VHF and AIS Again???

[BobE]

Spent the last hour searching archives and now I'm pestering you lot again.

Sorry but..

Just fitted an AIS engine from Nasa Coupled to a Standard Horizon CP175 and my laptop.
Works OK on Laptop but plotter screen is rather small..

I'm using an aerial on the pushpit but wonder about combining it with the masthead VHF antenna...
NASA say that they've had a couple of units "fried" when splitters failed whilst the VHF was transmitting and the full output went into the AIS...

They also say that the VHF output will be reduced all the time...

Problem I came across when installing was that at Lw in Christchurch using the low aerial (about 2m above water level) I could only get GPS signals and no targets ... As soon as I plugged the masthead aerial in I got ships in the traffic zones etc.

So I'm still enamoured with the splitter idea.
Does anyone know what the reduction in VHF output is in simple terms?

NASA spoke of decibels? I thought transmission powers were in watts.
(25 watts reduced by say 2 percent I could appreciate but by 2 decibels ???)

Any coments, help, advice, as ever appreciated from you fine people!!!
Cheers Bob E....

 

[Salty John]

Some basics here: http://www.saltyjohn.co.uk/resources/Mar...nstallation.pdf

Because the AIS signal comes from quite high up on the transmitting ship the fact that your antenna is located low down is less of a disadvantage than it would be in yacht to yacht communication.
I would have the same concerns regarding splitters that NASA has.
A pushpit mounted AIS antenna gives you redundancy if set up so that it can be plugged into the radio in case of a masthead antenna failing, as in a dismasting!

 

[rickp]

If my dodgy memory is right, a 3db insertion loss is about 50% of the signal. 2db loss is 36% of signal. A 1db loss ia about 20% of the signal.

So if you find a good splitter which less than 1db insertion loss, then its costing you about 20% (or less) of your VHF signal.

No doubt someone with a better understanding of this stuff will come and correct me /forums/images/graemlins/wink.gif

Rick

 

[jrt]

The power loss will depend on the particular splitter that you will be using, but it could be as high as 3dB ie half the power. Splitters are mainly used for two receivers on the same aerial, not a receiver and a transmitter. Take Nasa's advice and use a second aerial on the push pit (or of course if you have one, on the misson mast.) Don't put the AIS aerial within a metre of your VHF transmitting aerial (ie on the same mast) as it could still do damage.

Low signal strength in Christchurch bay. You are probably not picking it up on the pushpit aerial because Henigsbury Head (?) is in the way. Surely the only time that you will be using the AIS in anger is when you are out at sea with no headlands in the way. If you have a headland ahead, the ships are going to have to come round it before they get to you, and your pushpit aerial will pick up the signal in plenty of time for you to do whatever is deemed necessary.

 

[fireball]

We have the NASA AIS engine and the SH CP180i - although we have had some problems with the unit rebooting when trying to display target data - I've not managed to test it fully yet.

There is a marked difference in range with the pushpit mounted antenna to the masthead - with the pushpit one we get no targets when sat on our mooring (quite a way inside Chichester Harbour) but can get targets from the Bembridge anchorage with the masthead one.

I would not entertain a splitter on the VHF - Antenna line - I'd rather know that it is a single line with nothing to get in the way.
If you're really concerned about the range (I've tested up to 6Nm with the pushpit in the confines of the solent (6Nm would take us 1hr to cover)) of the pushpit antenna then you can either just swap the VHF/AIS antennas over, or install a second antenna up the mast (don't forget the separation mentioned above - vertical separation will do).

 

[tome]

Re: VHF and AIS Again???

We use a pullpit mounted antenna for AIS and have no problems at all picking up signals, well beyond the range we are interested in. Like others I wouldn't go for a splitter

If going for an additional mast antenna, why not consider one on the spreaders? This will give you very good separation from the masthead and plenty of range

 

[Talbot]

I started using a vtronix helical aerial (the short stubby one) for the AIS mounted on my pushpit. I was very unhappy with range, so I ditched the stubby aerial and installed a jgtech 1m whip. It doubled the range.

When I installed the AIS, I used bnc connectors, and I added a bnc connector to my vhf aerial, so I could swap the AIS/radio to whicheever aerial I wanted to use. It means I have a permanently installed emergency aerial for the vhf (inplace and tested) and also can get the additional range from the masthead for AIS when I want it. however I get 16nm now from the pushpit, and abt 40 from the masthead (on the bigger ships)

 

[BobE]

Thanks to John, Rick, jrt, Fireball,Tome, and Talbot for the informative replies...
It looks like I'll stick with the Taffrail mounted one metre thing.. Saves me around 100 notes!!
I appreciate all the comments and indeed had thought about some of them..
The main problem at the moment is testing that the AIS aerial is actually connected and working OK... I've got a mess of those fiddly data cables and every time I check something I cannot tell if the aerial works without going out of the harbour... Ok at Hw but only get GPS signals at Lw..

Still should have plenty of time to test the thing on the Cherboug Scuttle. /forums/images/graemlins/laugh.gif

Thanks once more...
Cheers Bob E.....

 

[rickp]

Re: VHF and AIS Again???

Have you noticed that we have a Class B equipped vessel in Northney now? G-Force was transmitting AIS when it returned on the weekend. Also see Fay's Fantasy at Port Solent periodically and that one never seems to turn its AIS transmitter off, even at berth. Class B could be chaos in a few years time....

Rick

 

[fireball]

You could always just plug it into the VHF and compare what you are receiving ... we get nothing whilst on our mooring - but I know the antenna works.