AIS/VHF aerials

[Fascadale]

I plan to fit AIS to my boat. I want to go the "additional aerial" route rather than sharing my existing aerial. The new aerial would serve the AIS and serve as a back up should the one on the mast top fail.

I notice that some aerials are specified as "AIS" and some as "VHF". Is there much difference or will one aerial do both AIS and VHF?

I'm also thinking about where to mount the extra aerial. The pushpit seems the obvious place: will this give me worthwhile AIS range?

(1.0m of aerial mounted say 1.3m above sea level, height of radar on small ship, say 10m up, combined dist. to horizon about 10 miles)(I think)

 

[nigelmercier]

I plan to fit AIS to my boat. I want to go the "additional aerial" route

Good plan.

Is there much difference or will one aerial do both AIS and VHF?

Not much difference, if you are going to transmit, perhaps an AIS antenna would be better.

I'm also thinking about where to mount the extra aerial. The pushpit seems the obvious place: will this give me worthwhile AIS range?

Yes, plenty.

 

[macad]

For me, the key component is the aerial for the AIS. While delivering yachts across the pond I've regularly had ships and a large yachts 70 miles away on the AIS before with a good highly positioned aerial and often see 30-50 foot yachts in line of sight not visible on AIS because they had a pushpit mounted wand or low cost aerial arrangement – and yes, the units were switched on. It could be that the gear works in coastal or flat water but I have been amazed at sea - worse that not having it if you think it is doing a fantastic job!

 

[Martin_J]

As Nigel says - A pushpit mounted aerial gives the additional reassurance that you have a spare aerial for the fixed VHF. The only thing you might find is that the AIS receiver socket is BNC so you could need a BNC to PL259 convertor to allow you to plug the AIS aerial cable into the VHF in emergency.

Pushpit mounted.>> The photo here shows that range of my AIS receiver with a pushpit mounted 1.1 whip is at least 30 miles

https://skydrive.live.com/embed?cid=D57B857B59D08B86&resid=D57B857B59D08B86%211806&authkey=AMJ9dXsKycTpUaU

The other reason that you will see big ships (and anything commercial) from further range is that class B transmissions from small boats are only 2watts. I think commercial vessels' class A transmit at 12 watts and much more frequently than the class B allows.

 

[Martin_J]

In terms of AIS aerial or not.. AIS would be tuned to the AIS frequencies which are within (but not central to) the usual VHF channels.

Using a better co-ax cable instead of what the chandleries sell will make a bigger difference.

 

[Salty John]

A good vhf antenna will cover 154 to 163 MHz without significant signal loss. It will be tuned to the centre of this range and the performance will gradually drop off as the frequency moves to right or left of this position. With a good antenna the extremities of this range will still have a good signal.
An AIS optimised antenna is one that is tuned to 163 MHz. As you move to right or left of this point the signal also deteriorates, with a good antenna you will still have good signal quality down to 154 MHz. Some will drop off quickly and be useless for voice transmission.
So, take your pick. An antenna that is to be used for voice and AIS, I'd recommend a vhf antenna with 154 to 163 bandwidth; if it's to send and receive AIS most of the time I'd choose an AIS optimised antenna with enough band width to handle voice.
The higher the better is the rule for vhf antennas, but don't use higher 'gain' than 3dbi on a boat - higher 'gain' antennas focus the signal to the point that, when the boat rolls, much of the signal is pointed to sky or sea and not to the horizon. Pulpit range will be typically 10 to 15 miles depending on the height of the other antenna, masthead will be 15 to 25 miles or more.

 

[Fascadale]

So, take your pick. An antenna that is to be used for voice and AIS, I'd recommend a vhf antenna with 154 to 163 bandwidth; if it's to send and receive AIS most of the time I'd choose an AIS optimised antenna with enough band width to handle voice.

I will not be sending AIS data, and I hope to very seldom (if ever) use it for voice: does this influence my choice?

 

[Salty John]

I'd say that points to an AIS optimised antenna.

I should declare a commercial interest, we're the European agent for Metz Communication. Here's our antenna and accessory page:
http://www.saltyjohn.co.uk/boat-products-antennas.htm
There are other fine antennas out there.

I'd suggest, if you're rail mounting the antenna, that you put a PL259 connector on the cable to the AIS engine and then, if your AIS takes a bnc connector, use an SO239/BNC adapter. That way if you want to use the antenna for your radio you can just switch over.

 

[mariner62]

Here's a bit of a diversion, but still within scope of the thread I believe

I have a TV antenna at the top of the mast (happened to be there when I bought the boat), and I'm wondering if it might work ok as an AIS receive (only) antenna. It would appear to cover the band, and I'm pretty sure it doesn't have an AIS notch filter built in.

Anyone tried such an idea?

 

[Boo2]

I'm also thinking about where to mount the extra aerial. The pushpit seems the obvious place: will this give me worthwhile AIS range?

(1.0m of aerial mounted say 1.3m above sea level, height of radar on small ship, say 10m up, combined dist. to horizon about 10 miles)(I think)

The SevenStar Sea Tracer user manual available as a .pdf file here http://www.sevenstarelectronics.com/images/SeaTracer User Manual Iss 1.8.pdf says :

The VHF antenna should be mounted at a minimum vertical distance of 3 metres from the head of any person standing on deck in order to meet international safety directives on Maximum Permissible Exposure (MPE) / Specific Absorption Rate (SAR). Failure to adhere to these limits could expose persons within the 3 metre radius to RF radiation in excess of the recommended MPE / SAR limits.

So that would seem to rule it out for pushpit mounting. That is a class B AIS transponder though, are you fitting an AIS that transmits or just a class A receiver ?

Boo2

 

[Fascadale]

The SevenStar Sea Tracer user manual available as a .pdf file here http://www.sevenstarelectronics.com/images/SeaTracer User Manual Iss 1.8.pdf says :

So that would seem to rule it out for pushpit mounting. That is a class B AIS transponder though, are you fitting an AIS that transmits or just a class A receiver ?

Boo2

A receiver only.

 

[Tex]

I have pushpit mounted AIS aerial and regularly pick up vessels at 25 miles, close ennough for my puposes.
One frustration is that, whilst picking up all mobile targets, static AIS such as lights and nav marks are not detected.

I was wondering about using the same aerial for a transponder but have concerns over the health risks of tranmissions to crew given the warnings about installation of a tranmitting VHF aerial.

 

[haydude]

I plan to fit AIS to my boat. I want to go the "additional aerial" route rather than sharing my existing aerial. The new aerial would serve the AIS and serve as a back up should the one on the mast top fail.

I notice that some aerials are specified as "AIS" and some as "VHF". Is there much difference or will one aerial do both AIS and VHF?

I'm also thinking about where to mount the extra aerial. The pushpit seems the obvious place: will this give me worthwhile AIS range?

(1.0m of aerial mounted say 1.3m above sea level, height of radar on small ship, say 10m up, combined dist. to horizon about 10 miles)(I think)

Commercially they say that AIS antennae are tuned to the AIS frequencies. I suspect that this is just "marketing ware". In practical terms a VHF antenna is perfectly suitable for AIS because the AIS frequencies are inside the VHF range, possibly not the opposite if the antenna has been tuned only to the AIS frequencies.

 

[haydude]

I have pushpit mounted AIS aerial and regularly pick up vessels at 25 miles, close ennough for my puposes.
One frustration is that, whilst picking up all mobile targets, static AIS such as lights and nav marks are not detected.

They are, they are, but they are likely filtered out by your display. Do you have a Raymarine C series? I have raised the issue with Raymarine several times, but Raymarine were unwilling to provide a fix.

If you watch the NMEA stream coming from your AIS receiver you will find also nav marks, but their record is ignored by Raymarine plotters.

 

[Tex]

They are, they are, but they are likely filtered out by your display. Do you have a Raymarine C series? I have raised the issue with Raymarine several times, but Raymarine were unwilling to provide a fix.

If you watch the NMEA stream coming from your AIS receiver you will find also nav marks, but their record is ignored by Raymarine plotters.

Yes, Raymarine C70. I suppose that given it is a discontinued MFD Raymarine are very unlikely to ever issue a fix.

 

[Salty John]

Commercially they say that AIS antennae are tuned to the AIS frequencies. I suspect that this is just "marketing ware". In practical terms a VHF antenna is perfectly suitable for AIS because the AIS frequencies are inside the VHF range, possibly not the opposite if the antenna has been tuned only to the AIS frequencies.

Actually there are several vhf antennas on the market that only cover, say, 155 MHz to 160 MHz. There are others that claim to cover 154 MHz to 163MHz but at the extremes of this range the VSWR is so high that it will trigger the 'antenna' fault alarm in some radios. So, you need to be sure that the vhf antenna covers 163 MHz at a reasonable VSWR - say, less than 1:2 or, better, 1:1.5.

An AIS optimised antenna, if it is to serve as a back-up radio antenna, needs to be able to reach 154 MHz at a reasonable VSWR, as above.

On the safety issue, the limits were tightened up a few years ago but even so amateur transmitter installations not exceeding 50w with a normal 3dbi gain antenna are considered safe, as are all 'click to transmit' radios.

 

[lw395]

The biggest danger from a VHF aerial is poking yourself in the eye with it. For this reason, it should be a couple of metres up, where it will also be less vulnerable to mooring ropes etc.

 

[mariner62]

Not sure whether to post here or a new thread, but this is related to AIS and VHF
Does anyone have any experience of using a masthead TV antenna as an AIS receiver. Its one of those omni-directional circular ones with specs which would seem to indicate it should cover the band ok. Would save a bit on installation. I'm thinking I could just T-off the signal to the AIS receiver. Only problem might be impedance mismatch as TV antenna probably 75Ohm and AIS looking for 50 Ohm setup?

 

[Ric]

I've been down this same decision process. Currently I have a dedicated antenna for the AIS mounted under the cockpit coaming at the rear of the cockpit locker, which doubles as an emergency VHF antenna. I experimented with putting it on the pulpit, but it made no discernible difference to performance so I decided that it was better tucked out of the way of flailing lines under the deck. Also, if on the pulpit, it would be just my luck to have it wiped out if ever I lost my primary VHF antenna in a dismasting.

However, I also experimented with connecting my AIS to my masthead VHF antenna. The performance difference was astonishing. I could see stations over 200 miles away. The ability to see stations at such distance is obviously of no relevance to collision avoidance - but it is useful information as I can see immediately who is within hailing distance on the VHF. I think that this could be very useful when well offshore as I could set my AIS alarm to 200 miles so that I am immediately alerted if any ships come into VHF hailing distance. I am therefore going to fit a splitter and use my primary VHF antenna for both VHF and AIS. I'll keep my under deck antenna for emergencies only.