an ais with 3rd antenna which will be ais antenna.
Old 04-12-2021, 10:12 PM
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Simrad RS40B has built-in transponder.
​​​​​​
Old 04-12-2021, 10:34 PM
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Originally Posted by smac999 View Post
you can not send from a vhf ( well I think there is one weird model now). Generally all vhf ais are receive only.

Buy 2 non ais vhf radios and vhf antennas. And buy an ais with 3rd antenna which will be ais antenna.
There are now a number of VHFs now that also have AIS transponders. Vesper Cortex, for example.
Old 04-13-2021, 03:58 AM
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By law you are only allowed to have one working AIS transmitter on any vessel.
I am not a big fan of splitting an antenna for AIS transmit.
Old 04-13-2021, 05:23 AM
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GUIDANCE ON ANTENNAS FOR VHF AND AIS

If you have two VHF Marine Band DSC radios, you need two antennas.

If you have an AIS transmitter-receiver, you need a third antenna or you need a very fast-acting automatic antenna switch that will switch one antenna between the AIS transmitter and one of the DSC radios.

Although these fast-acting automatic antenna switches are often sold as "splitters", during transmission of one transmitter connected to the antenna "splitter" the other transmitter will not be connect to the antenna at all.

In the case of transmitting on the VHF voice radio, that means the AIS receiver and transmitter will not be connected, and when the AIS is transmitting, the voice receiver and transmitter won't be connected. The antenna is "shared" only when neither the AIS transmitter or the voice transmitter is transmitting.

The AIS transmitter operates at a different part of the band than the ship voice radio transmitters, so the antenna for the AIS transmitter must be tuned differently than the antennas for the ship voice transmitters.

If a shared antenna is used, that antenna must be a special wide-band antenna that can work on both voice and AIS frequencies, and not an antenna that is narrow-band and tuned for just voice frequencies or just AIS frequencies.

Regarding antennas for VHF Marine Band--either voice or AIS--the most important element for the antenna is the height of the mounting base. On small boats, increasing the antenna height is more important than the length of the antenna. To increase the boat's range of radio communication, mount the antenna base as high as possible.

With regard to using an fast-acting automatic antenna switch to let a radio and an AIS use one antenna, consider the following:

--the cost of the automatic switch may be greater than the cost of a separate antenna;

--the cost of a special wide-band antenna will be greater than the cost of a standard antenna that is designed for just voice VHF or just AIS;

The essential advantage of using the fast-acting automatic antenna switch is that it eliminates a second antenna; this can be advantageous on small boats where room for antennas at high mounting height can be very limited.

Last edited by jhebert; 04-13-2021 at 09:43 AM.
Old 04-13-2021, 09:16 PM
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Originally Posted by bobeast View Post
There are now a number of VHFs now that also have AIS transponders. Vesper Cortex, for example.

cortex is fancy junk.
Old 04-13-2021, 10:31 PM
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Originally Posted by smac999 View Post
cortex is fancy junk.
I've had mine for about a year now, and it has been flawless. Did you have any specific issues with yours or are you just talking out of your.....opinion?
Old 04-14-2021, 04:23 AM
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Originally Posted by jhebert View Post
GUIDANCE ON ANTENNAS FOR VHF AND AIS

If you have two VHF Marine Band DSC radios, you need two antennas.

If you have an AIS transmitter-receiver, you need a third antenna or you need a very fast-acting automatic antenna switch that will switch one antenna between the AIS transmitter and one of the DSC radios.

Although these fast-acting automatic antenna switches are often sold as "splitters", during transmission of one transmitter connected to the antenna "splitter" the other transmitter will not be connect to the antenna at all.

In the case of transmitting on the VHF voice radio, that means the AIS receiver and transmitter will not be connected, and when the AIS is transmitting, the voice receiver and transmitter won't be connected. The antenna is "shared" only when neither the AIS transmitter or the voice transmitter is transmitting.

The AIS transmitter operates at a different part of the band than the ship voice radio transmitters, so the antenna for the AIS transmitter must be tuned differently than the antennas for the ship voice transmitters.

If a shared antenna is used, that antenna must be a special wide-band antenna that can work on both voice and AIS frequencies, and not an antenna that is narrow-band and tuned for just voice frequencies or just AIS frequencies.

Regarding antennas for VHF Marine Band--either voice or AIS--the most important element for the antenna is the height of the mounting base. On small boats, increasing the antenna height is more important than the length of the antenna. To increase the boat's range of radio communication, mount the antenna base as high as possible.

With regard to using an fast-acting automatic antenna switch to let a radio and an AIS use one antenna, consider the following:

--the cost of the automatic switch may be greater than the cost of a separate antenna;

--the cost of a special wide-band antenna will be greater than the cost of a standard antenna that is designed for just voice VHF or just AIS;

The essential advantage of using the fast-acting automatic antenna switch is that it eliminates a second antenna; this can be advantageous on small boats where room for antennas at high mounting height can be very limited.
I am sure you are correct in a narrow technical sense and I respect the radio expertise you show in your posts. However, I have never been able to discern any impact whatsoever on my VHF performance from the active antenna switch built in to the AIT5000. Since I have two radios with identical antennas mounted at the same height, it is easy for me to compare. One radio is sharing an antenna through the AIT5000 and the other is connected directly to its antenna. When the Sea Tow radio check station was still operating in my area, I ran a number of side-by-side tests of the two radios. They both could be heard and receive the response at virtually identical range. I have never had an issue with anyone with whom I communicated via VHF on the water with one radio doing better than the other. So what ever difference may exist in theory, on the water with properly installed cables and connectors and a quality antenna, it seems to make no difference.

As for the need for an AIS specific tuned antenna, that is also something that in real life adds limited value. My AIS transponder has no trouble at all receiving transmissions from large ships 30-40 miles away using the conventional VHF antenna. My signal shows up on the Marine Traffic ground station that is 30 miles away and friends have seen me on their AIS receivers over 10 miles away. An interesting question is whether the benefit of using a 17' VHF antenna mounted on a base 12 feet above the water outweighs the benefit of a shorter 4 or 8' AIS tuned antenna. On my boat, the space in the tower legs for cable is at a premium and my two big VHF antennas use leave no good place for a dedicated AIS antenna with the recommended separation so the active switch was a good solution that as I noted, appears to have had no negative impact on performance of either the VHF or the AIS transponder.
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Old 04-14-2021, 07:27 AM
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Originally Posted by Nomans View Post
I have never been able to discern any impact whatsoever on my VHF performance from the active antenna switch built in to the AIT5000.
I don't recall making any remarks about how a fast-acting automatic antenna switch would affect performance in a discernible way, and particularly no remarks about a specific model like an AIT5000.

The effect on transmission through a fast-acting automatic antenna switch would be limited to:

--a very short delay in operating the switchover; the transmitting signal has first to be detected so the switch can be operated; this interval must be very short in order to not mutilate the transmission by clipping off the initial portion; since there is probably some delay in the transmitter output signal rising to full intensity, and since the detector can begin to detect the signal at a very low level, the switch-over to providing the transmitter direct access to the antenna can likely be done without harm; for this reason, this delay is probably not "discernable" to the operator.

--introduction of some signal loss by routing the transmit signal through the switching device itself; this loss should be minimal in a well-designed and well-made device; the loss ought to be less than 0.5 dB. Such a small loss is probably not "discernable" to the operator.

The effect on reception through a fast-acting automatic antenna switch would be limited to some signal loss when the antenna is being shared between two receiver inputs, but this can be compensated for with a pre-amplifier in the device that adds enough gain to make up for the loss in the splitting of the signal; the result is there is no discernible effect, and there could be an improvement if the original receiver sensitivity was lacking. But any amplification of radio-frequency signals tends to add noise to the signal. To prevent adding noise from a pre-amplifier, the pre-amplifier must use a very good active amplifying device.

A further requirement for a pre-amplifier in the fast-acting automatic switch device is immunity to overload from strong signals. VHF Marine Band radios are already prone to problems occuring from strong signal inputs to their receivers, so any pre-amplification could cause more harm than good when very strong local signals--possibly even out-of-band signals--are present.

The degree to which making a voice transmission on the VHF Marine Band radio will cause a discernible effect on the operation of the AIS transmitter or AIS receiver is entirely dependent on circumstances and coincidences. If you happen to make a transmission on the voice radio at the same time as another vessel is transmitting its AIS information, then that transmission of AIS data from the other vessel will not be received and that data will be lost.

Also, if your AIS transmitter's slot for transmitting your vessel's AIS data occurs while you are transmitting on the voice radio, that transmission might be lost, unless the designers of the fast-acting automatic antenna switch give priority to the AIS transmitter; if that is the case then your voice transmission will be interrupted to let the AIS transmitter take over the antenna. Such priority for the AIS probably is (and if not probably should be) designed into the fast-acting automatic antenna switch.

I don't make any argument that a fast-acting automatic antenna switch CANNOT be useful. I just think prospective buyers should be aware of what those devices are doing. They are not really sharing one antenna between two transmitters. It is possible for two transmitters to transmit simultaneously into one antenna, but it would take much more circuitry and external gear than could be used on a small boat.

The overall effect on the radio system of using a fast-acting automatic antenna switch is to add complexity. When the device is external to the VHF Marine Band radio and the AIS transmitter-receiver, there are additional transmission line cable and connectors. The device must be powered. Typically if the device were to fail there could be harm to the attached transmitters. There could be significant signal loss on receive.

In a situation where a boat already has one VHF Marine Band radio with a dedicated antenna, then adding a second antenna with a fast-acting automatic switch to connect a second VHF Marine Band radio and an AIS transmitter-receiver is a reasonable configuration. The primary VHF Marine Band radio retains exclusive use of its antenna.

My other remarks about the requirement for the antenna to work with both voice and AIS frequencies is a significant limitation in the system, and any boater planning to use one antenna for both voice and AIS transmitters needs to be aware of the requirement.

Last edited by jhebert; 04-14-2021 at 09:55 AM.

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