Tune VHF antenna for AIS TX

[2copplane]

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.

 

[pvb]

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.

AIS uses frequencies within the same range as VHF communications, so a standard VHF marine antenna will work fine for AIS purposes.

 

[ShinyShoe]

A standard VHF marine antenna (dipole) should be tuned for CH16 as that is the most critical channel. That means it should be 90.97cm in total length. (if yours is shorter than that its not a simple dipole but bear with me - it doesn't matter the point will be the same... its the maths that differs)

AIS uses two frequencies, you don't control that, it just does. So which you tune a dedicated ant to would be hard to decide. But 88.1cm should be roughly right. SO *IN THEORY* shortening your antenna may make an improvement. But its going to be a marginal gain ('scuse the pun!). If you do have a dipole you'd need to reduce *each* end by ~ 1.45cm. Like Pete - I doubt thats going to make much difference. The fact you can receive shows you don't have a substantial issue. (if it does matter then using that same antenna as an emergency VHF will be less effective).

Your transmit as you say is 2W. At deck level - that will realistically be what you can see and nothing more (Handhelds do 5/6W and realistically you struggle to reach anything you can't actually see beyond maybe 1 mile away). If you want range - increase the height of the ant. Using the mast head I'd expect you to be reaching 5miles.

You say your mate is on Marine Traffic. I assume you are not. Did you check where MT was picking up his signal? AND WHEN. If you click on the symbol on the map so that the pop-up box appears it tells you both how long ago (e.g. 3 minutes) AND which mast picked it up. Click the link to the mast it will tell you where it is, what it normally covers etc. You can only really compare if BOTH you and your mate are switch on, left to run 15 minutes or so, and then and only then one of you can be seen in that 15 minute interval and the other can not.

 

[MikeCC]

The setup software or diagnostics screen should show AIS SWR.
Not worth messing around tuning antenna. Stern SWR sounds flaky - check connections both ends and, if a twist-on type, ensure centre core is soldered to pin.

 

[Yngmar]

I've done the antenna "tuning" bit with a standard whip VHF antenna. Did the math for how many mm to shorten it to bring it from the original 156 MHz (or was it 156.8? Can't remember, but it was printed on the box) to 162 MHz for AIS. Then pried the cap off the tip, cut the calculated length off (I think it was around 12 or 15 mm or something - please do your own math, this was a while ago) and epoxied the cap back on the new tip.

Seemed to improve our TX range. We also have a Matsutec HP33-A by the way.

 

[2copplane]

I've done the antenna "tuning" bit with a standard whip VHF antenna. Did the math for how many mm to shorten it to bring it from the original 156 MHz (or was it 156.8? Can't remember, but it was printed on the box) to 162 MHz for AIS. Then pried the cap off the tip, cut the calculated length off (I think it was around 12 or 15 mm or something - please do your own math, this was a while ago) and epoxied the cap back on the new tip.

Seemed to improve our TX range. We also have a Matsutec HP33-A by the way.

I’ve nothing to lose so will give it a go. Cheers.

 

[G12]

AIS uses frequencies within the same range as VHF communications, so a standard VHF marine antenna will work fine for AIS purposes.

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.

 

[G12]

I've done the antenna "tuning" bit with a standard whip VHF antenna. Did the math for how many mm to shorten it to bring it from the original 156 MHz (or was it 156.8? Can't remember, but it was printed on the box) to 162 MHz for AIS. Then pried the cap off the tip, cut the calculated length off (I think it was around 12 or 15 mm or something - please do your own math, this was a while ago) and epoxied the cap back on the new tip.

Seemed to improve our TX range. We also have a Matsutec HP33-A by the way.

If you're going to do the maths then remember to allow for the correct velocity factor of the item that you're trimming. Poor quality wire can be as bad as 0.6 whereas a good quality copper would be over 0.9.
Put simply you can't just lop off the difference between the two wavelengths in free space which the basic maths gives you.
156.8MHz is the norm for tuning a standard VHF antenna and surprise surprise that is the same frequency as VHF Channel 16 so your set gets the best (TX) performance there.

 

[pvb]

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.

Which bit do you think is incorrect? The AIS frequencies are within the overall range of VHF communications. And countless thousands of boaters have excellent AIS performance using an ordinary VHF marine antenna.

 

[lw395]

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.

162MHx is what? less than 4% above 156MHz. It's a simple antenna, not a crystal filter. The tuning of an antenna is not that selective. The losses due to being off-tune will be very small typically.
Take a portable VHF radio, add 4% to the telescopic aerial, I doubt you will be able to spot the performance difference.
3:1 VSWR anywhere in the band suggests it's broken.
Acceptable Rx performance but hopeless Tx could be caused by the transmitter sensing poor VSWR and shutting down/ reducing power, or something non-linear happening like the effects of damp or corrosion in the coax or aerial.
I'd suggest a close look at the coax, replace it if there is any sign of corroded braid. It needs to be kept dry.
A new antenna and coax is not really expensive.
It's not impossible that the transmitter is poorly, you could test with a borrowed 'emergency antenna' perhaps?

I would not muck sbout trying to tune up an antenna without the proper gear.
Even then, be aware that a yacht antenna is not exactly like a text book monopole, because it's mounted on a random stick on a yacht, rather than a textbook groundplane. The wavelength is nearly 2m, so a masthead or pushpit is electrically 'small'.

 

[G12]

Well I beg to differ. If you're trying to get a measly two watts out into the ether then you need to minimise all losses.

Yes, you can shove a basic antenna on and it'll work like most people do that don't know any better. It will work better with one that is correctly tuned however, that's why you can buy AIS specific antennas to start with.

"Take a portable VHF radio, add 4% to the telescopic aerial, I doubt you will be able to spot the performance difference." - This is completely incorrect - You need to SHORTEN the antenna for the higher AIS frequency for starters and as I mentioned earlier you can't just lop the difference in wavelength off, there is no account for velocitiy factor and other environmental factors in that calculation.

I should add that I'm probably fairly unique amongst yachties - I own a very good antenna analyser and I can see the difference of every change I make to any system. I practice what I preach too - My own AIS is 5W. This is fed to a dedicated masthead AIS antenna (Metz Manta AIS) via the lowest loss 8mm coax I could get at the time (M&P). The deck join is N type connectors to maintain the impedance. If I could have used a greater diameter cable I would have but sadly that was the fattest I could get up the conduit. The performance is better than the standard 10mm RG213 though. VSWR shows as 1.3:1 on the analyser. It is about 2.6:1 on the main VHF antenna at 162MHz. Strangely enough the main VHF antenna (Also a Metz Manta) shows lowest VSWR at 156.8MHz, who'd have thought it

 

[pvb]

Well I beg to differ. If you're trying to get a measly two watts out into the ether then you need to minimise all losses.

Yes, you can shove a basic antenna on and it'll work like most people do that don't know any better. It will work better with one that is correctly tuned however, that's why you can buy AIS specific antennas to start with.

But how much better does anyone realistically need it to be? What's the improvement in transmitting range? And is that essential - most cruising yachts only need maybe an AIS transmit range of a few miles.

Digital Yacht, who know a bit about AIS, say on their website "One of the most common questions we are asked by our AIS customers, is “what antenna should I use for my AIS ?”, so we thought we would post a short article to provide an answer. Basically there are two options; fit a second dedicated VHF antenna a suitable distance from the vessels main VHF antenna or use the main antenna for both VHF and AIS by fitting a special device called a “Splitter”. Generally our recommendation for yacht owners who want to fit an AIS, is to use a “Zero Loss” splitter like our SPL2000. By utilising the main VHF antenna at the top of the mast, you will definitely get maximum transmit range, plus the ease of installation often makes it a cheaper option than paying for a second antenna to be installed." They do say that a pushpit-mounted AIS antenna would benefit by being a specific AIS-tuned antenna.

 

[JumbleDuck]

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.

What is "much better" in dB?

 

[lw395]

But how much better does anyone realistically need it to be? What's the improvement in transmitting range? And is that essential - most cruising yachts only need maybe an AIS transmit range of a few miles.
...

Exactly.

The difference between 1.3 and 2.6 VSWRs is about 0.6dB of actual transmit power.
That is not the Op's problem.
If the Op is noticing a big reduction in Tx range, he has a much less subtle problem.

 

[G12]

Well personally, having spent time on various bridges I'd rather my AIS target popped up on an approaching ship's radar as early as possible. When you're doing 15kt+ in something porky then the more time you've got to play with the better. I did have one day in Biscay when an AISB target popped onto the radar at 2 miles, nothing prior to that. I make damn sure I keep a really good visual as well as electronic lookout and I hadn't seen this yacht who was well reefed, beating North East into a 4m sea. His white sails were hard to spot in amongst all the white water and I just hadn't seen him with the Mk1 eyeballs or on the radar which had probably removed him with the sea clutter. At two miles he was OK but if I'd known about him earlier I would have altered a bit for him. So - That is why I like mine to work as well as possible on TX as well as RX. I also made sure I got an AIS set that does the proper SOTDMA like the class A sets do.

Digital yacht - Well I'm not familiar with the architecture of the splitter but there may be a bit of impedance matching built in so that the AIS unit sees 50 ohms instead of what's really there from the antenna, or there may be none at all and they're happy to run with a not great VSWR. It's also another nice box they get to charge you for and notice that when the antenna is in the worst position on the pushpit that they recommend the one that radiates better....
The other thing with the splitter is that it makes the system quick to install so cheaper on labour which is often a big factor in people's overall costs. A proper antenna and good cable is still (considerably) less than a splitter if you're not paying for someone else to fit it.

Transmitting range does depend largely on antenna height (of both the TX and RX station) and the type of antenna so it is hard to quantify.

 

[lw395]

...
"Take a portable VHF radio, add 4% to the telescopic aerial, I doubt you will be able to spot the performance difference." - This is completely incorrect - You need to SHORTEN the antenna for the higher AIS frequency for starters and as I mentioned earlier you can't just lop the difference in wavelength off, there is no account for velocitiy factor and other environmental factors in that calculation....)

Did you not understand that the point was, a radio link is not going to be very sensitive to having the antenna slightly too long?

You are a long way from being the only person with a bit of RF test equipment around here, there are several Hams and at least two professionals.

 

[G12]

Did you not understand that the point was, a radio link is not going to be very sensitive to having the antenna slightly too long?

You are a long way from being the only person with a bit of RF test equipment around here, there are several Hams and at least two professionals.

I am a ham and I work with marine comms for a living hence why I have the kit.

 

[ShinyShoe]

I am a ham and I work with marine comms for a living hence why I have the kit.

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...?

 

[PaulRainbow]

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...?

Some good points here.

I don't have a hard an fast system for choosing the antenna for AIS. If the owner wants to use a splitter i'll fit one (although i'd usually suggest a separate antenna). In that case, it's obviously using the standard VHF antenna.

If my suggestion of a dedicated antenna is followed, i generally fit a standard VHF antenna at the pushpit. This would be wired so as to be a backup VHF antenna.

I've messed around swapping from masthead antenna to pushpit antenna, on particular boats, my own included, and i find it impossible to see a difference in received targets. Hard to check for TX.

When i'm installing class A systems on commercial vessels i always fit a separate AIS tuned antenna. No need to worry about dismasting and there is always a fully installed, fixed, backup VHF.

 

[PaulRainbow]

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.

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.