pvb
Well-Known Member
In theory, but the transmitting range is constrained by the max 2W power of Class B. The receiving range will obviously be better, but is anyone seriously interested in the course and speed of a ship 30 miles away?
Masthead VHF antennas
- Thread starter Hoolie
- Start date
In theory, but the transmitting range is constrained by the max 2W power of Class B. The receiving range will obviously be better, but is anyone seriously interested in the course and speed of a ship 30 miles away?
PaulRainbow
Well-Known Member
Digital Yacht give the best possible range as 8nm. Mounted on the pushpit give a horizon of about 4 miles. Ships bridge will have a much greater distance to horizon so would easily achieve the 8nm range. So you're right, might as well be on the pushpit.
The reason class A traffic is being picked up at greater distances is because they are transmitting at 12.5 watts.
RichardS
N/A
My masthead AIS antenna is at 20m so I would indeed see an identical Class B yacht at around 32 km, in theory (see above discussion). However, Class A ships include much larger ones where the antenna will be way above 20m, so I can see those at 40+ nm. Of course, there may well be many other Class A's with a lower antenna much closer than that but I can't see those so I don't know.
Richard
MikeCC
Well-Known Member
If you're going to use a splitter with a transponder, don't assume if it's an active type that it will be suitable. The ones priced around £75 are only for AIS receiver . The type for transponders are around £200. The difference is that they are dealing with two transmit devices with the added complication of one being digital, so making switch response time more critical.
With regard to antenna separation, I'd always understood that horizontal and vertical separation were required. I'm not sure that the 25W transmission to an adjacent AIS antenna will do the unit much good, but guess it depends on design of AIS.
With regard to antenna separation, I'd always understood that horizontal and vertical separation were required. I'm not sure that the 25W transmission to an adjacent AIS antenna will do the unit much good, but guess it depends on design of AIS.
JumbleDuck
Well-Known Member
Quibble: both can transmit at the same time, but the output from one will be dumped to heat.
Line of sight is not necessary. For example, when I was sailing past Tarbert on Loch Fyne last summer, I saw the CalMac ferry MV Finlaggan on my AIS as she was entering West Loch Tarbert on the other side of the Kintyre peninsula, and at my mooring in Port Bannatyne I can watch vessels entering and leaving Rothesay, which is in the next bay south.
RichardS
N/A
That is, indeed, a quibble of the third kind!
I've a screen shot posted a couple of years ago of when I was anchored in the bay outside Gouvia Marina of the cargo ships manouvering in and around Tripoli Harbour ....... 500 miles so deffo not line of sight.
Richard
RichardS
N/A
A Google search for "AIS Splitter" appears only to source the suitable active ones so there shouldn't be a problem.
Richard
BrianH
Well-Known Member
I have just taken a current screen snapshot while moored in my NE Adriatic marina using a half-wave, pushpit mounted antenna, partially screened by an adjacent wind-generator pole. The result is fairly typical, showing Class A targets as far away as Trieste and Koper at 30nm (extreme right, my boat extreme left). The only Class B is that due north of me at 4nm.
I have never seen the need for mast-head AIS antennas, or splitters, the range I get with a pushpit mounting is quite acceptable - at least for me.
I have never seen the need for mast-head AIS antennas, or splitters, the range I get with a pushpit mounting is quite acceptable - at least for me.
PaulRainbow
Well-Known Member
AIS is line of sight, same as VHF. Any reception to the contrary is caused by an anomaly, such as refraction or skip.
Alpha22
Well-Known Member
The concern about juxtaposition of antennas goes way beyond when they are transmitting. The physical presence of an effectively inert metallic vertical close to your main voice VHF antenna will grossly affect the SWR and therefore reduce the effective range, in different ways in different directions..... probably quite unpredictably.
Don't do it.
RichardS
N/A
That is technically true ....... but with AIS those anomalies or something else must be so prevalent that sailing in Croatia among its 1000, mainly moutaineous, islands means that I am surrounded by AIS targets on the other sides of moutains which are nowhere near line of sight even from the top of my mast.
This can be so confusing that I have to double-check on the AIS display with a chart overlay as on the radar-type display I might be able to see 10 close targets within 3 miles but only 2 boats are visible. This is seriously worrying until I switch on the overlay and see that the other 8 are all in hiding!
Richard
lw395
Well-Known Member
Not exactly.
The VHF horizon is quite a bit beyond line of sight, particularly over water. That's not really considered an anomaly.
Much the same as FM broadcast is pretty reliable without line of sight.
bbg
Well-Known Member
That is a very good point, and one that I considered when putting my antenna on the transom.
Your other point about "how far away do you care about" is also valid. If VHF is limited strictly to line of sight (which it isn't - lots of signals from "over the horizon"), an antenna on the pushpit will pick up (and theoretically be able to transmit to) a ship as soon as the ship's VHF antenna (which will be on the superstructure) is over the horizon. I challenge anyone to pick up a ship visually when all you can see is its VHF antenna.
For me, an AIS VHF antenna on or near the stern gives redundancy in the event of dismasting (make sure the cable is long enough that it can be removed from the AIS unit and plugged into the VHF) and gives sufficient range for the purpose of AIS. Yes, you could theoretically get more range with a masthead antenna, but I question whether such additional range is useful.
Hoolie
Well-Known Member
Thanks for the replies!
I do already have an AIS antenna on the pushpit for all the reasons given. I'll stick with that when we upgrade to an active transponder and not fork out £200 or so for a splitter.
I do already have an AIS antenna on the pushpit for all the reasons given. I'll stick with that when we upgrade to an active transponder and not fork out £200 or so for a splitter.
Salty John
Well-Known Member
VHF range is line of sight plus about 15 percent for refraction in normal conditions. The range is the combination of the horizon distances of receiver and transmitter plus 15%.
However, VHF radio signals can be reflected, refracted and diffracted by mountains/building, atmospheric conditions and interaction with sharply defined edges - ridges and so on. All these effects allow VHF signals to reach parts that the simple line-of-sight rule would seem to exclude. Unfortunately they are unpredictable so cannot be relied on.
On the subject of masthead clutter, it is a real problem. There are many different opinions: mobile antenna installers will tell you that the closest you can put two low power vhf antennas horizontally is a quarter wavelength - about 0.5m. Most manufacturers ask for 1m. Some say at least 3m, but that's usually for higher power antennas, both transmitting potentially at the same time.
Vertical separation helps a lot. Some people say they have success with pointing one antenna upwards and one downwards below it, but the mast then creates a shadow for the lower antenna.
I think the best advice is, if you can't get at least 1m horizontal separation at the masthead put the AIS antenna on the deck - elevated as high as is practical by using the puship, extension pole or radar arch.
As others have said, the deck level antenna provides a spare for the masthead antenna. For this reason a VHF antenna with bandwidth that covers 156 to 163 MHz is better than an AIS optimised antenna (one centre tuned to 162 MHz) for AIS so that it works well when needed for radio use.
Or use a good quality active splitter.
However, VHF radio signals can be reflected, refracted and diffracted by mountains/building, atmospheric conditions and interaction with sharply defined edges - ridges and so on. All these effects allow VHF signals to reach parts that the simple line-of-sight rule would seem to exclude. Unfortunately they are unpredictable so cannot be relied on.
On the subject of masthead clutter, it is a real problem. There are many different opinions: mobile antenna installers will tell you that the closest you can put two low power vhf antennas horizontally is a quarter wavelength - about 0.5m. Most manufacturers ask for 1m. Some say at least 3m, but that's usually for higher power antennas, both transmitting potentially at the same time.
Vertical separation helps a lot. Some people say they have success with pointing one antenna upwards and one downwards below it, but the mast then creates a shadow for the lower antenna.
I think the best advice is, if you can't get at least 1m horizontal separation at the masthead put the AIS antenna on the deck - elevated as high as is practical by using the puship, extension pole or radar arch.
As others have said, the deck level antenna provides a spare for the masthead antenna. For this reason a VHF antenna with bandwidth that covers 156 to 163 MHz is better than an AIS optimised antenna (one centre tuned to 162 MHz) for AIS so that it works well when needed for radio use.
Or use a good quality active splitter.
JumbleDuck
Well-Known Member
If refraction and skip work, then AIS is, like VHF, not line of sight. QED.
JumbleDuck
Well-Known Member
At the moment I have a VHF antenna at the masthead and an AIS one on the aft pulpit. It seems to work well. I have been looking for a manual coax transfer switch which would allow me to swap them round, but I haven't found one yet and I am not sure now that there would be any real benefits.
Alpha22
Well-Known Member
The only benefit would be convenience..... I wouldn't want the insertion losses of the switch and the extra plugs and sockets..... likely to reduce signal strength significantly.
Alpha22
Well-Known Member
The reason it is generally taken as line of sight is that refraction, skip and sporadic-e are anomalies..... they are irregular, dependant on atmospheric conditions and cannot be depended up on..... often they are very short term.
Line of sight - reliably and regularly.
JumbleDuck
Well-Known Member
With proper kit the losses shouldn't be too bad, but unfortunately it's either hard to find at these frequencies, or expensive, or both. My idea when I got the AIS was to use the masthead antenna for it (mainly to improve reception for Marine Traffic and thereby calm my other half's worries more) and swap the VHF up there for emergencies. However, since AIS works just fine at the pulpit, I'll leave it for now. Although I do like the idea of switching to a backup antenna by flicking a switch in the event of dismasting or other damage, rather than faffing about with plugs.
