AIS transponder AND integrated AIS receiver/VHF radio

[Auspicious]

Originally Posted by Martkimwat

I'm not sure that is correct, it is still line of site, and if your AIS antenna is down low, you will have much less range and be seen at a much lower range than if it is up high, at the top of the mast.

Let's look at numbers. Line of sight (LOS) radio horizon, nm is 1.23 x SQRT (height of eye, ft). Unit conversion is baked into the 1.23.

Compare an AIS antenna on a spreader at 50' with one on a radar pole at 14' with range to a commercial target at 100'.

Higher antenna 1.23*SQRT(50) + 1.23*SQRT(100) = 21 nm
Lower antenna = 17 nm

Not a big deal.

LOS to another sailboat with a low mounted antenna = 9 nm

That should be fine.

Originally Posted by Dsanduril

If you don't use a splitter then your own receiver, on a separate antenna, will receive your transmitted signal. That could mean, as noted above, that you see yourself as a collision risk. It could also result in an overload (probably temporary) of the receiver's input circuits depending on how close the receive antenna is to your transmit antenna.

Your description does not include attenuation through the splitter which reduces range and understandability. You neglect the failure mode of when the splitter fails and routes the output of one transmitter directly into other receiver. We aren't talking desense anymore - we're talking destructive damage.

[Cpt Pat]

Originally Posted by StoneCrab

The way I understand it, the transmission of AIS data only takes a few milliseconds, so the sharing is sort of a non event. Your radio can even broadcast the AIS info while you are transmitting. The interruption is so brief that you or the receiving station can't tell it happened. This is the magic of modern computing.

I'm not a salesman for Vesper, but I did get excited about their new Cortex product that I saw in fall. I was thinking about buying their XB-8000 AIS transceiver when I stopped in the booth, but when I saw the Cortex, I decided to hold off until the Cortex hit the market.

Vesper Cortex is a new integrated VHF with AIS transceiver and base maps onboard. It is due to hit the US market in June, 2020. The Cortex is unique in that the handset looks like a smartphone and is wirelessly connected to the base-station. Like a smartphone, it has a touchscreen which displays the AIS targets and allows for DSC calling by tapping on the target who you want to call. Pretty slick.

The base is also a multiplexor for NMEA 0183, NMEA 2000, and WiFi. Since it has a built in GPS and heading sensor for the AIS functions, it can do Anchorwatch. It also allows for monitoring of a built in barometer, NMEA 2000 sensors and five additional sensors ( 5 Inputs, Digital (0-24V), Analog (0-10V). Now if they can only figure out how to overlay RADAR on the display, it will be like having a MFD in your pocket.

The AIS transmission time slot is 26.67 milliseconds in length. It's audible if you are receiving a VHF station when an AIS transmission occurs. While that "hole" in reception may not seriously affect reception intelligibility, it is very annoying.

> Your radio can even broadcast the AIS info while you are transmitting.

Incorrect. The AIS transmission is re-routed to a dummy load when you are transmitting on VHF when using a standalone splitter. When the splitter is incorporated into a combined AIS/VHF transceiver, the AIS transmission is inhibited. Vesper specifications clearly state that VHF transmissions take precedence over AIS transmissions. And reception of that one AIS transmission - that will not be received by other vessels - may be critical in a traffic conflict situation. I suggest verifying for yourself what I just wrote with Vesper. Both transmitters transmitting to the same antenna at the same time would require a large and complex RF multiplexer. Otherwise, it violates physics.

The AIS receiver is also incapable of receiving AIS data packets while you are transmitting on VHF, which would also require an RF multiplexer. The standalone AIS/VHF splitters on the market are simple A/B switches - that allow receiving with both AIS and VHF only while neither is transmitting. But when one transmitting path is "on" the other path is "off" with priority being given to the VHF transmitter.

These issues are the reason behind why the IMO rules explicitly prohibit using splitters on compulsory AIS installations.

[Dsanduril]

I guess it depends on the splitter and the purpose. The Vesper splitter, for instance, has a 12dB gain for received AIS signals because it has a built-in amplifier. Insertion loss for the connectors on the transmit side of things is less than 0.2dB.

Yes, if your splitter can fail closed between two radios, well, you can probably kiss at least one of them goodbye. There are always tradeoffs.

Personally I do prefer a separate VHF and AIS antenna, with vertical separation. Both AIS and VHF in the marine environment are vertically polarized. Vertical separation means you can limit your de-sense to nearly zero. However, on most boats it leads to blindspots for the lower antenna where the supporting structure blocks, and possibly reflects radio signals. Another tradeoff.

[StoneCrab]

Re-reading the above posts...it seems that there is some confusion about how AIS works. AIS is just above the Marine VHF channels and is still VHF. The line of sight rules all still apply.

Not all splitters are built the same. Some have more losses than others. Ideally a splitter will detect the transmitting unit (AIS or VHF) and isolate the other line to prevent a blowout from the transmitting power getting blow straight into the receiver of the other unit.

An integrated system controls both for a single antenna solution. If the splitter contains an antenna amplifier, performance can actually be enhanced.

[Auspicious]

Originally Posted by Dsanduril

I guess it depends on the splitter and the purpose. The Vesper splitter, for instance, has a 12dB gain for received AIS signals because it has a built-in amplifier. Insertion loss for the connectors on the transmit side of things is less than 0.2dB.

The 12 dB gain does not take into account the impact of noise floor on signal-to-noise ratio. That affects intelligibility which leads to dropped bits which means lost messages.

I simply don't believe 0.2 dB real insertion loss. It certainly doesn't take into account radio final amp foldback that reduces power in the face of impedance bumps at the extra connectors.

Originally Posted by Dsanduril

Yes, if your splitter can fail closed between two radios, well, you can probably kiss at least one of them goodbye. There are always tradeoffs.

The switches are solid state - either PIN diodes or switching transistors. If/when those fail they can easily fail shorted. It's so much easier to just have a second antenna.

[slug]

Originally Posted by redneckrob

Like many people I originally bought a VHF radio that had an AIS receiver built into it. I just upgraded to a standalone SOTDMA AIS transceiver with an integrated splitter. I get the theory, and in theory there shouldn't be any issues with having the two going to the same antenna because of how the splitter works. But I was wondering if anyone has any real-world zexperience running a receiver and transceiver on the same antenna using a splitter and any potential issues?

AIS transmission is line of sight

The higher up the further you can broadcast

Since is not possible to fit two masthead ant on a small craft , your best solution is to use a splitter

A two ant system , with the AIS ant mounted on the top spreader, is another solution

Good project next time your mast gets pulled

Masthead ants frequently get blown off.... bird strike or lightning

A second ant on the spreader is handy

[Dsanduril]

Originally Posted by Auspicious

... It's so much easier to just have a second antenna.

If it was so much easier there wouldn't be a market for splitters and no one would be discussing their pros and cons.

[Cpt Pat]

Originally Posted by Auspicious

I simply don't believe 0.2 dB real insertion loss.

... And you are correct to not believe that claim. I measured more loss with the Vesper splitter: 25 watts in, 18 watts out = 1.4 dB loss.

I used a 50 ohm dummy load to remove the antenna and coax as factors. The measuring instrument was a model 43 Bird thruline watt meter with NIST-traceable calibration.

The 12 dB amplifier in the AIS receiver path only amplified noise (raised the noise floor), just as you surmise. I made that determination using a spectrum analyzer.

A more obscure problem I observed was intermittent dropped packets from other very nearby vessels. I hypothesize that when packets were dropped, those vessels were transmitting on a timeslot directly adjacent to mine and the splitter didn't switch back to receive mode quickly enough, dropping the packets. I'd need a sophisticated test lab to prove that, but I've seen strong anecdotal evidence. I haven't seen the problem since I removed the splitter from the transmission path over a year ago.

N8QH

[Cpt Pat]

Originally Posted by Dsanduril

If it was so much easier there wouldn't be a market for splitters and no one would be discussing their pros and cons.

People use splitters so they can maximize use of the valuable location of their masthead antenna.

And because they believe there is something to be gained by detecting vessels 20 miles or more away.

Whichever approach is "easier" is subject to taste and debate.

Whichever approach is more reliable needs no debate: complexity is evil. Splitters add more complexity than do separate antennas. And if one antenna fails, you still have service from the other antenna because the systems are decoupled.

[smac999]

the other issue you may have is showing yourself. as the radio will pick up the aisTX.

I had the issue with a garmin screen and garmin ais VHF + transmitting AIS. it was showing itself on the plotter. I was able to turn off the AIS on the garmin VHF.

on simrad screens, if you enter the mmsi into the plotter, it will ignor itself.

each system will behave differently.

[Dsanduril]

Originally Posted by Cpt Pat

Whichever approach is more reliable needs no debate: complexity is evil. Splitters add more complexity than do separate antennas. And if one antenna fails, you still have service from the other antenna because the systems are decoupled.

By that argument we'd all be in row boats. Sails and engines all add complexity and reduce reliability in comparison to oars. In another thread you posted that you have flown more than 100,000 miles. Those machines that keep you in the air are incredibly complex. The also tend to be pretty reliable. Complexity, in and of itself has very little bearing on reliability or even functionality.

Their systems generally have redundancy, which is one way to gain reliability in complex systems. Same as your dual-antenna approach provides redundancy. I'm not arguing against two antennas, but they are generally not "easier" and unless done well (which is not as easy to find as you might think) they may decrease rather than increase overall system reliability.

[Cpt Pat]

Originally Posted by Dsanduril

By that argument we'd all be in row boats. Sails and engines all add complexity and reduce reliability in comparison to oars. In another thread you posted that you have flown more than 100,000 miles. Those machines that keep you in the air are incredibly complex. The also tend to be pretty reliable. Complexity, in and of itself has very little bearing on reliability or even functionality.

Their systems generally have redundancy, which is one way to gain reliability in complex systems. Same as your dual-antenna approach provides redundancy. I'm not arguing against two antennas, but they are generally not "easier" and unless done well (which is not as easy to find as you might think) they may decrease rather than increase overall system reliability.

Yours is a well thought-out argument.

The basic "pillars" of a reliable service are:

1) Redundancy: have more than you need of something critical.
2) Diversity: have more than one way, or technology, for providing a critical service.
3) Duplication: have more than one identical copy of a critical service.
4) Isolation: isolate critical functions so if one fails, it doesn't impact the other(s).
5) Separation: physically separate critical services so a single physical casualty doesn't impact them all.

An "auxiliary" sailboat (one that has an engine) is a more reliable form of transportation than a rowboat (or a motorboat) because it has multiple independent forms of propulsion: wind and an engine. (And if you want to get around Viking-style: you could even include oars). It is more complex than a rowboat, but the complexity-to-reliability ratio is pretty much one-to-one in a virtuous progression. Each level of complexity adds one or more of the pillars of reliability. There doesn't need to be any unnecessary complexity built into a sailboat for it to be a sailboat.

The same is true of airplanes. A Cessna 150 has only as much complexity as is needed to fly. The Space Shuttle, on the other hand, was very complex and turned out to be less than reliable: with 2 total failures in 135 flights. Its complexity is what ended its usefulness. Capsule-based spacecraft (Soyuz/Apollo style spacecraft) are simpler and therefore safer and more reliable. The Air Force provided part of the funding for the space shuttle, and it insisted on funding only something with wings...

Comparing a sailboat to a rowboat, the sailboat has the advantages of:

Redundancy: you can motor-sail.
Diversity: sails and engines are very different technologies.
Duplication: fore and aft sails, that can be used independently.
Isolation: a dismasting doesn't disable the engine. An engine failure doesn't disable the sails.
Separation: This quality is limited, given the small space involved, but systems can be kept separate insofar as possible.

I've chosen the terms for the two enemies of reliability: complexity and coupling, from an excellent book titled: Normal Accidents, by Yale sociologist Charles Perrow. I recommend it.

Using a single antenna for both AIS and VHF doesn't provide isolation or separation. As the saying goes: all your eggs are in one basket. I'm not saying that's "wrong" because we all choose the level of risk that's acceptable. We all must do our own risk/benefit analysis. But I believe a single antenna does add a degree of avoidable risk - often at a higher cost than employing two antennas. A splitter may also add additional systems management burden and failure vulnerability: as does adding any active electronic system.

[Auspicious]

Hi Pat,

Originally Posted by Cpt Pat

I used a 50 ohm dummy load to remove the antenna and coax as factors. The measuring instrument was a model 43 Bird thruline watt meter with NIST-traceable calibration.

Definitely need 50Ω dummy loads for isolation and diagnosis. I only have an MFJ antenna analyzer. I like being able to quickly and easily find resonant frequencies. That's helped me find the very rare bad antenna tuner (this is like finding a unicorn).

Originally Posted by Cpt Pat

A more obscure problem I observed was intermittent dropped packets from other very nearby vessels. I hypothesize that when packets were dropped, those vessels were transmitting on a timeslot directly adjacent to mine and the splitter didn't switch back to receive mode quickly enough, dropping the packets.

Maybe. Definition of acronyms to follow are not for you. *grin* The other real possibility is that increased noise floor and therefore reduced signal to noise ratio (SNR) translates into a higher bit error rate (BER) that leads to bad messages (dropped packets). I suspect noise. The splitter doesn't switch between transmit and receive (T/R) on the same port. That's the T/R switch in the AIS transponder and that really should be fast enough on a decent box.

Originally Posted by Cpt Pat

I'd need a sophisticated test lab to prove that, but I've seen strong anecdotal evidence. I haven't seen the problem since I removed the splitter from the transmission path over a year ago.

Fast network analyzer and a bench receiver? *grin* Improvement after taking the splitter out makes me think NF > SNR > BER as the failure mode. You were there, not me. Regardless, avoiding splitters is a good rule of thumb.

In a previous life we used a "splitter" to take one input run it through a cooled LNA through balanced impedance matched distribution network to four outputs for a special application. I think that thing cost over $50k back in the 90s. I'd never do that again. Time marches on. Today I'd digitize the input and share the data stream. You'd have to deal with quantization problems but those can be managed.

[Pizzazz]

There are many reasons to have separate antennas for VHF/DSC and AIS for large ships and they are mandated. The only issue that needs to be addressed is the receiver ignoring own vessel MMSI which most receiver only radios can do.

You can mount the transmitter antenna low - the reason for this is just being considerate to the other ships around you. A large ship will have its antenna mounted high, so you will get 10-15 nm range. A small power boat (antenna mounted low) will see you 5-10 nm out. There is no reason for a further detection range, especially if you are small (under 60 ft). You are just polluting the AIS display of the other boats.

Thus, you have little to gain with a high mounted transmit antenna. Use the receiver in the (high mounted) VHF radio antenna and the transmitter of the stand alone AIS system mounted low.

Finally, have a hand held VHF as back up.

[Cpt Pat]

Originally Posted by Pizzazz

There are many reasons to have separate antennas for VHF/DSC and AIS for large ships and they are mandated. The only issue that needs to be addressed is the receiver ignoring own vessel MMSI which most receiver only radios can do.

You can mount the transmitter antenna low - the reason for this is just being considerate to the other ships around you. A large ship will have its antenna mounted high, so you will get 10-15 nm range. A small power boat (antenna mounted low) will see you 5-10 nm out. There is no reason for a further detection range, especially if you are small (under 60 ft). You are just polluting the AIS display of the other boats.

Thus, you have little to gain with a high mounted transmit antenna. Use the receiver in the (high mounted) VHF radio antenna and the transmitter of the stand alone AIS system mounted low.

Finally, have a hand held VHF as back up.

That configuration solves all the OP's issues!