VHF areils

[Island Time 1,214]

Here is a really basic diagram (sorry about my lack of artistic ability) that shows the problem high gain;

 

 

As you can see, when heeled, the transmission power goes into the sea and into the sky. Lower Gain works better when heeled.

 

If you need long range, VHF is no good anyway - Use HF, Sat Ph or EPIRB!

[Guest]

What an excellent drawing for us of little knowledge.

 

So why knot go for a 0db areil?

[Island Time 1,214]

The diagram is overstated to display what we are talking about better. A 3dbi gain antenna at the masthead will be best range and best angles. Odbi would limit your range unnecessarily...

[Guest]

Good on that.

[idlerboat 116]

Nice stuff guys...

Directional versus all weather....particulaly on a mast head mount and adverse weather (hard and constant heel).

I would like you to add about close steep terain and thunderstorms. Most vessels get into sh*t close to shore.

A simple high headland can kill vhf very quickly to a base station.

Needing (if they are there) a ship to ship relay back.

Comms with other vessels is a good thing.

I have never got the "independance" idea , where you dont stay in touch with other vessels. Including commercial ones.

(They are the best from my expirence).

 

 

121... that is true.

the second fx that aircraft traveling over your distress situation use to find you.

That is all commercial aircraft as well.

Remember that aircraft (as all you fly boys know) rely on these fx as well for their safety.

Just remember that in a true emergency situation you can use all means.....Do not treat this lightly. You will go to jail if you play around....

[wheels 543]

Sorry but that diagram is terribly inaccurate. It's a great diagram and good with describing the differences with gain and dispersion. But the patterns are way to critical and they do not "terminate" their coverage pattern like that. A 6dB antennnae will still give a good "throw" to Horizon while on a heel. I am not saying a Whip should not be used however. With a greater chance of anything being caught and broken or whipped off the rail, a Whip is probably going to be the most rugged as against a large fibreglass rod that has mount that can be broken. Ours was.

[too_tall 15]

Sorry but that diagram is terribly inaccurate.

 

 

IT noted this was very basic - and just to give an idea of what he is trying to explain. Which I thought it actually does quite well.

 

KM, I would expect that a 3dB on the stern would be suitable for AIS - giving that it would be say, 2 m above the waterline, and a commercial ship would have their antenna probably a minimum of 10 meters ( depending on size ) Without actually bothering to do the calculations, I would assume a range in average conditions exceeding 10 nautical miles from that to an elevated base station. 1/2 to someone standing with their handheld 3 meters above the water ( most handhelds work over that kind of range with a little elevation in my experience but they do vary markedly ). A small 3dB whip on the stern is going to be more robust than a bit fibreglass whip will ever be, whilst keeping the whip antenna stored with your fishing rods for emergency use. I know that when fishing we always leave the handheld set to a local fishing chatterbox fq, and we can be at Mayor Is and occasionally still be hearing people discussing things at they approach the marinas, 20nm or so away.

 

However, if safety is the prime concern, VHF really is not the way I would like to go. As noted before, Satphone, epirb, 121.5 AB and a handheld for when rescuers get close are all more likely to be of use if your really in trouble than a VHF with an antenna on the stern. Why are you in trouble? More likely than not, weather. You will be spending maybe 1/2 your time _under_ the wave peaks, where VHF is going to be damn near useless. If your boat is still in one piece enough to use said VHF that is. If the boat is in one piece, and close enough to land or another boat, then VHF might be your first choice. But if your really in the poo, the EPIRB should probably be your first thought.

 

A little OT, but when cruising, we always have an epirb and a handheld clipped to anyone who is on deck, as well as a good torch.... That VHF is quite useful should someone go for a midnight swim. Safety first over anything else.

[too_tall 15]

With a greater chance of anything being caught and broken or whipped off the rail, a Whip is probably going to be the most rugged as against a large fibreglass rod that has mount that can be broken. Ours was.

 

We have to replace the fibreglass antenna on our fishing boat probably every second year - they just are not that tough if things get caught on them - such as a line with something big on the end, or the road cover. I cant see the rear of a yacht being that much better!. You can get stainless folding mounts for them, which I have thought about. But then I suspect the antenna will just snap.

[Island Time 1,214]

Yep, as usual you are right Wheels! My diagram is technically inaccurate! It was supposed to illustrate the problem, not be a definitive technical description of the issue. It is considerably simplified for clarity. I apologize if I did not make this clear enough!

 

A 3DBI gain Stainless whip on the stern rail would also be fine - but a bit less range (when upright) than a 6dbi one. I found that, with the old 3dbi Stainless whip on the Stb rail aft, as an AIS antenna, I'd get subtantially less range (down to about 2 miles sometimes) from the AIS when on Port tack, as the rail was almost at sea level. On STB tack it was much higer, and I once got over 50nm!

 

I changed that antenna for a 2.5m 6dbi glass one, due to the AIS playing up, (not receiving anything) but it turns out it was not the antenna! Saving for a new AIS now

[Guest]

There is a theme of jumping from heavily technical to simple speak going on

It's good in a way as the simple sets the scene and then the techo expands on that... or at least the amount of techo I understand, which is a shite load more after reading the simple that it would have been without that. I think I'm getting a good picture here.

 

Pacific Areils have this in there book when I went for a suss.

AIS is a marine tracking system used for identifying and locating vessels by electronically

exchanging data with nearby ships. The AIS unit uses a standardised transceiver to

broadcast the vessel’s position and name as well as speed, course and destination.

The frequencies used to transmit this data are higher than marine VHF, so using a standard

marine VHF antenna will compromise the performance of the AIS system as will using a

splitter to run your AIS and VHF from the same antenna.

We recommend that you use a dedicated AIS antenna to ensure the best possible signal

reception and transmission to maximise your safety.

 

Any comments on that? No panic if there isn't.

[Island Time 1,214]

Yep, they are right, of course. The AIS freq/s are a little diff to normal VHF. So an antenna tuned specifically for the AIS is a bit better, but a normal VHF one will work for AIS, and Vice versa...

 

So, if you want to have the best possible setup, the 3dbi gain SS one on the masthead, and a separate

AIS ss one on the Transom. It will prob be a 3dbi gain SS one as well, just tuned for the diff freq/s

[wheels 543]

All I am trying to say, is that it is being implied by both words and the Diagram, that 6dB and even 9dB antenna's will not work when a yacht is heeled and that is simply not true. But hey, I'll just butt out then.

[Island Time 1,214]

Perhaps it is me saying too much Wheels. You understand this better than I do! I'm not a radio engineer. Everything you have said is correct. Please don't let me put you off, you are a valuable asset to this site..

Matt

[RedLine 0]

Hi KM

 

Am also thinking of Adding AIS, and as its at a different freq than VHF will add a simple rail mounted Helical Antenna for AIS, then feed this into the Plotter to get AIS on the Chart Plotter. Downside is there is limited range on Rail Mounted, upside is there is a spare emergency aerial you can use for the VHF if you lose the rig. What gear are you planning to run the AIS into??

[Guest]

G'day Rdeline, I wasn't going to but I think AIS just makes me more visible to big bits of steel so I'm pretty sure I will. So just a transmitter or maybe a small plotter with it built in if that's a cheaper option. A mate got a small simple no name plotter that had it in and it worked fine. I have no real need to see out as if I hit a container ship it would probably never notice but if it happens the other way around I'll be fish food, hence maybe just the transmitter. That 'plotter or knot' side of the lectrics is still a work in progress hence the sus now to give myself more time to ponder.

 

What's a Helical Antenna?

 

I'll suss that small plotter I mentioned. He got it from Korea and while still a no name it came from a factory that makes names. It was basic but the built in K Map was OK and quite usable. It also took C Map. It cost him something like $850 delivered to Whangarei so hell cheap. I've seen AIS transmitters at 1200 so the plotter could be the cheaper option and have some extra bonus at the same time. I think it was branded ONWA, I'll check. I think IT knows the name so may have more of an idea on goodness or knot, I've ever only played with one unit.

[Island Time 1,214]

KM & Redline, I'd suggest for low power Plotter and AIS a low power Netbook - Trade me has them under $300.00. Load Linux and OpenCPN, load the NZ Charts, All free. With the right power setup, you can get it down to about 10w - like a single bulb. It can be connected to a speaker, and provide a loud alarm if a vessel approaches with AIS!

Cost does NOT include the AIS Transponder!

[rigger 47]

Am also thinking of Adding AIS, as its at a different freq than VHF

regarding AIS

 

Did you know that you can get a VHF radio to receive AIS signal and input it into a computer and decipher the data / signal.

 

AIS uses the high side of the duplex from two VHF radio "channels" 87B and 88B.

 

The ships I have been involved in as an owners rep used the exact same spec antennas for the VHF radios and the AIS units. Range on the VHFs was good and the AIS unit was picking up targets at +80nm in open ocean. HoA ~25m.

 

You may be able to get antenna that is specific for AIS but it if these people do not specify a special VHF antenna you may not need to - http://www.km.kongsberg.com/ks/web/nokb ... penElement

[idlerboat 116]

Further to what rigger has said, and if you dont feel confident in playing with your existing vhf radio........

As rigger has stated, the AIS frequency is part of the standard range on a vhf radio.

Sure you could tune an ariel closer to those frequencies but in the scale of signal reception it is simply not needed.

An AIS alert will go off long before you see the vessel.

IF the ariel is at the masthead.

A specialy tuned ariel at a low height defeats the purpose.

If you want the second ariel for redunency then have two broad range units so if you lose the mast head you still have the best possible vhf transmission.

 

BAUD RATE AND AIS

read this before you buy an Ais unit !

Standard Horizon make a good quality vhf that is full dsc plus ais input. There are two versions. The later one allows for single outputs.

Ais by the fact that it was a comercial protocol runs a differant baud rate than the normal outputs usualy used by marine instruments.

To put it simply if you are injecting the ais into a chart plotter for overlays, make sure that your chart plotter has multiple input capability and handles two differant baud rates on the differant channels.

Sometimes called "high speed and low speed" baud rates in marine protocols.

Otherwise you will need a multiplexer to combine the various inputs to one baud rate output.

(A NZ company makes one of the best on the market).

This is a very common problem when adding AIS to older existing systems. (with multiple NMEA inputs).

 

 

The gx 2150 radio sorts the baud rate problem in the unit.

 

disclamer....i dont have shares/ or even own any standard horizon products......I do however have a lot of experience in the instalation of marine electronics.

 

The latest products (new and expensive full bells and whistles chart plotters) have largely sorted this problem out.

[idlerboat 116]

Sorry...because of the less than helpful edit time out rate this has to be a second post.

 

 

as a ps....I would always recomend a good radar instalation and the knowlege and practice to use it before AIS.

It promotes far better seamanship and collision avoidence, will "see" far more targets (Including them that aint boats !)

...and can still do most of the tracking features (using MARPA) or similar of an AIS unit.

 

Finaly...I promise you that you will turn the AIS proximity alarm off when you are anywhere near a busy port !.....

[rigger 47]

Finaly...I promise you that you will turn the AIS proximity alarm off when you are anywhere near a busy port !.....

 

First thing most users learn.... how to silence the 'annoying' alarms....