AIS Antenna Height (part II)

[BrianH]

Rather than resurrect an old thread I am starting another with the same name. Similar concerns to the OP of the original thread, I was concerned about the coverage of an AIS transponder with only a pushpit antenna mounting. My new system has now been in operation for three weeks and I am reassured. The antenna is a standard Glomex VHF 1/2 wave mounted with the base fixed to the lower horizontal bar where it meets a vertical one and offers the most secure mounting.

Moored in my marina in northern Italy I am receiving ships on average 15nm distant (often well over 20nm) and I am consistently shown on Marinetraffic, the receiving station of which is at Koper in Slovenia, some 28nm distant and is at 1028m.

I am aware that reception by a receiver at over 1000 metres high is no proper assurance of being seen by a ship - but I am more concerned with seeing the ship ... the receiver is the important part of my strategy - active avoidance. The transponder part is more for my home-bound wife to see I am still afloat.

In the image below, a snapshot of the netbook screen, running OpenCPN, the ship outbound from the Gulf of Trieste is 13.5nm distant and the closer, anchored ones, are 9nm. The Amec Camino-101 is mounted on a locker shelf above the netbook. The active GPS antenna for the transponder is just lying on the shelf adjacent to the unit and the signal strength indicator of the OCPN screen, is just about at maximum.

 

[yoda]

The one thing I have noticed is that some large ships have 'wooded' arcs where the transmitted signal just doesn't get through. Fortunately these seem to be (as far as I have seen) in the stern arcs and therefore tend not to be too much of a problem.

Yoda

 

[BrianH]

The one thing I have noticed is that some large ships have 'wooded' arcs where the transmitted signal just doesn't get through. Fortunately these seem to be (as far as I have seen) in the stern arcs and therefore tend not to be too much of a problem.

"Wooded"? You mean you can't see the ship for the trees?

If you have noticed arcs of diminished reception it could be due to the ship's antenna placement where the aft superstructure may be blocking - or if a gantry, absorbing - the signal.

As I am moored well inland I note that I sometimes lose the signal from ships passing beyond the high buildings that line the coast. The VHF propagation principle of 'line of sight' is not always relevant with occasional atmospheric anomalies, but does have a bearing.

I notice that ships at longer distances are still often lacking their static data so clearly I am only intermittently receiving the six-minute static data sentences on weaker signals. But infinitely better than with the single, alternating-channel receiver just replaced.

 

[lw395]

The one thing I have noticed is that some large ships have 'wooded' arcs where the transmitted signal just doesn't get through. Fortunately these seem to be (as far as I have seen) in the stern arcs and therefore tend not to be too much of a problem.

Yoda

Likewise the antenna on a yacht could have a dead area looking forwards if it's mounted too low at the stern.
I also feel there is some merit in raising it so it's above any wave height.
Realistically though, the main benefit of being up on a pole is being away from other people's mooring lines when rafted.

Personally though, I wouldn't be relying on a non-waterproof laptop on any vessel under about 2000 tons.
Things can get very wet in rough weather.

 

[bedouin]

If your primary concern is seeing ships then I am not sure it is much of an issue. Surely for your purposes you only need to be able to pick them up at 5-6 miles range and almost any mounting will do that.

For detecting ships the main influence on the range will be the height at which their antenna is mounted, not yours (think line of sight between the two). Raising your antenna from say 1m to 5m will only increase the range by about 5 miles.

 

[BrianH]

Personally though, I wouldn't be relying on a non-waterproof laptop on any vessel under about 2000 tons.
Things can get very wet in rough weather.

I have been using one below on the chart table for many years now. Even in "rough weather" - and even where I sail I have experienced quite a lot of that - it remains dry, being protected by a pilot-house with its own chart table on which is a Garmin plotter, for which the laptop is a backup. So, no "relying" on it at all.

 

[BrianH]

If your primary concern is seeing ships then I am not sure it is much of an issue. Surely for your purposes you only need to be able to pick them up at 5-6 miles range and almost any mounting will do that.

For detecting ships the main influence on the range will be the height at which their antenna is mounted, not yours (think line of sight between the two). Raising your antenna from say 1m to 5m will only increase the range by about 5 miles.

Good point, I must admit that hadn't occurred to me and that a metre or two is inconsequential. But in the previous thread there were some who maintained a masthead AIS antenna was preferable; I am reassured my installation is adequate enough for me.

 

[Daydream believer]

I have the same AIS set & impressed with the construction etc. I also have paid the extra £ 90-00 for the silent mode switch as I see no point in adding to clutter when i am just sailing locally. The plotter it is connected to is a Lorenze 5X as I prefer not to use a laptop

My GPS aerial is on the pushpit but my transmitter aerial is mounted on an old sailboard boom. I have cut it off just where it starts to curve in & fitted it to the pushpit with a couple of "U" bolts set at 45 degrees to the cross rail & vertical post. I used a sail board boom for 3 reasons
1) Cost- I am sure many of us have a sailboard in the garage
2) Quality of material- nice & strong with good anodising. It is 32mm diam covered in neoprene with a 25mm diam gold anodised extension piece
3) it was an extending boom so if I want another 700mm of height I can just draw the centre out a bit thus raising the aerial

As has been mentioned it is better to be clear of dock lines so height helps. The aerial can double up as a VHF aerial in the event of a dissmasting
I also have the other half of the boom fitted & that carries an Echomax active radar reflector. The second post is on the opposite quarter of the stern so that the aerial does not affect the reflector

 

[Koeketiene]

Personally though, I wouldn't be relying on a non-waterproof laptop on any vessel under about 2000 tons.
Things can get very wet in rough weather.

Surely that depends on the type of boat.
Two weeks ago I did a Channel X-ing in a F7-8, and even though the decks were awash at times it remained bone-dry below throughout. (44' - 13 GRT sloop)
AIS tranceiver and Open CPN were a great help in the busier bits. Was reassuring to see the 'big boys' take avoiding action.

 

[BrianH]

I have the same AIS set & impressed with the construction etc.

It is indeed a physically robust unit, but I note that my supplier, Milltech Marine, has discontinued it, supplying now the Camino-108. Perhaps why I could buy it last November for $399.

I also have paid the extra £ 90-00 for the silent mode switch as I see no point in adding to clutter when i am just sailing locally. The plotter it is connected to is a Lorenze 5X as I prefer not to use a laptop

As my plotter is somewhat antique it cannot overlay AIS targets, hence the laptop, with which I can invoke the Amec utility to silence the transmissions. It does have the luxury of a much larger screen and, being a single Atom-powered netbook, frugal with the Amps.

My GPS aerial is on the pushpit but my transmitter aerial is mounted on an old sailboard boom.

An active GPS antenna does not need external mounting in the average plastic boat, as my system shows by just lying in the locker next to the unit, the cable neatly coiled; I get instant fixes and never lose a solid signal. By the same token, I have used a GPS USB adapter in the laptop, and also in other yachts, below decks with 100% reception. The aforesaid Garmin plotter, by no means latest technology, has an internal GPS antenna working under a pilothouse roof equally well.

As has been mentioned it is better to be clear of dock lines so height helps.

A valid point, a metre-long appendage is always something else to catch a line. But mine is sited on the quarter where I already have a wind generator mast and the outboard storage bracket, so it just adds to the general mess that I work hard to avoid with mooring lines.

The aerial can double up as a VHF aerial in the event of a dissmasting

... wherever sited. I had thought I might be able to use the original, short, receive-only, AIS antenna fitted on an outrigger from the wind generator pole, but it clearly didn't work so I had to source a normal one locally and site on the pushpit. I was tempted to buy a stubby version to replace the old one on the pole but bumped into the Italian Glomex manager in the marina who convinced me of the benefits of a 1/2 wave model (from his "30 years of installation experience" - "what is it you want, performance or convenience?" A good salesman).