AIS Transponder - aerial/antenna location?

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neil1967

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I am considering buying a class B AIS transponder for my yacht, and have been considering where I would mount the aerial. The AIT2000 instructions state that the aerial should be mounted 3.5m above the deck to avoid hazardous emissions - I assume all AIS Class B's output the same power and therefore this recommendation is probably common across all class B transponders(?). Are they just covering themselves against future litigation, or is this a real issue? I appreciate that better range is achieved with a masthead fitting, but that would entail an active splitter for my VHF aerial and I would like to keep that as simple as possible - less to go wrong. Ideally I would use a pushpit mounted aerial, possibly on a small pole. Any thoughts?

Neil
 
I can't give you an engineer's opinion, but mine has worked fine with a splitter, which was the simplest installation. I routinely pick up ships at 40-50 miles and other people have picked me up at a similar range. On one occasion, I crossed from Lowestoft to Ijmuiden and stayed on the website throughout, and on other occasions they have lost me for a while in the middle.
 
My AIS transponder antenna is on a short pole mounted on the stern gantry so base is about 3m above the deck. Regularly picked up ships at 10 to 20 miles, though a number of Spanish fishing boats would only display at 5 miles or less, often well after I had seen them visually. With this set up I have a spare antenna for the VHF should the masthead unit fail or fall off (as one did some years ago due to corrosion in the base of the unit). The leads from both antennas will reach and fit both the fixed VHF and AIS units at the chart table.
 
We ran our (receiver only) AIS off the VHF aerial with a splitter, getting signals from 50 miles and often further. When we had a problem with it, which we thought was due to the aerial being faulty, we ran the same AIS engine off a pushpit mounted whip aerial. The range was not quite as great, but 20-30 miles is enough for most purposes. With no aerial at all, we got signals from up to about 5 miles away!
 
Am surprised there have not been more replies to this thread as there must be many who have the aerial on the taffrail and would be concerned that the RF transmissions might be a health risk. This is the location where I have mounted ours but have not actually got the set yet. Am not concerned about range as I am sure we will get at least 6 miles (both ways) which should be plenty for our type of sailing but I am concerned about the health risk.

Are there any experts able to comment please ?
 
I am not a radio expert but used to teach electronic navigation to MN deck officers:

Class B AIS transmit power is restricted to 2Watts (Class A 12.5 W)
Transmission is a very short burst at 30 sec intervals (3 min if speed less than 2kn)

For comparison handheld VHFs typicaly have TX power of up to 5 or 6 watts with the antenna very close to the operator's head.
 
As others have said, the higher it is the longer the range mainly for others receiving you rather than you seeing big ships. I doubt having such a low power near you will do any harm but of course remember that unlike a vhf, it is transmitting frequently.
As for my choice, it was easiest to use a VHF antenna splitter, so mine is at the top of the mast. I looked at having it separate but the costs and inconvenience of fitting extra cables and antenna meant I went for the splitter option. Of course having a separate aerial for AIS might be useful in emergency which takes out the mast aerial but there are other solutions to this.
 
Class B transponders emit a burst of data lasting only a few milliseconds, every 30 seconds. They transmit at only 2W.

In comparison, handheld VHF radios can transmit at 5W or 6W, and are held right next to your head, and can be used for much longer periods. The FCC approves handheld VHFs for use up to a 50% duty cycle, as long as the aerial is at least 1" from your head. So concern over Class B RF emissions seems misplaced!

Both of these, of course, pale into insignificance compared with mobile phones....
 
o/t but on a similar theme - This summer I saw a large gin palace with their radome mounted at the aft end of the flying bridge and maybe only a foot above the head height of the helming position and only 2-3m away. Thought this was a no-no or maybe it was one of the new digital radars .....
 
I agonised about this before deciding to fit a Raymarine splitter with new AIS transceiver this spring. In the end this was the simple option and it has worked well. No need for extra aerials and no danger of interference from fitting a second aerial beside the existing one (big no no) on top of my only mast. I have not been able to notice any deterioration on the VHF radio so very happy with the new splitter.
 
doraymefa said:
I agonised about this before deciding to fit a Raymarine splitter with new AIS transceiver this spring. In the end this was the simple option and it has worked well. No need for extra aerials and no danger of interference from fitting a second aerial beside the existing one (big no no) on top of my only mast. I have not been able to notice any deterioration on the VHF radio so very happy with the new splitter.
Click to expand...

It is the easiest solution but are you really happy about splitting the VHF signal! You will continue to agonise!

Trialled our new setup this weekend using the tuned for AIS Metz antenna on the Stern rail.

Thank you for your advice Salty J .

Picking up plenty of targets at 10 miles out and even had short contacts with large marine traffic 25 miles out.
 
My intention with my post was to state that, with hind sight, I am happy with the splitter arrangement and had no need to agonise in the first place. Sorry if I was not clear.
 
TSB240 said:
It is the easiest solution but are you really happy about splitting the VHF signal! You will continue to agonise!

Trialled our new setup this weekend using the tuned for AIS Metz antenna on the Stern rail.

Thank you for your advice Salty J .

Picking up plenty of targets at 10 miles out and even had short contacts with large marine traffic 25 miles out.
Click to expand...

You may be picking ships up but are they picking up your weaker transmissions? Experience from speaking to a ship a while back was that they were able to see my transmissions from over 20 miles away. My AIS aerial is at the top of the mast via a splitter. Of course you could take the view that (a) they do need to see you from so far away and (b) they might turn off class B reception anyway (though I am sceptical as to whether they would).
 
On the STCW NRAST courses I used to run for MN deck officers our guideline was to detect and start to plot targets at around 6 or 5 miles so that required course alterations could be assessed and alterations made before the target was 3 miles away. In open water we expected a minimum CPA of 1 mile.
In practice a small vessel needs to provide a consistent AIS signal and/or RADAR return out to at least six miles in any conditions. Intermittent signals are likely to be filtered out by the ship's processors.
To calculate the range your AIS antenna will get use the following formula where d = distance to your radio horizon in nautical miles and h = height of center of your antenna above waterline in feet.
d = 1.2246x√hf
The effective range is your radio horizon plus the ship's radio horizon so even a low antenna should be effective in calm conditions. However, if your antenna is low down, say mounted on the stern pulpit, it may be masked by waves when the stern is in a trough effectively giving intermittent transmissons.
I would suggest that a minimum antenna height is more than the maximum wave height you expect to sail in.

Recently, I have received class B AIS from fishing vessels while on pasage between Ireland and the Azores at 5 miles plus with my antenna about 4.0 m above the waterline.
 
there seems to have been no mention of the frequencies used by these devices.


Marine VHF is between 156.0 and 162.025 MHz;
AIS Channel A 161.975Mhz (87B) Channel B 162.025Mhz (88B). So there is a partially overlapping spectrum.


The range of frequencies most risky for humans is range of about 80 and 100 MHz, and exposure to this range is potentially damaging to two areas of the body: the eyes and the testes.


The danger comes from the heating effects of broadcast radiation, and VHF radios at a max of 5Watts, and AIS Class A at 12.5W, and Class B at 2W are not regarded as especially strong sources.



"Vertical whip type antennas radiate in a flattened doughnut pattern. So, if you're directly above or below it, there is little or no radiaton. If on the other hand you're directly along side it, you'll get the full radiation right through you."

Wooslehunter. http://www.ybw.com/forums/archive/index.php/t-316285.html That thread discusses the issues in depth.



I would be reluctant to mount a VHF or AIS aerial on the pushpit or taffrail at head height (except for emergencies), but on a gantry
above head height would be acceptable, and give a slightly longer range.
 
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As with most things it depends what you want from it. I want maximum range to detect and be detected (my family watch my progress on Marinetraffic.com even when 20+ miles to sea). The better splitters are powered and actually amplify both the AIS and VHF signals so neither suffers. My antenna (16+m up) is matched to the splitter (the Cockatoos broke my VHF-only one). I was happy to pay for a good splitter, antenna and AIS unit (Vesper - no connection). Cheers, Andrew
 
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