pandroid
Well-Known Member
AIS is only 2 watts. You won't do that much damage with that.
Did you know that mobile phones work on the same frequency as your microwave oven?
Flipping stupid AIS aerial - what am I doing wrong?
- Thread starter Rivers & creeks
- Start date
Did you know that mobile phones work on the same frequency as your microwave oven?
Salty John
Well-Known Member
Marine band transmitters are limited to 25w output. The FCC in the USA doesn't even require an evaluation for outputs less than 50w. Even above 50w the exposure limits would be difficult to achieve unless you were constantly on the radio with your eyes close to the antenna.
I'm sure the guidelines will be similar for the UK and EU but don't have them to hand. I'm also sure that manufacturers will state safe operating distances of a meter or so to cover themselves against litigation.
I suspect a handheld, where the antenna is in your face, would be more dangerous.
I'm sure the guidelines will be similar for the UK and EU but don't have them to hand. I'm also sure that manufacturers will state safe operating distances of a meter or so to cover themselves against litigation.
I suspect a handheld, where the antenna is in your face, would be more dangerous.
JumbleDuck
Well-Known Member
I looked into it when I put my AIS antenna on the rear pulpit, and came to the conclusion that the power involved is far too low to do any harm.
Norman_E
Well-Known Member
If you buy the correct (expensive) splitter!
Javelin
Well-Known Member
I have a AIS specifically tuned antenna which should make a slight difference over a standard VHF antenna.
Again there are a couple of reasons I don't use a splitter/
Firstly the cost of the low loss versions is in my mind excessive and frankly I don't want anything to detract from my VHF performance.
Secondly I saw an opportunity to build in redundancy having a second non mast mounted antenna just in case I lost my rig.
In practice I get ais contacts 15 to 25 miles away.
The ones I actually want to know about are less than 10 miles away so I can't see the point of seeing contacts beyond what I'm already getting out at sea.
If I want to see others when coastal sailing I just look up marine traffic on the phone or tablet.
Again there are a couple of reasons I don't use a splitter/
Firstly the cost of the low loss versions is in my mind excessive and frankly I don't want anything to detract from my VHF performance.
Secondly I saw an opportunity to build in redundancy having a second non mast mounted antenna just in case I lost my rig.
In practice I get ais contacts 15 to 25 miles away.
The ones I actually want to know about are less than 10 miles away so I can't see the point of seeing contacts beyond what I'm already getting out at sea.
If I want to see others when coastal sailing I just look up marine traffic on the phone or tablet.
lw395
Well-Known Member
The issue is not the maximum distance at which you sometimes see other vessels.
It should be the distance at which you are always going to see a vessel you care about with a margin.
Seeing 90% of ships at say 10 miles might make you feel comfortable, but that's false if you don't see the 10th ship until too late.
With a low-mounted antenna, it seems possible to have a near-blind spot at some angles.
It should be the distance at which you are always going to see a vessel you care about with a margin.
Seeing 90% of ships at say 10 miles might make you feel comfortable, but that's false if you don't see the 10th ship until too late.
With a low-mounted antenna, it seems possible to have a near-blind spot at some angles.
Javelin
Well-Known Member
I've only noticed two contacts that at the time I thought I should be able to see on ais.
One turned out to be a British Destroyer and the other was a freighter 30 miles off Harwich that was getting a real grilling on ch16 by an irate radio operator on board a Harwich/Hoek ferry demanding to know why his ais was not transmitting.
One turned out to be a British Destroyer and the other was a freighter 30 miles off Harwich that was getting a real grilling on ch16 by an irate radio operator on board a Harwich/Hoek ferry demanding to know why his ais was not transmitting.
[70521]
Well-Known Member
True, but it might still be cheaper than fitting a new antenna.
PaulRainbow
Well-Known Member
Don't get why there would be a blind spot if mounted low, do you ?
Again, how on Earth do the poor old mobo boys manage without a 45ft stick to mount their antennas on ?
RIBW
Well-Known Member
Having spent many hours in anechoic chambers testing the radiation patterns of VHF antennas on satellites, I now think of VHF radiation as much 'art' as 'science'. (We needed to be sure that we could communicate with the satellite no matter the final orientation of its antennas).
An antenna, sited amongst the metallic clutter that is the average leisure boat cockpit, will have its radiation pattern modified by that clutter. Inevitably, this will cause both gain and near-nulls in certain directions . If perfect nulls happen then there will be no RF signal in that direction at specific frequencies and could cause AIS 'blindness'. However, boats corkscrew, nulls will probably not be perfect etc. The chances of missing a ClassA AIS transmission in these circumstances are, to me, sufficiently small to be acceptable.
I have chosen to have both my main and AIS VHF antennas mounted on an arm on my WindGen mast at the pushpit. That way, I can maintain my VHF systems without calling in expensive riggers who may have little understanding of VHF technology.
Doing the theory (see Salty John above), I have theoretical line-of-sight in mid English Channel even out west.
Cheers
Bob
PS Now, who wants a discussion about whether boats should have vertically diverse antennas to eliminate the possibility of radiation pattern nulls caused by reflections from the sea surface
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RobbieW
Well-Known Member
My AIS antenna (rx only) was similarly positioned until 2015, on a stub off a 2" stainless tube carrying the wind gen. It was about 3m above sea level and 2-3" aft of the tube. I found I wasnt seeing the targets I thought I should ahead of me, stuff behind seemed ok. Put in a splitter to the VHF mast head antenna and my AIS visibility has dramatically improved, as has speed to acquire and retention of targets.
BrianH
Well-Known Member
With a pushpit-mounted antenna I have never noticed that ... on what basis do you make that assumption?
If the 10th ship is missed at 10nm I would suggest it would not be missed at a more realistically relevant 5nm.
Those with a transponder are also interested that large commercial ships see them at such distances - I have observed course changes from such vessels at distances of 2-3nm from my position in the middle of the Adriatic, which gives me reason to believe they are acting on seeing my target with CPA data rather than on visual information of much less certitude.
I posted the OCPN screenshot below two weeks ago in a similar-themed thread that showed my reception range approaching my destination in NE Italy (left of screen). It was not a day with exceptional long-distance propagation. It shows the cluster of moored vessels around Trieste (right of screen) at well in excess of 20nm as were others further south in the Adriatic (off-screen). The furthest ship was 37.3nm. The antenna is a half-wave, shunt-fed Metz (cut to AIS frequencies) mounted on the pushpit.
MikeCC
Well-Known Member
On original post, I'd suggest hooking the stern antenna up to the VHF with an SWR meter and seeing what you get. Also, most AIS transceivers have an SWR diagnostic onboard but you may need to connect to PC to see that. There may even be a flashing error light with a sequence to indicate the fault. Are you getting the poor results in open water with known targets in range? Is core of VHF cable crimped or soldered on to centre pin?
Rivers & creeks
Well-Known Member
Hi MikeCC, yes problem in open water as well. Centre wire isn't soldered.
lw395
Well-Known Member
Like RIBW, I have had to measure a few antennas in my time.
When you install an aerial in a piece of equipment, it's not uncommon to get big variations of gain in various directions.
6dB, which would halve the range from say 10 to 5NM is not what I'd call a big variation in this context. 20 or 30dB is common, often the limit of the null you can plot is due to the difficulty in measuring it!
It's a compromise you have to make.
But it is important to understand the difference between frequently seeing some ships at 10 miles and reliably seeing all ships.
A better placed antenna not only gives you more range (which isn't always ever so useful) but also some margin in hand, to 'see' around headlands or over small islands to some extent. If nothing else, there's more to look at when you're in the marina....
Bru
Well-Known Member
A better placed antenna not only gives you more range (which isn't always ever so useful) but also some margin in hand, to 'see' around headlands or over small islands to some extent. If nothing else, there's more to look at when you're in the marina....
And lots more people can see you when you're in the marina
BrianH
Well-Known Member
Thank you for your response. I'm interested because my antenna is mounted on the rail only 40cm from a 50mm diameter wind generator mast and I expected either overall attenuation or shielding in that specific direction, which does not seem borne out in practice.Like RIBW, I have had to measure a few antennas in my time.
When you install an aerial in a piece of equipment, it's not uncommon to get big variations of gain in various directions.
6dB, which would halve the range from say 10 to 5NM is not what I'd call a big variation in this context. 20 or 30dB is common, often the limit of the null you can plot is due to the difficulty in measuring it!
It's a compromise you have to make.
But it is important to understand the difference between frequently seeing some ships at 10 miles and reliably seeing all ships.
A better placed antenna not only gives you more range (which isn't always ever so useful) but also some margin in hand, to 'see' around headlands or over small islands to some extent. If nothing else, there's more to look at when you're in the marina....
I put this down to VHF frequencies being generally robust but perhaps sophisticated test equipment would reveal more than my casual empirical observations.
With reference "to 'see' around headlands or over small islands to some extent" ... such as the screenshot below with a different receiver and software - but the same antenna mounted in the identical constellation. A ship traveling the other side of a mountainous island gave a consistent target. Yes, only 2.9nm away at the time of the screenshot but as can be seen by the track, it had been reported much earlier.
RIBW
Well-Known Member
Or possibly 'gain' ? - think of a TV yagi on a house rooftop. The active element sits behind many metal elements to improve the directivity (=gain) of the antenna. The quid pro quo is that it causes nulls off-beam.
As a general comment on this thread:-
To emphasise my point about VHF being partly 'art', the variables in any one point to point link include distance, the sensitivity of the receiver, the transmit power, both Tx & Rx antenna polar diagrams (as influenced by thir surrounding metalwork), the atmospheric pressure (and any ducting), sea state (determines the strength of any multipath reflected from the sea), the height of any intervening land (bear in mind that a hill can be used, with the correct antenna heights at Tx & Rx to improve reception - requires analysis of the fresnel zones and the shape of the hill top) and .....etc, etc.
As lw395 says "It's a compromise you have to make."
My compromise is weighted by a preference for maintainabilty and 'reasonable' AIS reception. This is tempered by my cross-channels 10+ years ago with the old NASA 'radar' AIS and a £2.99 Maplin whip attached - all at deck level in the cockpit. It was perfectly adequate for the purpose of avoiding high-speed crossing commercial traffic.
Each to his own, eh?!
Cheers
Bob
st599
Well-Known Member
A directional splitter will, wouldn't fancy using a cheap resistive/capacitive one.
Whether it's a good idea or not is another discussion.
st599
Well-Known Member
Thank you for your response. I'm interested because my antenna is mounted on the rail only 40cm from a 50mm diameter wind generator mast and I expected either overall attenuation or shielding in that specific direction, which does not seem borne out in practice.
I put this down to VHF frequencies being generally robust but perhaps sophisticated test equipment would reveal more than my casual empirical observations.
With reference "to 'see' around headlands or over small islands to some extent" ... such as the screenshot below with a different receiver and software - but the same antenna mounted in the identical constellation. A ship traveling the other side of a mountainous island gave a consistent target. Yes, only 2.9nm away at the time of the screenshot but as can be seen by the track, it had been reported much earlier.
You probably have what is known as knife-edge diffraction happening along the mountain ridge.
