Tune VHF antenna for AIS TX

[superheat6k]

Don't mess around, fit a 1m VHF antenna, like the mobos use. It'll work fine for AIS and work well as a backup VHF antenna. Or, fit the same type of antenna that's tuned for AIS, will still work OK as a backup VHF antenna. Install it so the PL259 connector will reach the VHF, or install a patch cable, ready in case the worst happens.

This is sound advice IMHO as the 1m Glomex I fitted on my Mobo for the AIS has worked absolutely fine, and far simpler and cheaper than a splitter. OK mine is on the side of the flybridge, but it would easily attach to the pushpin rail of a yacht and wouldn't be too much lower.

My view of AIS is about me being able to see the potential risks early rather than the secondary bonus that a large vessel might see me earlier, as I am sure many have class B sets transmissions blocked anyway.

 

[2copplane]

The masthead antenna I plan to tune when tested on ch16 with an SWR Meter is 1.5:1 which I understand to be very good?
So if my antenna is all good then why is my TX range worse than nearly every boat in the marina? KIV it’s a stop a 16m mast.

Also I have installed on a neighbouring boat and am then appear Appear on Marine Traffic so the unit appear to TX ok.

 

[greeny]

Ok, so a question to the antenna guys out there. If there is excess coax between your set and the antenna (and i mean maybe 15m of it) should you cut it short and remake the connection or leave it coiled up unused under the bulkhead. Will cutting it to correct length improve the performance at all.

 

[lw395]

The trouble when people say this, is that it's impossible to tell if they are sharing their genuine wisdom or their years of self preaching about why you need to pay a pro. Yes if I was paying a pro to install AIS on a VTS building or a major ship bridge, I'd expect they supply a tuned antenna.

On a yacht there are a whole host of compromises. That ant may become my backup VHF ant - how will detuning it for 16 affect it when I am dismasted etc.

4% is mentioned, but I don't know if that is purely the measurement (my calculation was ~ 15mm on ~ 910mm ant so only 1.6%) but what does any of that translate to in real terms as a distance? 3 land miles is ~5000m. 4% is 200m, so if that's the gain to the value I'd question the value of doing it. But of course rule of squares applies etc

Personally I'd say digital yacht splitter adds another point of failure and double the systems relying on that single point. Plus a possible place for losses.

If someone wants to really show the benefits of this can we see some real world data...?

The 4% is the difference between AIS frequency and CH16 frequency. So AIS wavelength will be 4% less than Ch16 wavelength.
There is no direct logic saying the reduction in range will be 4%.
Basically the range will be horizon plus a bit. The 'bit' increases slowly as you increase power or efficiency or antenna gain.
So maybe a mediocre antenna with 1 watt and 50% efficiency gets you 5 miles for given antenna heights, but going to 5 watts only gets you an extra couple of miles. To only get 3 miles things have to be quite sick.
The real life range gains between a basically sound antenna and an excellent antenna are generally small.
If you are serious about range, then height is the key for VHF.
All assuming there are no obstructions. Scattering of signals around say a marina is either lots of hard sums, or 'suck it and see'.

 

[ShinyShoe]

The 4% is the difference between AIS frequency and CH16 frequency.

...

The 'bit' increases slowly as you increase power or efficiency or antenna gain.
So maybe a mediocre antenna with 1 watt and 50% efficiency gets you 5 miles for given antenna heights, but going to 5 watts only gets you an extra couple of miles. To only get 3 miles things have to be quite sick.

which is what the OP is reporting, although... Handheld to handheld is often 2-3miles max. But that's low power and low height.

If you are serious about range, then height is the key for VHF.

absolutely..

This is sound advice IMHO as the 1m Glomex I fitted on my Mobo for the AIS has worked absolutely fine, and far simpler and cheaper than a splitter. OK mine is on the side of the flybridge, but it would easily attach to the pushpin rail of a yacht and wouldn't be too much lower.

1m on the rail? What's the boom clearance like? Do mobo ants not transmit more horizontally - how does heel on the boat affect it?

The masthead antenna I plan to tune when tested on ch16 with an SWR Meter is 1.5:1 which I understand to be very good?
So if my antenna is all good then why is my TX range worse than nearly every boat in the marina? KIV it’s a stop a 16m mast.

Also I have installed on a neighbouring boat and am then appear Appear on Marine Traffic so the unit appear to TX ok.

So let's be exactly clear what you have tried? Same AIS unit connected to:

Your 1.5:1 16m masthead*
Your 3:1 pushpit rubber duck
Your mates ??m masthead

And the two mastheads don't compare? How much higher is your mates? Is it AIS tuned?

How are you assessing reception?

How are you powering the AIS? Wouldn't be the first time we've heard of a VHF that was starved of amps and couldn't output the power.

What range are you getting and what range do you want...

Ok, so a question to the antenna guys out there. If there is excess coax between your set and the antenna (and i mean maybe 15m of it) should you cut it short and remake the connection or leave it coiled up unused under the bulkhead. Will cutting it to correct length improve the performance at all.

15m... Remove it you will sail faster! Just from the weight!!

In reality it depends on the exact cable and if you are replacing the plug like for like. 15m is A LOT... Is there a reason for so much?

 

[greeny]

15m... Remove it you will sail faster! Just from the weight!!

In reality it depends on the exact cable and if you are replacing the plug like for like. 15m is A LOT... Is there a reason for so much?[/QUOTE]

I "inherited" the antennae second hand from a donor and it came with circa 20m of coax included. No other reason. The antenna now lives on the pushpit so 5 m is probably all i actually need.

 

[TSB240]

My Matsutec AIS transponder doesn’t seem to have a great TX range. I’ve a stern stubby antenna and a Masthead whip one. I’ve tests the antennas with a SWR meter and the stern was 3 and the masthead 1.5, however this was using the vhf radio for the TX, not the AIS.

I’ve tried my transponder on a mates boat next door and it was being seen on marine traffic. I’m picking up boats 40 mile always so receive works great.

My thoughts are the old vhf antenna isn’t tuned for AIS so the 2watt transmission is degraded? Short of replacing the aerial could I do anything to tweak it a bit for AIS, I’m thinking shorten by a few mm or lengthen? If it doesn’t work I can fit a new one.

I am not a ham......
I have had the exact experience that you have with a dedicated stern rail mounted Stubby AIS aerial on a friends boat. This was on a new version of a Matsutec . On a cruise together I lost sight of his transmission if we were more than two miles apart. I use a dedicated Metz Ais mounted at about 2m above the stern rail on trailing arm from my Radar mount. I was picking other rail mounted boats with AIS at over 8 miles. I had a spare standard Metz vhf that I loaned him and we made a clamp to fit it to his back stay at about 2 m above the stern rail. Massive improvement to 10 miles. I would suggest you bin that cr*p stubby aerial and get yourself a Metz. Use a Dedicated AIS as even when that is used in emergency vhf role with 25 watts it's going to have a better range than a hand held. The cable supplied preconnected with the stubby aerial was absolute rubbish. He would probably have transmitted better using a piece of wet string! Steve.

 

[G12]

Don't mess around, fit a 1m VHF antenna, like the mobos use.

Motorboat antennas are great on motorboats - These antennas exhibit much less of an isotropic radiation pattern and have superior performance when vertical..... The problem arises on a heeled yacht which makes the antenna far from vertical. Still pretty good for RX but on TX it'll be firing more of your RF out to the sky and down to the sea. The longer the antenna the greater this effect (put simply). I have a VHF antenna on my house that is about 5 metres tall, it exhibits considerable gain (8.3dBd) but it'd be useless on anything other than a building as the RF is emitted in a pancake fashion. It does mean I can Use a repeater 30 miles away using only half a watt though (my house is also on a hill which helps).

To be fair a 1m GRP antenna isn't going to suffer *too* badly with this but it is still something to consider. As far as I remember it is even mentioned in the syllabus for the GMDSS GOC.

 

[Allan]

Can I pick up on a couple of points mentioned here? How do you decide what is good quality co-ax? A couple of centuries ago I used to install TV aerials and the reels we used were marked low loss but I never saw any specification. (They were always referred to as Joe Loss, which dates me!). Someone above mentioned fitting two aerials, "suitably spaced". How does one decide on a distance between them?
Allan

 

[lw395]

Motorboat antennas are great on motorboats - These antennas exhibit much less of an isotropic radiation pattern and have superior performance when vertical..... The problem arises on a heeled yacht which makes the antenna far from vertical. Still pretty good for RX but on TX it'll be firing more of your RF out to the sky and down to the sea.....

The antennas in question are reciprocal devices and exhibit the same gain/pattern in receive as transmit.
High gain antennas are a bad idea as you say because of sailing boats heeling, but also their tendency to interact with metal rigging wires.

 

[rszemeti]

That's not actually correct. A dedicated AIS antenna will be tuned for 162MHz and not 156MHz and therefore will offer much better performance in terms of TX, especially where only 2W is being applied and as much of that needs to be radiated as possible. I would also add that it is very important to use the best quality coaxial cable you can. This will affect both TX and RX.

The point about "much better" performance is ... dubious. It is very difficult to detect the difference between a 2:1 SWR and a 1.2:1 SWR in practice ... it makes suprisingly little difference when all is said and done. It's also worth noting that with 3dB or more of loss on the RG58 going up a typical mast, even with a broken aerial, you will never see worse than a 3:1 SWR on a meter at the bottom, as the losses in the cable mask the true nature of the real problem at the masthead.

I would however agree that decent quality cable makes a big difference. My masthead aerial is wired in RG213U instead of the usual RG58 horror, and it makes a useful imorovement of around 2dB. I was easily able to hear Holyhead and Milford Haven coast guard from the Irish coast, 5+7 or better.

 

[TSB240]

Can I pick up on a couple of points mentioned here? How do you decide what is good quality co-ax? A couple of centuries ago I used to install TV aerials and the reels we used were marked low loss but I never saw any specification. (They were always referred to as Joe Loss, which dates me!). Someone above mentioned fitting two aerials, "suitably spaced". How does one decide on a distance between them?
Allan

I always use RG8X coax for all marine VHF applications. RG58 is commonly supplied with many preconnected antennas. I cant quote the comparable loss figures but a quick Google will tell you. Salty John's web site is also a mine of useful information. He is always there on the end of the phone should you want free expert advice.

 

[JumbleDuck]

The point about "much better" performance is ... dubious.

I am still hoping that G12 will quantify his "much better performance" claim in dB.

 

[lw395]

I am still hoping that G12 will quantify his "much better performance" claim in dB.

Bear in mind that the last AIS I fiddled with had a sensitivity of -110dBm.
The transmit power of 2W is +33dBm.
So, total path loss is up to 143dB.

People not involved in the dark art see a 3dB cable loss and think OMG that's half the power!
Whereas to us, it's 2% of the dynamic range.
In a good system design, you want several bits of 3dB you can 'find' when something steals all the margins.

And sometimes, for reasons I won't go into, some radios work better with a bit of loss between antenna and front end.

 

[superheat6k]

When I tip the aerial down to pass under the bridge at high water and then forget to tip it back up again, I still see loads of AIS targets, and presume they can see me - and after the typical trip my history on Marine Traffic is all shown, so I guess my transmissions have bene received.

 

[G12]

High gain antennas are a bad idea as you say because of sailing boats heeling, but also their tendency to interact with metal rigging wires.

Interesting thought, that's not something I'd considered for a masthead antenna but thinking about it it is entirely possible.

 

[G12]

I am still hoping that G12 will quantify his "much better performance" claim in dB.

Check post 14 right after you asked the question in post 13.

 

[JumbleDuck]

Check post 14 right after you asked the question in post 13.

Post 14 is by someone else and answers a completely different question. So I'd still like to know what - in dB - you meant by "A dedicated AIS antenna will be tuned for 162MHz and not 156MHz and therefore will offer much better performance in terms of TX"

 

[PaulRainbow]

When I tip the aerial down to pass under the bridge at high water and then forget to tip it back up again, I still see loads of AIS targets, and presume they can see me - and after the typical trip my history on Marine Traffic is all shown, so I guess my transmissions have bene received.

Had the same thing when i had mobos, the "issue" of GRP type antennas being less than vertical is vastly overstated in my experience.

A 1m antenna at the pushpit is hardly going to be on its ear with some heeling. Anyone would think mobos don't heel or pitch.

 

[G12]

Post 14 is by someone else and answers a completely different question. So I'd still like to know what - in dB - you meant by "A dedicated AIS antenna will be tuned for 162MHz and not 156MHz and therefore will offer much better performance in terms of TX"

That's exactly what he's answered by using the difference in the two VSWR's to calculate the loss in TX between the two different antennas. 0.8dB in that particular instance wasn' it? That's about 27%. (I haven't checked the maths but it sounds about right) Every single antenna installation is different so the numbers will vary on every installation but if you're running an antenna that isn't properly resonant then the system will be inefficient. I can't just turn around and say to you "you will gain X amount by doing this" but I can guarantee you that a properly tuned resonant antenna is a more efficient radiator than something that is running at the edge of it's bandwidth (where a chunk of power isn't being radiated) so you will gain something useful. If you won't take it from me for some reason then I'm sure there will be plenty of others who will confirm that.

By the way, I do deal with this stuff as part of my day job as I mentioned before but I don't work with yachts and I have nothing to sell or gain by stating this information. I'm just trying to say that there's a better way of doing things than the typical lazy installation and there are performance rewards to be had by using a proper antenna and proper cable.
Suggested further reading if you're really that interested, and yes I do own a copy.
https://www.amazon.co.uk/ARRL-Antenna-Radio-Communications-Softcover/dp/1625950446