Another AIS Antenna Question!

aknight

aknight

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Apologies, I have run a search for this but even the most recent threads didn't appear to reach a clear conclusion.

I took advantage of a deal at SIBS to pick up a NASA AIS 'Radar' for my small yacht, which is currently undergoing a refit. Having seen the antenna options discussed in earlier threads, I am inclined to distrust the idea of using a splitter so that the AIS can share the existing, masthead VHF antennna connected to the VHF DSC on board. The idea of mounting a helical antenna at the spreaders is viable, as the mast is coming down soon for other work to be carried out, but I am concerned about the vulnerability of the antenna to damage from the sails on certain points of sail.

By far my preferred route would be to install the AIS antenna on the pushpit. Has anyone got direct experience of doing so, and if so, could they tell me what kind of AIS performance has been achieved, e.g. range, signal strength, etc? In the case of my yacht, the antenna base would be situated between 5' and 5'6 above the waterline.

Two other subsidiary questions that are troubling me, if I go down the route of installing the antenna on the pushpit are:

(1) Is it feasible to use a modestly-sized helical antenna (as opposed to a whip antenna) in a bid to reduce the risk of it becoming knocked/fouled? If so, do any forumites have experience of any particular brands or models?

(2) I am guessing the answer to this is no, but will the antenna suffer from (or indeed cause) interference with GPS antennae or (NASA) Navtex antennae? It will be sharing the pushpit with both on my boat.

Any views/experiences would be greatly appreciated.
 
While compact helical antennae work they can never be as good especially on receive as a longer (whip) antenna. But I have no experience.
An antenna on the spreader may be better certainly higher however may suffer from sheilding from the mast and stay wires.
My spreaders loose paint on front and back from halyards, The mainsail presses on the spreaders when running however generally only the tip can be contacted by the jib. It is a fractional rig with aft swept spreaders. So an antenna should be mounted about halfway out from mast to tip and slightly forward. It should be OK then provided halyards not in use are clipped forward rather than to the mast base. good luck olewill
 
What range do you really need? I imagine you want it for anit-collision not "ship spotting" so you should not need much of antenna.
 
Range,
Based on line of sight figures you add together the distances to horizon for the heights of both antennas. Yours at 5' - 5'6 will be around 2.5 miles. The antenna height on a ship will vary but if it's 20 meters high that will have a range of just over 9 miles. In this case your detection range will be 11.5 miles ish. If you mounted your antenna on the spreaders (I'm guessing 15' above water) you would only get another 2 miles. As previously mentioned a helical will not perform as good as a whip.

As for interference to GPS & Navtex, you are not transmitting through your AIS antenna and neither are your GPS & Navtex so only interference would be from sheilding or slight modification of the reception patterns - negligible if more than a 6" to a foot away.
 
I've just been playing with onevia both a splitter and the masthead direct. It wasn't overly keen on the splitter and not much more keen on working direct. Bizarrely, although its operation was erratic to say the least, it would often pick ships up from 50 miles away, names and everything - but then struggle with those closer to home.
If mounted on the stern somewhere I guess it would pick up a reasonable amount.... if anything at all. We were then struck by lightning so it doesnt work at all now! Doh!
 
I have a stubby helical on the pushpit and pick up targets >16nm. At one stage I have some targets around 30nm but that was unusual.

In my opinion that is more than enough.

Cheers

Wayne
 
With a whip aerial mounted on the pushpit, close to HF and gps aerials which are unaffected, the ranges it works over are:
>10nm and a message from a reasonable size ship will be received every 45-60 secs (the rest are lost because of receive errors)
<10nm and the same ship's messages will be received almost without any losses

What a whip rather than a helical antenna will give you is a more assured picture in the 8-10nm range. Bear in mind that the Nasa engine, as do most others, will receive on only one of the two transmitting channels so you lose half of the available transmissions. Hence the need to get as reliable a picture as possible at the longest practicable range.
 
Thanks to all. It looks from the experience of others (and the helpful trig - not my strong point) as though pushpit mounting is a viable alternative to the other methods, then.

In response to JimG's question, yes I am only looking for early warning of potential threats, not for ship-spotting, so a range of say 8-10 miles ought to be sufficient and would give me at least 15 minutes warning of the approach of the fastest merchant vessels.

The point about whip antennae being more effective than helicals was interesting. Does that principle hold true even if I fit a short whip antenna, say 12" to 18"? I may be able to site one in such a way as to minimise the risk of damage.

Thanks again.
 
We have a whip antenna on the pullpit and seeing 15M+ range. During a recent period of high pressure, we were seeing ships at ranges up to 48M whilst tucked away in our marina!

AIS_LRange.JPG
 
Andrew

I ahve the helical option on the pushpit, so it is almost totally out of the way and secure. However, I am seeing quality information from ships only <10m away. This is fine if you pick them up soon enough and concentrate on making a decision - did this several times on a recent night crossing to CHerbourg.

However, based on others experience, I am planning to replace it with a longer whip over the winter.
 
AIS info superimposed on \'proper\' RADARs

Andrew, I think that your question has been well answered and you won't mind if I make a small digression....I am wondering if anyone has any information on ordinary, proper, radars that have an AIS option or capability. A radar with ARPA and AIS with, possibly, plotter overlay would be a mighty piece of kit! Far more than I need, but when I next buy a radar I guess that's what I will be looking for.
 
Re: AIS info superimposed on \'proper\' RADARs

You're looking at one in my picture

The Raymarine C or E series have combined radar (including MARPA) and chartplotter capabilities with AIS via an extarnal receiver. The picture shows the AIS overlay on the chartplotter, here's a zoomed version of shipping in the E Solent

AIS_2.JPG


and this is a poor one of the radar/AIS overlay. The angular misalignment is due to the fact that we've not yet interfaced the fast heading sensor - this is essential for both MARPA and accurate AIS overlay

AIS_Radar.jpg
 
Re: AIS info superimposed on \'proper\' RADARs

Wow! I want one! Very tempting but I really cannot justify spending the money all the while the present kit is working. In any case, I'd have to swap my extensive, expensive C-Map cartography for Navionics, I suppose.

The fast heading sensor is the rate 'gyro' compass, is it? I could usefully add that to my existing old Raytheon MARPA radar. Is ARPA as opposed to MARPA available yet? It would be good for lesser-skilled watch keepers and could trigger an alarm more effectively than MARPA especially for smaller targets that don't paint until the last couple of miles.

Have you used the AIS function in anger yet? How do you find it? Is it useful? Would you buy a standalone AIS 'radar' if you had no plans to upgrade your radar system?
 
Re: youse AIS anorak .....

jings, can dae better ... Ah weel ... 28 Aug 2005 0400, ships at 85nm range ..

oddly, all these ships were picked up to the SSE only (toward Le Harve) when abt 4 miles north of cherbourg. in other words none at this range in any other direction. Flat calm, clear sky & high pressure directly overhead ...
 
Re: AIS info superimposed on \'proper\' RADARs

are you sure that it was the nab tower you aligned the radar to? could have been one of the forts, no?! easy mistake to make and .. and ...
 
Re: AIS info superimposed on \'proper\' RADARs

The nice man at the Raymarine stand at SIBS yesterday let me play with the C70 (the smallest model), layering in the radar and AIS info over the top of the chart. Absolutely fantastic - I was very tempted and I doubt anyone could reasonably be disappointed with it. Then I remembered the lack of space, either on my yacht or in my budget , and forced myself, with gritted teeth, to buy the NASA unit...oh well. /forums/images/graemlins/frown.gif

Thanks to everyone for all the advice - I'll go for the whip antenna on the pushpit.
 
Re: AIS info superimposed on \'proper\' RADARs

Don't you have a radar?
 
Re: AIS info superimposed on \'proper\' RADARs

[ QUOTE ]


Have you used the AIS function in anger yet? How do you find it? Is it useful? Would you buy a standalone AIS 'radar' if you had no plans to upgrade your radar system?

[/ QUOTE ]i've done exactly this - with bog standard radar, no MARPA, bought AIS engine and stuffed it into the laptop (with Maptech CNP charts).

used it in anger twice - Heading for CHerbourg overnight last week had a ship showing CPA less than 0.2m, 8 miles away, doing 19knots carrying hazardous cargo! Called up on CH16 when he was 3 miles away, confirmed he could see us, and that he would alter course to go behind us.

All worked perfectly, and a great comfort to know there was no risk.
 
Re: AIS info superimposed on \'proper\' RADARs

[ QUOTE ]
are you sure that it was the nab tower you aligned the radar to?

[/ QUOTE ]

How very dare you, Douglas !
 
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