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Whoa! What a great dissertation. Thanks.
One question that has nagged me since we got AIS:
Our Raymarine chartplotter has several symbols for targets: one for big commercial, one for sail, one for small motor, one for passenger, and a generic symbol, perhaps useful if a transmitting vessel’s AIS has not identified itself as to vessel type. Sometimes I can see a generic AIS target, and when I click on it, all it will give me is distance and MMSI number. As it gets closer, it will then morph into a specific symbol, say, sailboat. Then if I click I get everything, vessel name, speed, course, etc.
Most other digital communications are sent in packets, which, as I see it, if true for AIS, the packet should be quite compact and contain all of the data. I either receive the packet, or error checking tells me that I don’t. Why, at some distances, do I get only partial information?
Not that it’s that important, just curious. Thanks.
SM 240 Aletes
There are a bunch of issues here and far too many to respond to each email sent individually so I’ll try to put down some basic principles and hopefully all will become clear.
- VHF band antennas can interfere with each other (or any other similar shaped long metal object) if they are within a wavelength of each other which for VHF is about 2 metres at the same plane (I.e. they won’t interfere if they are stacked one on top of each other (which really isn’t practical but it is true). This means that an antenna on top of the Mizzen will NOT interfere in any significant way with one on top of the Main but two Antennas on the same mast will interfere badly and should never be done.
- AIS transmits over VHF level frequencies (which are generally around 156-162MHz) but the VHF frequency range that we use for communications (156-157Mhz) and the AIS range (162Mhz) are fairly widely separated within the VHF band (with a few exceptions - the VHF weather channel for instance is around 162Mhz as well.)
- VHF Antennas are tuned for very specific frequencies…. People think that because they have a VHF antenna and an antenna splitter they are good…. If you have a typical Shakespeare 4’ antenna, it has a working range of 155.3 to 158.3Mhz while keeping the Standing Wave Ratio less than 3:1 (1:1 is perfect but less than 3 to 1 can be acceptable. The SWR is a measure of how much power is sent out by the antenna and how much is reflected back (and lost). The VHF antennas are generally optimized for VHF channel 16 (156.8Mhz) and the SWR of the antenna is optimized there but falls off rapidly on either side of that - think of a parabolic curve… the further away from the optimal tuning, the worse it gets exponentially. For instance the typical 4’ Shakespeare 5400 has a pretty good SWR of 1.18:1 at Channel 16 but it can only hold the SWR between 2 to 1 within 5Mhz of that and it falls off the cliff beyond that. That’s why they sell dedicated AIS antennas or antennas that are tuned midway between the two bands and have a broad enough SWR curve to be able to keep both within 3:1). The Shakespeare 5396 AIS antenna is tuned optimally at 158Mhz but again can keep the SWR within 2:1 down to 153Mhz and up to 163Mhz… so not optimal for Channel 16 or AIS but OK for both. Takeaway, do not use a standard VHF antenna for AIS as it is very suboptimal and you may end up losing much of your power (which is already only 5W compared to the 25W of your VHF).
- Antenna splitters allow you to use the VHF antenna (or a combined VHF/AIS antenna WHEN YOU ARE NOT TRANSMITTING but they still can’t fix your antenna tuning. I know we don’t spend a lot of time actually transmitting but it is worth noting that your AIS system is running blind when you transmit on VHF if you use a splitter and if you understand how AIS works, each station gets a small slice of time to transmit its particulars before moving on to the next…. So it wouldn’t be impossible to cover up a boat that might be in your danger zone with back and forth transmissions.
- VHF signals are line of sight (well they will actually go 4/3 times the radius but let's not get too picky) so height matters (getting the antenna up helps extend the range that your antenna can see over the horizon at sea level. Unfortunately, VHF antennas are also vertically polarized which means they are VERY directional (they send their signal out perpendicular to their axis and they are much better when longer. This means that a 4’ antenna with a 3dB gain will have half the power of an 8’ antenna with a 6dB gain (or an 8' has twice the radiated power of a 4') and a 16’ antenna will have a 9dB gain (twice again or 4 times that of a 4’ antenna). Unfortunately, as the power goes up, the signal also gets flatter in plane. The 4’ antenna’s signal is roughly spherical but as you go up in power, the signal shape extends out perpendicularly from the antenna so it gets worse and worse when the antenna is pointed away from the source. That’s why you don’t see 8' & 16’ antennas on sailboats (or monohulls at least) because we put them up on the mast and the rolling of the boat can inadvertently “aim” the antenna away from our targets to the point where you can’t pick up much at all in rough seas. So, we live with 4’ antennas but try to put them up as high as we can to extend the range. (This is why the Coast Guard can broadcast and receive so well from shore… their antennas are turned and optimized for both height and range).
- Our VHF radios can typically broadcast at 25W or 1W while our class B AIS radios are limited to only 5W (unlike the Class A used on commercial ships). Similarly, your handheld VHF is probably setup to broadcast at 1W but has a 5W high power setting. It should be noted that the watt rating does not affect range but it does provide the ability to overcome another signal (or noise) weaker signal and take over the channel. At 25W, your 4’ VHF 3dB antenna at the top of your mast can easily provide VHF communications to the horizon (as seen from the top of your mast) but you may have more issues with the 5W Class B AIS in noisy conditions. You probably notice this yourself when you transmit on low power around the marine but have to switch to high power offshore.
- The distance in nautical miles that your antenna can see can be approximated by the formula 1.22 * square root of the height of the antenna above the surface in feet. A Super Maramu has a mast height of about 66’ so an antenna there could see about 9.9nm. "Hang on… “ I can hear you saying, "I can talk to people way farther out than that…" yes, but you have to factor in the height of THEIR antennas too.. If they were also in a Super Maramu you could talk to them out to about 20 miles. From pictures, it appears to me that the mizzen on a Super Maramu is about 80% of the height of the main so let’s say it’s at 53’. That means an AIS antenna on the mizzen could “only” see 8.9 miles (plus what ever the height of the transmitting antenna was which is independent). So while putting the antenna on the main gives you one more mile of range, it isn’t significant in the scheme of things. Bouncing off the troposphere also can get you some additional range but that is very atmospheric dependant so we shouldn’t count on it.
- While DSC does mean Digital Selective Calling, VHF radios so equipped still send out the “digital” signal over the analog VHF much like an old style modem would translate our computers binary information over our telephone lines - think of the analog signal like a bunch of sine waves all intertwined but the digital signals are just two specific frequencies sent over them like the old modem squeals you would hear on your fax machines. So the Digital signals are really just super fast 1’s and 0’s transmitted over analog radio waves that your radio can discriminate and make sense of. Clear as mud?
So looking at all that, what are the best options for VHF and AIS?
- Best - A 4’ dedicated and optimized antenna for VHF on the main mast so you can transmit to the coast guard when needed. Luckily your Amel already has this. Add to this a 4’ dedicated AIS SPECIFIC antenna (optimized for 162MHz) on the Mizzen but a generic “AIS” antenna that is optimized for both bands would do as well. No splitter.
- Less optimally, add a splitter and then replace the old VHF optimized antenna for a VHF & AIS optimized one (I.e optimized midway between the two bands) at the top of the main mast
- Worst, your old VHF tuned antenna at the main with a splitter.
I should mention that you can easily make all of the above optimizations completely irrelevant by having had wiring/connections to your antenna(s). The Offshore Safety Regulations used for offshore racing dictate that your coaxial cable have a signal loss of no more than 2.2dB (a 40% signal loss) between your radio and your antenna. If we use an optimistic length of only 25 metres as the length of the cable from the radio up to the antenna, then the standard RG-58 coax cable would generate 4.7dB of loss assuming no connection or splitter losses. If you use RG-8X, you are at 3.8dB and with less common RG-8U, you are at 2.0dB…. So you really should be using at least LMR-400 cable (1.3dB) which I suspect very few are. Of course one bad connection with the crappy standard PL259 bayonet connectors that ship with the standard Shakespeare antennas puts it all for naught as well… you should really make sure you fill them with dielectric grease and then waterproof them with tape/heat shrink.
I know this is a lot to absorb but having been in situations where we weren’t seen even at a couple of miles, I’ve done a LOT of study on how to create the best setup and distilled it here as much as I could. Practical Sailor has done a good series to get you started on this and you can learn a lot by following all the links.
Brent Cameron, Future Amel owner & Amel Owner's Registry Moderator
-- Brent Cameron
Future Amel Owner & Amel Owner Registry Moderator
Oro-Medonte, Ontario, Canada