AIS Aeriel Question?

[Trident 24]

In the meantime...

I'm sure a proper sparky will be along before too long, until they do here's my opinion:

The 'frequency range' of an aerial is usually a normal distribution or bell curve, the published figures are normally taken to be the half power points above & below the design frequency.

A narrow frequency range indicates a 'peaky' curve which implies a very efficient aerial that rapidly drops off each side. A wider range implies a broader (and lower) curve that sacrifices efficiency at the design frequency to gain similar efficiencies at most frequencies.

As the frequency in use moves away from the peak there is an increasing risk of damage to the radio, to prevent this the power of the output is designed to drop off.

The same is true for SSB radios..and as there is no way you will get an efficient aerial on board, you will have an antenna tuner to 'con' the radio into providing it's full power into whatever dodgy wire you have.

It is quite easy to transmit out to the horizon with only a watt or so of power, the only purpose of extra power is to 'punch through' interference & competing signals, so the reduction of power mentioned above isn't a major problem.

If I were choosing between 2 aerials I would take a broadband one over a narrowband one...but I wouldn't lose any sleep over it if I were not offered the choice. Siting the aerials will have a much greater impact than their design.

I have a whip at the mast & one on the pushpit, both have their leads ending at the radio by the chart table so I can connect to either, I have an adaptor to allow my handheld to connect to either. I can't remember the brand or design spec for either.

I can think of one situation where a dedicated AIS aerial would be useful; if high performance were compulsory in an area of high interference.
The peaky curve at the AIS frequency would allow maximum power out and would enhance the reception on that frequency by reducing the received power of adjacent signals.

 

[whipper_snapper]

Oh well since YOU guarantee it that's fine then...

Thanks

But seriously. I have transmitted and received vhf over the horizon many many times with aerials with FAR greater difference between the optimal and actual frequency. This includes aerials of different design. I have transmitted throughout the amateur 2m band on a marine vhf aerial and vice versa.

the difference between ch16 and ais is absolutely negligible, there is a tiny difference in transmitted power and no difference in range achieved.

It'll work

 

[pappaecho]

Instead of wasting a lot of beer token on an expensive aerial, I would do a test with a cheapo one to see what effectm when on the pushpit.
The AIS system which you have needs to be tested to see how far it can "see" ships. Within limits, the higher the aerial the further it will be able to see.
If for example you decide than you are happy with say a 10 mile limit, then if the pushpit gives you enough range then go for it.
If, on te other hand your boat does 30 knots, then 10 miles may not be enough!

 

[xeitosaphil]

AIS Aerial Question

Thanks guy’s for all the input; it seems a lively debate and one which doesn’t seem to have any definite conclusions, which I am very surprised at?
With all the interest and advertising associated with AIS, I would have thought that Companies advertising aerials for AIS, as being backups for VHF Radio transmissions, that there would have been documented evidence and performance figures to support their claims? It would appear that, it maybe no more than an advertising hype, just to enhance their products, to corner a prospective purchaser.

From those who seem to know far more than myself, it would appear that I need a dedicated AIS aerial to maximise possible reception and range of targets, and a separate back up VHF aerial, although the pushpit mounted VHF aerial will have reduced range due to its height, but perhaps just enough hopefully, should ever the need arise to use it?

I think, as Luystd hinted at, that this is a subject that should be taken up by YM or PBO in which to find some definite results, as there must be a lot of people with misconceptions as to their equipments capabilities?

 

[fireball]

Thanks

But seriously. I have transmitted and received vhf over the horizon many many times with aerials with FAR greater difference between the optimal and actual frequency. This includes aerials of different design. I have transmitted throughout the amateur 2m band on a maine vhf aerial and vice versa.

the difference between ch16 and ais is absolutely negligible, there is a tiny difference in transmitted power and no difference in range achieved.

It'll work

Well that makes at least 2 of us with our heads screwed on the right way. I didn't get a tuned ais antenna because it was more expensive and the difference in reception would be negligible. If I wanted greater receiving range I would move an antenna to up the mast but then I'd need another for emergency.
I wouldn't loose any sleep on whatever antenna I had on the stern providin it was giving an acceptable range for its primary purpose.

 

[lenseman]

Any whip aerial will work over a wide range of frequencies but the 'tuned' length is 'technically correct' for only one frequency since the wavelength is derived from the frequency.

However the drop in efficiency for frequencies either side of the tuned one is small otherwise no one aerial would cover the whole marine vhf band. Ditto your TV aerial etc etc

AIS specific aerials are a marketing ploy* - a standard vhf one will work just as well in practice and can double as a backup.

* the extra range if any of an AIS specific one is harldy relevant - what difference does it make if it picks up vessels from an extra few miles distant? . . . . . .

Ian, you have to be so careful on this thread talking like that!

You will have forum members who are much younger than your good self, possibly still wearing short trousers (because they still sail dinghies and get wet), they will come along soon and tell you that you obviously have no idea what you are talking about!

They will, of course, probably have an Honours Degree or a 2:1 in Radio Frequency Engineering and RF Propagation and know all about Intermodulation Products (2f¹±f²), Tuned Lengths, Third Harmonic Distortion, Wave-Guide Beyond Cut-Off, Resonant Frequency and Free Space Path Loss?

They might even hold a LRC or VHF marine licence and a ham radio licence to boot, heaven forbid but the still talk utter drivel!

Ho-Hum.

Be very careful, even people living on the East Africa coast and others have been put in their place. You have been warned!

 

[Playtime]

From those who seem to know far more than myself, it would appear that I need a dedicated AIS aerial to maximise possible reception and range of targets, and a separate back up VHF aerial, although the pushpit mounted VHF aerial will have reduced range due to its height, but perhaps just enough hopefully, should ever the need arise to use it?

Get a good quality broadband VHF aerial on the pushpit for AIS. It will be perfectly adequate with 12-15 miles reception on AIS and will give better performance as a DSC VHF back-up in extremis. You don't need AIS and back-up VHF antennas.

 

[Sandyman]

I have recently fitted a DY AIS receiver with the DIGITAL YACHT AA10 aerial on pushpit.
(Linked to a C70)

I think the results so far are quite acceptable, picking up contacts at least 15 miles away.

Similar set up. Standard VHF aerial atop the mizzen. Contacts far in excess of 15 Nm.
Good to have CPA's at that range. Plenty of time to work out a course of action to avoid a collision

Hey Lustyd, I guarantee it as well.

David. Me thinka you isa takin da peish. Wouldn't expect that from you

 

[lenseman]

. . . . David. Me thinka you isa takin da peish. Wouldn't expect that from you

Nah! I wouldn't do that. You will get me talking about Sporadic E and Transequatorial Propagation (TEP) next.

Mind you, I do know about a piece of wet string. I hear the work just as well as an AIS antenna.

 

[Hostage]

Is it not the case that in receive mode ariels are fairly tolerant whereas in transmit mode they need to be tuned.

If that is the case then I would lean towards a VHF radio ariel which in any case covers up to 161 mhz and suffer a slight loss on the AIS receive side.

But then I am not a techie!

 

[Salty John]

Marine VHF antennae

The following comments on marine VHF antennas (aerials) are intended to be informative and not self serving. I hope most of you will accept that. The information is fairly basic so I apologise to the RF techies on the forum, I trust they’ll forgive me for oversimplifying some issues in the interests of clarity.

We are talking here about marine VHF whip antennas – the ones with the can at the bottom and a whip of about 1m in length. These antennas are the most popular in the leisure boating world because their radiation pattern is sufficiently broad that on an unstable base such as a boat there is always at least some part of the pattern pointing at the horizon.

If you understand ‘gain’ or don’t care about it, skip this section! These antennas are said to have a 3dBi gain. The little i stands for isotropic because the ‘gain’ of the antenna is 3dB compared to a theoretical antenna called an isotropic radiator. This method of ‘rating’ antenna gain is the one most prevalent in the marine industry. In other industries this antenna would be a ‘unity gain’ or ‘0 gain’ antenna. Most 1m whips are 3dBi even though the i is sometimes missed off and sometimes canny retailers call them 6dB gain antennas.

Those 2m, 5m and 9m fibreglass antennas that look like fishing rods have higher ‘gain’ by focusing the radiation pattern – instead of the pattern looking like a football it looks like a doughnut and, as the gain increases, like an LP record. As the radiation pattern becomes more focused so the tendency increases for it to point at the sea or the sky as the boat rolls – so you have higher gain but it often isn’t pointed where you want it, at the horizon.

By the way, the antennas we are talking about are omnidirectional antennas - the radiation pattern is the same all round the antenna. An example of a unidirectional antenna would be a satellite dish where the signal is gathered from one direction.

Ok, enough about gain. Marine VHF antennas should have a frequency range from 156 MHz to 163 MHz. That covers all the frequencies we are interested in.

But, the important thing is that the performance at all parts of that range, end to end, should be within the specified limits. By this I mean that the specified SWR (don’t worry about it, think of it as a measure of signal quality) applies over the full range of frequencies. It’s no good if the SWR is a very good 1.2:1 at the centre of the range but rapidly falls off to an unacceptable 2.5:1 at the edges. If that happened the performance on AIS frequencies, 162 MHz, would be unacceptable.

All good quality VHF antennas have acceptable performance over the full frequency range and that is why they are perfectly acceptable for AIS use. It is why we recommend you use a normal VHF antenna for your AIS system particularly if you might also want it to serve at some time as your radio antenna.

Now, someone said that the introduction of AIS antennas is a marketing ploy. I might disagree with the term ‘ploy’ and what that implies, but I do agree that the AIS antenna was introduced to fill a largely imagined, or created, need. But, those that only want an antenna for AIS purposes would probably want one that was ‘optimised’ for AIS use should such an antenna be available. So perhaps ‘ploy’ is harsh.

The problem is, does that antenna continue to have acceptable performance at the radio frequencies, or is it compromised by being tuned to 162 MHz rather than to the emergency channel, 156.8 MHz? You need to seek assurances from the manufacturer that radio performance will not be unacceptable or you should not rely on this AIS antenna as a back up for the radio antenna.

So, use a good quality VHF antenna for AIS and it will give good performance across the marine frequencies. If you want an antenna only for AIS use, by all means get an AIS antenna and you will get a slightly better quality signal at AIS frequencies.

VHF range is based on the height of both the transmitting antenna and the receiving antenna. Because AIS signals are usually received from high up on large ships the range is surprisingly good even if your antenna is on the yachts rail – 12 to 15 miles is typical and that is quite enough for AIS. Radio communication between two rail mounted antennas is never going to be much more than 5 – 7 miles, hence the use of masthead mounting where 25 miles is not unreasonable.

Sometimes, when atmospheric conditions are conducive, VHF ranges of hundreds of miles have been reported, but this can’t be relied upon.

Other factors to consider when choosing a VHF antenna would be build quality, (cheap antennas have internal coils and other components that can distort when they heat up, changing the frequency unhelpfully), UV resistance, resistance to bird strikes and cable quality (5mm RG58 cable is unacceptable for all but the very shortest runs but is factory crimped to some cheap quality antennas).

I hope I’ve helped to shine a little light on this subject.

 

[Chalker]

Salty John understands the issues very well.
I suspect we share similar professionl radio backgrounds.
Personally I am using a normal Marine VHF antenna for my AIS Transponder - because I had one available.
John

 

[Sandyman]

In a 'nut shell' Salty John, and if I may say so very well explained. BZ

David how about you explaining to the good peeps on here what SWR means

 

[BlueChip]

The other antenna I tried was a Heliflex Stub antenna. this didn't produce good results at all for AIS. We have the same one on the club rib and the range is limited - although this may be to do with relative heights - I have used my vessel (with 60' mast) as a relay on more than one occasion despite being further away from the Rib than the base station.

.

I also got poor results with a stub antenna on the pushpit even when raised 1m on a pole.

 

[lustyd]

Now, someone said that the introduction of AIS antennas is a marketing ploy. I might disagree with the term ‘ploy’ and what that implies, but I do agree that the AIS antenna was introduced to fill a largely imagined, or created, need. But, those that only want an antenna for AIS purposes would probably want one that was ‘optimised’ for AIS use should such an antenna be available. So perhaps ‘ploy’ is harsh.

Thanks Salty John, very well explained, although it still leaves us with the conclusion that they can't be trusted without an assurance from the manufacturer.
I thought the AIS specific ones were also designed to be smaller than the main VHF ones? In my mind that was the main driver, to get a small aerial which isn't in the way on a pushpit.

 

[lustyd]

In a 'nut shell' Salty John, and if I may say so very well explained. BZ

David how about you explaining to the good peeps on here what SWR means

I presume this is it...?
http://en.wikipedia.org/wiki/Standing_wave_ratio

 

[Sandyman]

I also got poor results with a stub antenna on the pushpit even when raised 1m on a pole.

I replaced one of these stubby aerials which was sitted on my mizzen spreader with a standard whip because the stubby went U/S.
Wasn't until I replaced it that I came to see just how many close range targets were not picked up using the stubby. In my case it was something like a 10 fold increase of targets I had previously not seen, which under some circumstances, I may have wished to be able to see.
I have been suspect over the use of these stubby aerials for some time & wonder how many other users out there get poor results without realising it ?

Just a thought, could this be another reason why some peeps have little faith in the system ?

 

[Sandyman]

I presume this is it...?
http://en.wikipedia.org/wiki/Standing_wave_ratio

Your presumption is correct. Still think David should describe SWR in more simpler terms,
what it means & what it indicates to the user

Dont be shy David

 

[Salty John]

I replaced one of these stubby aerials which was sitted on my mizzen spreader with a standard whip because the stubby went U/S.
Wasn't until I replaced it that I came to see just how many close range targets were not picked up using the stubby. In my case it was something like a 10 fold increase of targets I had previously not seen, which under some circumstances, I may have wished to be able to see.
I have been suspect over the use of these stubby aerials for some time & wonder how many other users out there get poor results without realising it ?

Just a thought, could this be another reason why some peeps have little faith in the system ?

Those stubby aerials typically have a 0 DBi gain compared to the 3 DBi of the 'standard' whip you replaced it with. Stubby helical aerials seem to be aimed at the RIB or small power boat market where there may be a lower expectation of long distance performance.

 

[xeitosaphil]

AIS Aerial Questions

Thanks Salty John for a very in-depth explanation on the aerial question, and to all the other contributors on this thread, it has been very interesting and useful.
I have made the decision to mount a Standard VHF aerial probably a Metz Manta on the stern pushpit next to the GPS aerial. Others have said that they get between 10 and 15miles range, so I may get same? which really, is all I was hoping for and would be very happy with.
It will be reassuring to know, I have the definite back up of a proper VHF transmitting aerial should the need arise to use it, even if it is of limited range?