Tuning VHF antenna for use with AIS

[Nicholson58]

Originally Posted by CarstenWL

Hi Steve,

the frequencies used for AIS are

AIS 1 - 161,975 MHz (Kanal 87B) and AIS 2 - 162,025 MHz (Kanal 88B) (as you might know)

For transmitting on VHF from the ship you use 156MHz - 157.4MHz. The 'normal VHF antenna' will cover this range with some sacrifice but there might be antennas out there that are more narrowband than others so an AIS antenna 'tuned for 162MHz' might help.

As you said you might also look for the fault in the cable and or connectors.

Regards,

Carsten

Cable should be low impedance. Old & high impedance cable will likely influence results.


Length also is a factor. My antenna is 80 feet up and another 30 feet to the AIS. I run the proper COAX. The Watchmate 850 sees commercial vessels 60 to 75 miles out. Class B is picked up about 5-10 miles. Good enough.

[Olde Chief]

One thing not address here is the fact that the boat you want to find out about may not have their system turned on, or it may be hiding behind a larger ship or be further away.

[Tikka]

AIS Cat "B" transmitter power is limited to 2Watts in Australia.
Not sure what is allowed in US but it is obvious, that 2W OUT
is very very limited range. That would be from my experience
equivalent of hand held VHF transceiver.
You know, hand held has limited range.

[SpiritofGlenans]

Let me first say that I know nothing about the technical aspects of antennae, relying on the seller to supply me with the correct one for the job.
My comments on the posts in this thread are that I believe an AIS Rx range of about 10-12nm is sufficient for collision avoidance purposes. That kind of range can be achieved with a rail-mounted antenna. The advantage of thsi is that in the event of a dismasting you will still have the ability to transmit.

[transmitterdan]

Originally Posted by Tikka

AIS Cat "B" transmitter power is limited to 2Watts in Australia.
Not sure what is allowed in US but it is obvious, that 2W OUT
is very very limited range. That would be from my experience
equivalent of hand held VHF transceiver.
You know, hand held has limited range.

Tikka,

The range of AIS is significantly more than a handheld VHF radio. The AIS signal is digitally encoded and requires less power to be decoded by the receiver. The range of AIS at 2W is about the same as voice at 10W.

[rgleason]

Which would get a better signal?
Good 3db Base coil antenna tuned to AIS at 20' above water with shorter cable one connection in the middle and no switch.
Or Good 3db VHF on 40' mast shared with VHF via a switch with connector at the bottom of the mast.
Both using the best coaxial canle available.

[rgleason]

Whoops I meant 10' for the 3db AIS.

[btrayfors]

In my experience a well matched AIS antenna is important, both for reception and, especially, for transmission. I have personally experienced this with both my shipboard installation and a shore-based installation which collects AIS signals.

First, do not use a splitter unless you absolutely have to. I don't care how good the specs look; splitter use is simply not good practice IMHO.

Next, get the very best SWR match you can for the AIS. The first antenna I put up for my AIS (stern rail mount) was not well matched. I found that it's performance varied with weather conditions: a bit of rain and it would degrade significantly. It was a Metz VHF antenna with a s/s whip, which I trimmed using an antenna analyzer until SWR was 1:1. No more problems, even in the rain.

Antenna height is important if you want long-distance reception/transmission. However, I find that the AIS antenna on the stern rail -- now well matched to the AIS frequencies -- provides plenty of performance for what I'm interested in, i.e., to see and be seen.

Bill

[Olde Chief]

In my opinion the AIS with the 3db gain antenna on a 40' mast would work the best, but a standard VHF antenna would work almost as well and would be cheaper. Why, because the higher you go the more of the signal you would get, under normal conditions. Always remember in the back of your mind WEATHER/ATMOSPHERICS are always in the ball game at the lower power levels, and normally when operating in the VHF frequency, the MAX Range is line of sight, from the Antenna.

[Olde Chief]

I don't know what kind of weight my comments make on this subject, I do have a lot of experience here. I have worked frequencies from 100 KHz to 500 MHz, 25 watts to over a 1 Mega Watt, as a technician and an operator.


But as a Sailor, I will try to help anyone I can, short of getting into the equipment. Hope I have helped with this AIS with out generating more confusion.


Just remember if you buy good equipment and have it installed correctly, and keep it DRY, it will work as it is suppose to. Read and try to understand what your manuals tell you.

[Extra Class]

In regards to VHF antennas, SWR, cables, connectors and coax ... there is a good bit of correct information and some misinformation.
Lets face it. If you know this subject ... the answers are easy. If not, you are just guessing. The subject is very easy to learn ... but I will bet you that most people ... will not.
A perfect SWR (VSWR) = Voltage Standing Wave Ratio ... will read 1.0 to 1.0 . Some times it is impossible to actually achieve it. So anywhere from 1.1 to 1.5 is acceptable 2 to 1 is not so good and anything above 2 to 3:1 is really not good. How to measure it? A simple (low cost VHF SWR meter) will do. Some HF meters do not perform very well on VHF.
Best advice. If you can get in touch with an Amateur Radio operator (not a CBer) He will most probably be able ..or might know of someone with the necessary equipment to help you. Try to find someone with at least a General or an Extra Class license. You can easily learn more about it yourself .. by starting with a ARRL (American Radio Relay League) Technician Class license manual. A vhf whip antenna is 'TUNED' simply by either lengthening or shortening it. BUT ... ou need to know what you are doing . (Very simple !) More info?
73
Joe W4CBJ Extra Class St. Pete, FL

[a64pilot]

Supposedly the Vesper splitter improves performance, I don't know how, but that is the claim.

"skip" is not all that un-common, I've received AIS signals from considerably more than 100 miles on occasion, seems most often to be bridges, buoys and other stationary objects for some reason though, I've assumed its just atmospheric conditions are good for skip is all.

Seems skip is more prevalent at night?

[Tikka]

A Class B transponder transmits at 2 Watts so even in perfect line of sight conditions, the best range you can expect is about 8NM. It is important to check if the station is within 5-8NM of your location and also if there are any other Class B vessels displayed near you. If you are more than 5-8NM from the receiving station or there is not good clear line of sight between the station and your location, then the chances are that the station is not picking up your transmission or any other class B transmissions from vessels in your marina.

It should be noted that Commercial vessels and some larger pleasure craft have Class A Transponders that transmit at 12.5 Watts and have a much greater transmit range 20-25NM, so these vessels maybe displayed near your location, even if your Class B transmission is not received.

Finally, it can take an hour or so for a new AIS (MMSI number) to be recognised and stored in the database of these online services so if you are confident that you are in range of an online receiving station, leave your transponder on for a few hours to give the online service time to detect and record you in their database.

[Cadence]

Originally Posted by a64pilot

Supposedly the Vesper splitter improves performance, I don't know how, but that is the claim.

"skip" is not all that un-common, I've received AIS signals from considerably more than 100 miles on occasion, seems most often to be bridges, buoys and other stationary objects for some reason though, I've assumed its just atmospheric conditions are good for skip is all.

Seems skip is more prevalent at night?

Skip is more prevalent at night I think it has something to do with the ozone layer? I wouldn't rely on it. 100 mile doesn't sound like a skip but could be? One of the HAMS could probably answer that. I've spoken to Norway and Oz from SC, USA on 13 watts SSB. It sounds as AIS is line of sight for reliability? I doubt I would ever speak to the same person again on a skip?

[Paul Elliott]

VHF "skip" is very different than HF skip. At VHF, it's usually tropospheric ducting that does provides the extended range (the troposphere starts at the surface, and goes up to about 11 miles). Often the marine-layer fog can cause ducting and provide VHF propagation well beyond 1000 miles. 100 mile AIS distances are common when these conditions exist.

HF skip is an ionospheric phenomenon.

The lower power of Class-B AIS does reduce the reliable range to perhaps 6-10 miles, and unreliably much further.

The main reason that a handheld VHF has poor range is due to the lousy antenna. Put a good antenna on a handheld and the range improves dramatically. Put that antenna up on a spreader or masthead and it does even better.

Marine whip VHF antennas are usually tuned for channel 16, and the SWR can be very good. The SWR can be extremely bad at the AIS frequencies, and this will make a significant difference in AIS performance, especially on the transmitted signal of a transponder. I think having an antenna that is tuned for the AIS channels is a very good idea.

As has been mentioned, at VHF normal cable loss can make a bad SWR (at the antenna, where it counts) appear to be acceptable. To get an accurate measure of your antenna's performance you either have to measure SWR at the antenna, or know how to correct for cable loss.