View Full Version : VHF Antenna location for AIS

10-08-06, 10:20
Following Jimi's thread about the NASA AIS 'radar' versus the engine into a PC, which resulted in him buying a PC, this is just a suggestion about the antenna location.

I have a rubber 'stumpy' mounted on the pushpit. This was the easiest place to mount it and run the cable and I didn't want a whippy antenna getting in the way when mooring or accessing the MOB gear.

It has to be at least 3 feet (IIRC) from the radio antenna, so top of the mast is not practical.

I can receive signals from ships in the channel approx 8 miles away, so if they are doing 20knots, I have about 20 minutes to deal with them.

At home in the marina, (which is virtually landlocked) I can't pick up anything despite being near one of the busiest shipping ports in the UK.

In hindsight, I believe the best location for the antenna is on the spreaders, to give the extra height and hence range.

Comments and Suggestions?

10-08-06, 10:24
Im going to experiment with my emergency antenna before fixing upon a location

10-08-06, 10:24

Can't you wear it on your hat? With your height you should be able to pickem up as they come into the Dover Straits.

10-08-06, 10:38
Sound plan - i for one would be interested in reading the outcome.

10-08-06, 10:56
Young Para has some experience of this and reckons that the stumpy things, like the one we have on RS, are crap and reckons that one of them long whippy things are a better bet. I'm a technoklutz so don't ask me why.

10-08-06, 11:02
Funnily, I bought a NASA AIS yesterday and picked up the emergency antenna from the boat to experiment. Need to make up a lead with BNC connectors

I have it downstairs in the lab as i want to check how NMEA data passes through the NASA receiver - it has a normal 4800 input which it multiplexes onto the fast (38400) AIS output. This should let me use it with my E80 display

Will get a chance to play later and hope to try it this weekend...

10-08-06, 11:05
As you know - I'm also thinking about going down the AIS engine route and been thinking about where to place the antenna ...

Spreaders would be great - gets the added height without getting in the way of everything else - although mounting bracket would need to be so it sits directly above or below the spreader so it doesn't get mashed up by the genoa on a beat or mainsail on a run!
My problem with getting it on the spreader is the cabling - there is a conduit that runs all the way up the mast - exiting a cable 1/2way up is obviously not going to be easy.

The other option - if the antenna needs to be 3' appart - this is on the horizontal plane - so what is to stop you mounting a stub antenna at the top of the mast BENEATH the existing VHF antenna ? This wouldn't require a huge bracket, but may need to be far enough away from the mast to avoid shadowing from the mast?

10-08-06, 11:46
Vertical separation works well (we often use this for telemetry) but spreaders would be my first choice

10-08-06, 11:51
I went through this process yesterday and here is my input.

Mounted a stubby aerial on pushpit (Vtronix I think).

I did not leave Gosport marina (G Pontoon about half way out) and was able to pickup and track the IOW ferries all the way to Wooten, Ships off Stokes Bay, Ships anchored off Bembridge and a whole lot of craft within Portsmouth harbour.

For those that do not know the area, there is a fair bit of land between the marina and Wooten.

I was rather impressed. Will know more about the range over the next two weeks as we head west.

The book of words (Sorry, sheet of words) that came with the NASA engine recons that the range should be LineOfSight + 30%



10-08-06, 12:53
I started with a helical aerial (small stumpy) and also achieved about 7nm. I was not happy about that so purchased a 1m whip from JGTech) and sited it on my davits. I know achieve abt 12-16nm (depending on vessel size) and reckon that that is fine. If I want a longer range for any reason, my system is set up so I can swap over aerials to VHF and to AIS, thus getting an emergency aerial as well.

10-08-06, 12:55
tome, if you add in the gps signal at 4800, it effectively slows down the AIS engine to that speed.

10-08-06, 13:23
Another possibility is the use of an AIS/VHF antenna splitter. This allows use of the normal masthead VHF antenna for AIS reception and normally gives a range of 30+ miles. Thes costs are not much greater than a new VHF arial with cabling to the spreaders.

10-08-06, 13:28
VHF/AIS splitter Like the sound of that, anyone got any experience of using it?

10-08-06, 13:29
but it does reduce power to your vhf by 50% and in an emergency could make a significant difference on whether your call for help is heard.

Besides it introduces an additional point of failure on a piece of safety equipment.

10-08-06, 13:33
No it doesn't - it multiplexes the 4800 baud GPS data onto the 38400 baud output stream interspersed with the AIS sentences

There's no way of getting AIS data out at 4880 baud - there's too much of it

Salty John
10-08-06, 13:34
Two points:
1. Because AIS is always receiving from a very tall structure, say a 160' tall mast on a ship, and line of sight range is a function of the root of the combined height of transmitter and receiver, the effect of raising the receiving antenna from the pushpit to the spreaders is not as significant as it would be if trying to communicate with small vessels, as with your voice VHF. The difference in theoretical range between 9' at the pushpit and 49' at the spreaders, receiving from a 160' transmitter, is about 26 miles versus 21 miles.
2. Putting the AIS antenna on the spreaders deprives you of an emergency antenna if you lose the mast.
To avoid the inconvenience of having a 1m whip getting in the way it could be mounted on a demountable bracket that would allow you to reposition the antenna when not in use - say pointing down alongside a pushpit upright.

10-08-06, 13:44
I presume you have just proven that, but it does disagree with the findings of another forumite. Personally I use a virtual comm port software fix to be able to share the gps inside the laptop. This also gets around a specific problem with shipplotter software which will not record the gps feed when it is buried in the rest of the AIS data - it does use it for display, just doesnt record it.

10-08-06, 14:04
Agree but need to add that it only adds the Position sentence to the output. No other NMEA sentences are passed.



10-08-06, 14:06
Two excellent points js, although AIS is required for anything commercial over 300tons, so many vessels will have their transmit antenna lower than 160', more like 60'. The trig still applies in a similar way.

My only other thought is that in rough conditions, say a 10ft swell in the channel, an antenna on the pushpit is going to spend c.50% of the time below the peaks which cause a degraded if not total loss of signal.

Based on yours and other comments,, it seems like a longer antenna at pushpit level is the way to go for me, although I look forward to the outcome of tome and jimi's trials.

10-08-06, 14:10

I have it downstairs in the lab as i want to check how NMEA data passes through the NASA receiver - it has a normal 4800 input which it multiplexes onto the fast (38400) AIS output. This should let me use it with my E80 display

[/ QUOTE ]

Doesnt seem to work for me Tom. I have taken the NMEA output form the Raymarine mux into the GPS input on the AIS engine. Then output from AIS at 38400baud into the PC. I get GPS position sentences, and the Maptech software calculates COG and SOG from that, but I am losing all other data, particuarly HDG which would be useful.

Of course, I could have done something wrong and you will come back to tell me how do it properly !!

10-08-06, 14:19
I spoke to NASA re this yesterday and they confirmed that they only pass position data. All other sentences are dropped.


10-08-06, 14:41
Insertion loss of AIS/VHF splitters is smaller than -1db , while -3db would be necessary for a 50% loss.
So this small loss (of around 10%) should be weighed against greater AIS range - which is important esp. with fast-moving high-speed craft and single-channel receivers.

10-08-06, 15:18
Yes just tested that a RMC sentence input at 4800 baud emerges in the data stream at 38,400 baud

Also from the NASA documentation:

"NMEA RMC sentences received at...4800 baud will be copied through on the 38,400 baud output channel whenever they are available"

10-08-06, 15:19
Just tested this and agree, only the GPS RMC sentence is passed through

10-08-06, 15:35
I wouldn't compromise the vital function of a VHF set by splitting the antenna regardless of insertion loss. As Talbot says, you're adding another point of failure

10-08-06, 16:14
you'll find that the GPS stream is interrupted quite regularly .. i presume that the NASA engine gets indigestion when stuffed with AIS decoding ...

10-08-06, 16:45
I saw this - I was outputting RMC's at 1 Hz and there were gaps of up to 6 secs when they failed to get through to the output. This was with the antenna disconnected so no live AIS to process...

10-08-06, 16:48
perhaps that explains the apparent slow data rate observed.

10-08-06, 16:48
Mmmmm. Why bother using a fast processor when a slow one will do the job? /forums/images/graemlins/crazy.gif

10-08-06, 16:56
hmm ... you sure? with my aerial disconnected just trundles out GPS at same rate it comes in

10-08-06, 19:16
Not sure if this one would cause a 3db loss, from its current specs it seems to switch when you transmit. It looks a well engineered bit of kit, as does the AIS receiver made by the same people:


To add further input on the range question, my home monitoring antenna is about 16m above sea level and the normal range is 26-28nm, even over a range of hills and that is using a broadband communications receiver rather than a dedicated unit. (With exceptional propagation contidions I have achieved 147nm!) Although I'm sure a spreader mounted antenna would be a lot better than a pushpit mounted one, the former may suffer range degradation if there is a rain soaked sail in the propagation path.

10-08-06, 19:56
Pal of mine fitted his aerial on the spreaders and found he had a blind spot so has had to have a rethink. I used my emergency aerial on the end of a brush stale tied to the pulpit and this worked surprisingly well, picking up transmissions from 16 miles. I found that the longer ranges were only used when things were "quiet", not much about, but when it was busy 8 miles and less was much more workable.


10-08-06, 20:29
109 buys 3 x VHF aerials and then you have an emergency aerial.

10-08-06, 21:51
Insertion loss of AIS/VHF splitters is smaller than -1db , while -3db would be necessary for a 50% loss.

If both the AIS and the VHF are to both receive at the same time then the signal is split 50% each way on receive and that is a 3db loss and will affect the distance you can work with your VHF (mostly in ship to ship and non high powered shore stations) and will reduce the range of the AIS (but some of that recovered if there is an AIS antenna height gain one would not otherwise have had).

Obviously to all, I expect, there is no split when you transmit on the VHF else the AIS will be fried, the only loss will be the splitter's insertion loss (maybe -1db as you say). However, when transmitting on the VHF the AIS will not receive.

For myself, I would never put a splitter in my VHF transmission line for what I would have thought were obvious reliability reasons associated with safety equipment. Furthermore, from the few tests I have done (those with AM/FM splitters) and experience in other services there is nothing that can equal the performance of 2 separate antenna systems.

Frankly I cannot imagine why anyone who uses their boat enuff to justify the installation of an AIS would revert to using a splitter and not do the job properly (unless the AIS is really just another toy bought to play with rather than rely on). As Talbot mentions, a set up using a separate VHF antenna that can also serve as a backup VHF antenna is ideal in my opinion.