AIS range issue...

[pongoglo]

I have a NASA AIS 2 connected to a Garmin 298C plotter/sounder. The AIS seems to interface fine with the Garmin but the detection range is very very poor. I am only picking up AIS targets at 1.5 miles or less? I believe the problem might lie with my antenna connection? Im using a pushpit mounted 2m Glomex antenna which should give me a detection range of 12 miles or more, however Im using a standard TV aerial extension cable between the Glomex lead and the AIS BNC fitting? Do I need a specific marine VHF cable, or do you think the problem might lie with the connectors? Any advice gratefully received!

 

[Salty John]

You need a 50 ohm impedence coaxial cable, TV cable is 75 ohm.

 

[R32Stbrigid]

I have a pushpit mounted aerial and regularely see AIS targets at 12+ miles, So I am sure SaltyJohn is correct .....

 

[fireball]

Whilst my learned friends are pointing towards your antenna and cabling can I suggest that you test your engine using a mast head antenna.
With my masthead antenna I can receive ships from ~35Nm away, but this was after having the engine replaced by NASA following a range issue where I could only "see" ships less than 5Nm away ...
It would be really handy if you could test against another known good engine/display on your current set up - if your engine is still giving poor range then contact NASA as it is likely to be an engine fault - this appears not to be uncommon!!

 

[pongoglo]

Thanks for that. any idea where I can source a 50 Ohm cable - (pref with connectors fitted!)?

 

[BrianH]

I have my antenna (supplied with the AIS receiver along with 5M of cable) mounted on an aft mast about 2m high and receive reports on average between 6nm to 10nm. The maximum I have seen has been 23nm.

Someone in the marina said I should mount it at the masthead as he had seen a demonstration where reports were being received from ships 50nm away. I answered that I wasn't interested in ships 50nm away - I only wanted to know about the ship on a collision course in my immediate vicinity. Why clutter your reporting list with irrelevant reports that you may never get close to?

 

[maxi77]

Try Maplin who I think do cut lenths and a wide range of connectors.

I there is a radio ham or CB radio store near you they will be able to supply as well.

Avoid marine electronics places as their prices make your eyes water.

 

[pongoglo]

Ok. Maplins do it. Ive also noted that my BNC connectors are 75 Ohm, so that might be causing the problem as well? If I dont get any joy with the 50 ohm cable, and replacement connectors I will take up Fireballs suggestion and return to NASA.

 

[pongoglo]

I agree with Barnacle. Too great a range would be an impediment. Even with the 1.5 to 2 NM range Im getting at the mo Im finding I get sufficient warning at night to be useful, but 5 -12 NM would be nice! What I am finding is that a lot of contacts are comin up as 'unknowns', and the system can take up to 10 mins to identify the target by name. Has anyone else found this?

 

[fireball]

No no no no no ... don't just return ... theyll send it back saying there is nothing wrong with it .... YOU have to test it first!! Based in Portsmouth there should be loads of others with similar setup, it doesn't take long to do, but will give you peace of mind that it is your cabling at fault and no the engine ... or vis versa!

 

[fireball]

10 mins is perfectly normal - which is why you want to detect them further out .... then you've got their names when they become a problem ...

 

[BrianH]

[ QUOTE ]
What I am finding is that a lot of contacts are comin up as 'unknowns', and the system can take up to 10 mins to identify the target by name. Has anyone else found this?

[/ QUOTE ]
Static data messages are sent every 6 minutes and if you miss one - easy to do with non-parallel dual channel receivers like your NASA - you will have to wait for the next.

Not a problem with the dynamic data messages as they are being sent every few seconds, the frequency period depending on the ship's status and speed.

 

[alan]

Whilst using the incorrect cable is not good practice, I doubt that this is the problem. Poor connections could be the cause but as suggested you should try it with a mast head antenna and see if there is a dramatic difference. With my Easy-AIS and a 1m Glomex antenna on the pushpit I have seen ships at 200nM ..... and regularly see them at 20nM. The range you are getting is very poor and it would seem like a receiver sensitivity problem to me.

Alan.

 

[Salty John]

VHF range is 'line of sight' and is a function of the height of the transmitting and receiving antennas. Line of sight means two observers at the same height as two antennas would be able to see each other - if land gets in the way or the straight line between the two points is interupted by the horizon then the signal will be lost.
The formula is Distance = 1.23 x root H1 + H2. D is in nm, H is antenna height above sea level in ft.
Typically, a VHF transmission from a ship with it's antenna array at 100' above sea level will be picked up by a pulpit mounted antenna at 10' above sea level at a distance of 16 miles. Smaller ships will have an antenna set somewhat lower, say 40 -50', and this would drop the range to 12 miles.
Sometimes atmospheric conditions introduce weird results that can increase these distances substantially but such phenomena can't be relied on.

 

[Talbot]

[ QUOTE ]
I only wanted to know about the ship on a collision course in my immediate vicinity.

[/ QUOTE ]

That is fine except at 6nmm range, a vessel doing 25kts only takes 14 minutes to get to you. you might be doing something up forward and miss this for 7mins which means he is awfully close before you do something.

I prefer to manae the situatin by having a good range (20+ miles), then have software that manages the warning based on CPA and arrival time (www.shipplotter.com) This enables me to assess the situation, and alleviate a potential concern before that problem gets into visual range.

 

[peterb]

[ QUOTE ]
The formula is Distance = 1.23 x root H1 + H2. D is in nm, H is antenna height above sea level in ft.

[/ QUOTE ]

Not quite right. Your horizon distance is 1.23 x root H1; the ship's horizon distance is 1.23 x root H2; adding the two together gives 1.23 x (root H1 + root H2). If you're quoting heights in metres, then instead of 1.23 use 2.2

 

[Eygthene]

I had exactly this problem with my NASA AIS "Radar" unit and a Glomex whip antenna mounted on the pushpit. With the mast-head antenna I "saw" ships at 25 miles but only 2 miles with the pushpit one. It turned out to be a bad connection where the cable is connected to the Glomex base. When I rectified the assembly of the cable to this connector, the range from the pushpit went to 20 miles. Your 75ohm cable will not cause the problem you describe, take a careful look at the inner and outer conductors where they are clamped inside the connectors (both ends) and make sure there is (a) a good physical contact between wire and metal and (b) no possibility of any wire from the outer making contact with the inner. I'm pretty confident this will solve your problem.

As said above, if you can make a temporary connection to the cable to your mast-head antenna instead you will soon see if the performance of the AIS unit is OK or not.

 

[achwilan]

mast-head antenna + navicom RT650 + raymarine C70 = usual range of 30-40 miles with helpful real-time computation of targets' cpa/tcpa.
But non standard propagation can increase the range above 100 miles... allows to get interesting pictures of channel traffic!

 

[BrianH]

[ QUOTE ]
[ QUOTE ]
I only wanted to know about the ship on a collision course in my immediate vicinity.

[/ QUOTE ]

That is fine except at 6nmm range, a vessel doing 25kts only takes 14 minutes to get to you. you might be doing something up forward and miss this for 7mins which means he is awfully close before you do something.

I prefer to manae the situatin by having a good range (20+ miles), then have software that manages the warning based on CPA and arrival time

[/ QUOTE ]
This is a valid point for a limited number/frequency of potential threats. I agree that the ideal would be 20nm reception and let the software manage a filtering process based on proximity and CPA.

However, from the experiences of ranges in this thread, this implies a masthead antenna with VHF radio/AIS splitter complications. I agree this is probably the ideal way to go but my present setup has been so useful and effective over the past three years I am unlikely to change it anytime soon, or until I next have the mast down.

But it also means that Mobos will always have a distance limitation .....

 

[Graham_Wright]

[ QUOTE ]
Whilst my learned friends are pointing towards your antenna and cabling can I suggest that you test your engine using a mast head antenna.


[/ QUOTE ]

Now I am worried. The masthead generally carries the VHF aerial which blasts out 25 watts. Does that not overload the AIS receiver? I believe the frequencies are similar.