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Re: Too many VHF aerials?
Linda Melton <lindajmelton@...>
Mark,
I have an emergency aerial just in case I lose both masts.
The main mast carries the VHF and the Mizzen the AIS. These are interchangeable with either the radio or the transponder.
Suggest trying you mizzen aerial out on your AIS and buying an emergency aerial. Easiest solution.
Ian
Ocean Hobo
SN96
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Re: Too many VHF aerials?
Brent Cameron
Thanks Scott/Ryan/JP.
Scort, I had seen your post on the bend radius of LMR-400 and why you chose RG8X but thought you had them backwards (I’m sort of dyslexic myself so let it slide). You are absolutely right about not violating the minimum bend radius but If you look at the RG8X specs you’ll see that it has a pretty wide (2.5”)minimum bend radius because it has a very compressible dialectic unlike the TMS LMR-400 which has got a 1” minimum bend radius and is very flexible.
The difference in signal loss between the two is fairly significant. If you have say an 90’ cable run, with zero losses at the connectors (possible with good installation techniques) and a good antenna with a 1.5:1 SWR at Channel 16, your 25W radio will only be sending out only 9.9W of power with RG8x while with LMR-400, you would be sending out 17.7W. For your little Class B AIS transceiver, the difference is even bigger.. 0.7W for RG8x and 1.4W for LMR-400 (they transmit at only 2W) despite being 5W radios (they also monitor 2 other channels concurrently).
Now, lest everyone go ripping out all their old COAX cables…. Remember that the little old Voyager spacecraft only has a 25W radio and it’s still communicating with us from beyond the solar system now! Power doesn’t increase your range… but it does help you increase your signal strength to overpower background noise and other signals (and squelch settings turned too low). In a storm with lots of lightning, you might just want that extra power someday.
-- Brent Cameron
Future Amel Owner & Amel Owner Registry Moderator
Oro-Medonte, Ontario, Canada
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Replacement Corian for SM2K dishwasher lid
Last season, the Aurora pattern Corian on the dishwasher lid broke. I've found someone who can make a replacement. I have the dimensions (43.7cm square) with screw holes centered 3.3cm from each corner. Because the Corian in mounted in a lipped space, I need radius of the curve on the 4 corners. If someone is on their SM2K, I'd very much appreciate the measurement. Two lines perpendicular to the edge at the point where the curved corner begins on each side. The distance from the edge to the point where the two lines intersect gives the "radius" of the curved corners and with that a new "cover" can be made.
Related, the removable cover for the port galley counter was broken several years ago. We bought a matching "Corian" lid sold as a cutting board with a slot for a handle made a perfect replacement.
Thank you.
Drew Gaffney
SV Revelation SM390
Lying Carloforte, Sardinia
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Re: Too many VHF aerials?
Brent Cameron
I haven’t used the Raymarine system with AIS but I believe that your AIS already has all the information to show the details on the targets but keeps them hidden to reduce clutter.
AIS systems use a scheme called Time Division Multiple Access much like your cellphones do so they can all transmit and receive using only a very a few channels and not step all over each other.
Class A systems use Self-Organizing TDMA where there are something like 4500 slots spread equally across two channels available over a minute and each transmitter ‘reserves a slot’ and all receivers need to keep a map of all of the other transmitters. It takes two channels of receive to accomplish this. In Class A systems, this transmission Is continuous (I.e. once every 2-10 seconds depending on vessel's speed while underway down to every 3 minutes for a ship at anchor, they update their slot information). They use 4 channels (one transmit, two receive (duplicates to avoid bandwidth collision /noise issues), and finally a DSC channel. They also require a highly synchronized time source (delivered from the connected GPS). Now, to answer your question, they transmit in this “slot” the Vessel’s MMSI, Nav Status (at anchor, underway using engine, etc), rate of turn, SOG, Position, COG, and true heading…. That gets transmitted every time. Once every 6 minutes or so it sends the Ship’s name, radio call sign, type, dimensions, destination and all the other stuff we see on our screens when we ask for the detail.
Class B systems on the other hand are much simpler and can use either Carrier-Sense or Self Organizing TDMA (much less frequently). Carrier-Sense works by listening for an empty slot where the signal is no bigger than the background noise and then it transmits on that but might use a different slot the next time. Class B systems by default only transmit (at 2W) once every 30 seconds while underway and every three minutes when the SOG is less than 2 knots. They only use three channels… one transmit and two DSC enabled receive channels. Because they have much less bandwidth available, with this scheme, they have restricted the packets into only 4 different types of messages. The first is a burst safely message that your system can send when a button is pushed. The second is the standard position report info (sent every 30 seconds-3 minutes), the third contains the rest of the position information (ship type, dimensions, etc.) but only is sent when it is polled by the coast guard, and the fourth is extended information sent every six minutes (but this is broken down into two separate messages sent 1 minute apart due to bandwidth issues). The system was designed so that it’s messages operate a much less priority than Class A and are the first to get dropped (by distance away from you) as things get really busy.
Your life vest MOB AIS beacons actually use a third standard… Pre-Annouce TDMA in which they pick a random slot and transmit on that (because they want to keep the battery going long enough they can’t be listening for free slots or reserving slots with SO-TDMA.). They only send position and identifier.
So, to answer your question Tom, when you click on it, Raymarine is showing you the detail that it already has gathered from the various sources over the course of the last 6 minutes or so. Of course if it hasn’t yet got the extended detail, it might not show immediately. That’s also why your screen hangs on to lost targets for some period of time as well…. It has to remember them for some timeout period set in your settings.
The system is pretty cool but it does have a bunch of issues, not the least of which is that it isn’t secure and can easily be jammed and spoofed so we should always take it with a grain of salt. Also, although ships over 300T are required to carry it (plus all passenger ships regardless of size), it isn’t required to be turned on for military vessels or fishing boats (outside of the EU). Apropos of nothing at all, Iranian Oil Tankers have been known to turn theirs off as well!
As a final warning, I also know that commercial ships with their Class A systems often also have the option to turn off class B targets entirely to reduce “clutter” so don’t ever assume that they can see you even with the all the right antennas, setups, etc. especially around busy harbours. I can just imagine what the systems are gong through at ports like Singapore!
The day is coming when all of this will be on satellite but the big problem has been that because of their elevation, satellites can see many more vessels than fit in the normal slots designed for line of sight visual range… so they are having to launch fleets of them with restricted antennas to scan only portions of the sky (or build satellites with dozens of receivers and antennas each focused on specific locations).
As scary as all of this is, they have similar issues with airplanes using the ADSB system - which the FAA is using to replace Air Traffic Control Radar eventually! Sorry you asked?
Brent Cameron, Future Amel owner & Amel Owner's Registry Moderator
-- Brent Cameron
Future Amel Owner & Amel Owner Registry Moderator
Oro-Medonte, Ontario, Canada
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Re: Too many VHF aerials?
Brent - Fantastic information! The distinction between an AIS antenna and a VHF antenna is subtle but important and something I didn't know. One thing I will add is that I went for RG-8X. The reason is the minimum bend radius spec for each type of coax. The RG-58X we have from Amel is junk. Nothing more I can say about that. I went for RG-8X because the minimum bend radius is 2.5inches / 63mm and by my rough eye calcs, that's the bend you have to make to exit the mast and enter the boat. There's no way around that I found. RG-8U has a bigger required radius and LM-400 has an even larger bend radius requirement. If you examine the routing through the base of the mast, there's almost no way you can achieve that. And my understanding is that signal attenuation shoots up dramatically when you violate the min bend radius, negating much of the benefit of low loss cable. I also skipped the junction connector as the coax enters the boat and instead replaced the coax from antenna all the way to the VHF. I also went with proper soldered terminals. I figure if I ever need to remove the mast, I can cut and re-terminate the coax then. For now, I'll enjoy the reduced risk of signal attenuation from and additional connector. Also, the Shakespeare 5215 that I purchased connects the rigging to bonding, which was only done by Amel on boats built after 2009. The connection was made through the PL259 connector "outer" which connects to the shielding foil, which then goes to the ICOM VHF case, which then is connected to the bonding system.This revealed a small negative leak (Mass- light) which I traced to the bow light wiring that had been causing surface rust on my rigging. That was a good find. Caveat: I am no engineer and don't play one on TV. -- Scott 2007 A54 #69 SV Tengah http://www.svtengah.com
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Re: Too many VHF aerials?
That was a great post, Brent.
Tom, to answer your question, the AIS messages are packets, but they're not as compact as you're envisioning. The transmission speed is slow enough that the designers decided to split the data up into many messages (27 types), and not all of them transmit at the same rate. The messages concerning position, speed, heading, etc are sent every 2 seconds to 3 minutes (depending on class of transceiver and speed of the vessel), but the messages that contain the name, size, type, etc are transmitted only every 6 minutes ( source). You might not receive every transmission due to interference, so sometimes there can be a considerable wait before you receive the message identifying an AIS target.
Ryan and Kelly SM 233 Iteration Boston, MA, USA
Bravo!
Jp, Santorin #51 Vanille
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
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Brent Cameron
Future Amel Owner & Amel Owner Registry Moderator
Oro-Medonte, Ontario, Canada
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Re: Amel's suggestion to run the Volvo D3-110 (A54) daily while on passage
Scott, Jamie Wendell here on Phantom.
As you may recall when we met last year, I told you I had saltwater migration into my D3-110 on my Amel 54. No one could positively identify why that happened, but the engine was toast. There is a long thread from 2015/2016 about that.
I replaced with a D3-150 and reworked the exhaust from the muffler to the discharge flapper. I upped the size to 90mm and replaced the flapper to match. The original was 90 to the muffler but only 76 downstream. I also shortened the loop significantly.
I have had no problems since.
Hope all is well with you guys. Enjoy PR.
Maybe see you next winter when I head back down.
Jamie
Phantom A54
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Re: Too many VHF aerials?
Bravo!
Jp, Santorin #51 Vanille
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
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Re: Too many VHF aerials?
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.
Tom Peacock 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 CameronFuture Amel Owner & Amel Owner Registry Moderator Oro-Medonte, Ontario, Canada
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Re: Too many VHF aerials?
Brent Cameron
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
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Re: Too many VHF aerials?
Alan Leslie
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Re: Sidepower SP 155 TCi needed
Sharing is caring, so here are the wiring diagrams and schematics for the A54 bow thruster that I got from Amel. I also added the instructions to replace the sacrificial flexible coupling, which I have had to do. Hope this helps someone. -- Scott 2007 A54 #69 SV Tengah http://www.svtengah.com
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Re: Too many VHF aerials?
I just replaced my main mast VHF antenna and traced all the other ones. On our A54 main mast, there are two Banten aerials, one for the VHF and the other for the FM/AM radio. On the mizzen, we have the AIS antenna, connected to our Furuno FA-50. As you may know, AIS transmits on VHF frequencies. The second antenna on the mizzen is the weather fax, which I don't really use. Based on my understanding, it's important that the VHF and AIS antennas are at least half a wavelength (a meter) apart horizontally and ideally on a different vertical plane. That's how Amel installed it on my A54. So if you're going to add an AIS antenna, put it on the mizzen, which it seems you're planning on doing. Having VHF and AIS on the same mast will negatively impact both. That said, in reality, Class B AIS doesn't transmit that often and we don't blabber that much on VHF that I'd be too worried about that affecting AIS that often. :) -- Scott 2007 A54 #69 SV Tengah http://www.svtengah.com
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Re: Too many VHF aerials?
Arno Luijten
Why do you say too many? Are they interfering with each other. We have two on te main mast and one on the mizzen; AIS, VHF and WeatherFax. They seem to get along. In my marina the is currently a motorboat with 11 of these white sticks, make me wonder what they are for. But it also has 4 domes and 2 Radars. The owner must be very important :-)
Regards,
Arno Luijten
SV Luna,
A54-121
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Re: Amel's suggestion to run the Volvo D3-110 (A54) daily while on passage
Thank to Oliver and also to Scott. Great question! Porter A54-154 Excuse the errors. Sent from my IPhone Www.fouribis.com
On Jan 7, 2020, at 11:18 AM, Scott SV Tengah <Scott.nguyen@...> wrote:
Fantastic information from Olivier. Thank you. I presume the flap is a specific Amel part or can we buy it elsewhere? I am totally for supporting Amel but the shipping costs from France get a bit high for a small part. :) On another note, what is the general guideline for using the Volvo D3-110 and the Onan when the boat is heeled over? I do recall reading the Onan spec that it shouldn't be used at more than 10 degrees constant with short bursts to 20degrees. I can definitely hear a change in genset note when the boat is rolling and it is not related to the exhaust outlet going underwater. Back in my car racing days, I was very aware of oil starvation when the oil sloshes to the side of the oil sump while cornering. The solution there was either baffles (helps) or a dry sump. Is this a concern with our marine engines/genset? -- Scott 2007 A54 #69 SV Tengah http://www.svtengah.com
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We bought a set of deep cycle lead acid batteries from West Marine in Newport Rhode Island in 2009, got 8 years out of them. They would have lasted longer if I hadn't messed up by leaving the freezer open and switched on when I left the boat on its mooring for 6 weeks. I leave two fridges on to use the power from wind gen and solar but hit the wrong breaker. They were labelled "West Marine". I replaced them with Trojans in New Zealand Regards Danny SM 299 Ocean Pearl
On 08 January 2020 at 04:15 Chuck_Kim_Joy <clacey9@...> wrote:
Have not tried West Marine. Looking for lead acid deep cycle. Preferably Trojan good price good battery. Thanks for your reply. Chuck Joy #388 On Tue, Jan 7, 2020 at 11:08 AM Mark Erdos < mcerdos@...> wrote: Have you tried the local Costco? West Marine? It may help if you state the type of batteries you seek. With best regards, Mark Skipper Sailing Vessel - Cream Puff - SM2K - #275 Currently cruising - Vista Mar, Panama www.creampuff.us Greetings All and Happy New Year! In Puerto Rico now and looking for batteries. Can anyone recommend a supplier. Hoping Gary S. will pipe in I know he spends a lot of time here. Thanks in advance.
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Re: Amel's suggestion to run the Volvo D3-110 (A54) daily while on passage
Mark Erdos
Although not fitted with a Volvo or a 54, Cream puff is fitted
with a Vetus Waterlock muffler. Even if you do not have the Vetus model, the
setup for the water-lock is probably very similar. The muffler is fitted with a
drain screw. Undoing this screw while underway would absolutely eliminated any
chance of water back-flowing into the engine. If you opt to do this, it might
be advisable to attach a hose pointed toward the bilge. And, of course, do not
run the engine with the water-lock drain open.
So, for those
of you not wanting to run the engine at all while underway on long passages,
this is an alternate option.
With best
regards,
Mark
Skipper
Sailing Vessel
- Cream Puff - SM2K - #275
Currently
cruising - Vista Mar, Panama
www.creampuff.us
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Re: Too many VHF aerials?
Hi Mark,
Do you have a Class A VHF? Those are the type that use two antennas for DSC, but I've never seen one in the flesh -- most recreational boats use Class D, with a shared antenna.
On our boat, we have one antenna on each mast. The main mast is split between the nav station VHF and AIS, and the mizzen is connected to the helm VHF. Both VHFs have DSC. This setup has worked well for us. If the splitter fails, it's easy to connect the nav station VHF directly to the radio, and I believe we also have a jumper between the helm and nav station so we can swap the antennas to either radio by plugging a few wires (that's what the wire is labeled as by the previous owner... I haven't tried). And we also have a handheld as a third backup.
I think having the AIS on the main mast for better range outweighs any drawbacks of having a splitter.
Ryan and Kelly SM 233 Iteration Boston, MA, USA
Hi Mark,
We also have one VHF on the main mast, and one on the mizzen. Maybe I have misunderstood the configuration, but here is how I think we have it:
The main mast antenna is hooked up to our VHF analog radio. That radio also has a button for DSC (the D standing for digital), but I always assumed that the DSC signal was transmitted via the same antenna as the analog VHF signal, not on a separate antenna. At least, when I installed the radio, there was only one antenna output terminal on the radio, so I don’t see how it would use two antennas.
Our mizzen antenna hooks up directly to the AIS transceiver, no other devices use it.
If the either VHF antenna failed, we also have a spare antenna on board that could be partially hoisted for elevation.
Tom Peacock SM 240 Aletes Martinique
On my main mast there is the main VHF Radio aerial, on the mizzen are the DSC aerial and the Emergency VHF aerial and I would like to add an AIS aerial to avoid a splitter. I am considering replacing the Emergency aerial with the AIS and using a bolt on emergency. How have other owners coped with this proliferation? Mark Porter Sea Hobo SHARKI #96
CARDIFF Marine Village
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On my main mast there is the main VHF Radio aerial, on the mizzen are the DSC aerial and the Emergency VHF aerial and I would like to add an AIS aerial to avoid a splitter. I am considering replacing the Emergency aerial with the AIS and using a bolt on emergency. How have other owners coped with or avoided this proliferation?
Mark Porter
Sea Hobo
SHARKI #96
CARDIFF Marine Village
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Re: Windsreen replacement
Arno Luijten
Hi Paul, Last spring I got an offer to have them replaced for a little over 900 euro by:
SARL caraibe menuiserie le Carénage
97290 LE MARIN
Tél : 0596537663
Tél portable : 0696257604
Fax : 0596537663
Site web :
Email : serge@...I was not able to get to Martinique at that time so I have no idea about quality, etc. Regards, Arno Luijten SV Luna, A54-121
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