VHF/AIS antenna tuning, VSWR

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GTom

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I was wondering how important is to choose the correctly tuned antenna? VHF channel 16 is 156MHz, AIS is 162MHz, what do I loose if I e.g. buy a VHF-tuned antenna for the AIS, do serve as a best possible backup VHF in case the mast comes down?

Antenna manufacturers rarely provide a VSWR-Frequency plot...
 
GTom said:
I was wondering how important is to choose the correctly tuned antenna? VHF channel 16 is 156MHz, AIS is 162MHz, what do I loose if I e.g. buy a VHF-tuned antenna for the AIS, do serve as a best possible backup VHF in case the mast comes down?

Antenna manufacturers rarely provide a VSWR-Frequency plot...
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There have been threads about this before and my take-away from those would be that it won't make any noticeable difference.

Richard
 
RichardS said:
There have been threads about this before and my take-away from those would be that it won't make any noticeable difference.

Richard
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I agree with Richard. Most of us are using VHF tuned antennas for AIS and they work just fine. There are AIS tuned antennas, which i have fitted for customers and i can't tell the difference.
 
A reasonable marine VHF antenna will almost certainly be designed to accommodate a much wider range than 156 - 162 MHz. Here's one example (Comrod AV7M).

vswr.png
 
Martin_J said:
It turns out that the issue in that other thread was a badly installed PL259 plug.

As others have said - Any VHF antenna tuned perfectly for CH16 will also be good at AIS frequencies and vice versa. They're not that far apart in the overall radio spectrum.
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Right, didn't check the second page of that story :D. Also gone trough quite a few "SWR error AIS" search hits around the internet, have yet to find one where things went that south only because of antenna tuning.

Still, I'think stitching a 159MHz antenna could be a good idea.
 
GTom said:
Right, didn't check the second page of that story :D. Also gone trough quite a few "SWR error AIS" search hits around the internet, have yet to find one where things went that south only because of antenna tuning.

Still, I'think stitching a 159MHz antenna could be a good idea.
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We use a standard vhf antenna dedicated to the AIS. We get up to 40nm receive range on ships and 10nm transmission range with our antenna mounted at the top of the mizzen mast. There is no other electronics or electrics at the top of the mizzen mast. I cant see the need for an AIS antenna tuned to the specific frequency
 
geem said:
We use a standard vhf antenna dedicated to the AIS. We get up to 40nm receive range on ships and 10nm transmission range with our antenna mounted at the top of the mizzen mast. There is no other electronics or electrics at the top of the mizzen mast. I cant see the need for an AIS antenna tuned to the specific frequency
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Having only one mast I am puzzled where to put a second AIS antenna. You have the advantage of a direct antenna-radio connection, without a ~1dB loss splitter and some loss on the additional connectors.:encouragement:.
 
GTom said:
Having only one mast I am puzzled where to put a second AIS antenna. You have the advantage of a direct antenna-radio connection, without a ~1dB loss splitter and some loss on the additional connectors.:encouragement:.
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You also get an emergency VHF antenna should you lose the masthead one (or, heaven forfend, the mast). Our AIS antenna is on a comms post sticking above the aft rail. It shares this post with the GPS, the ordinary AM/FM radio, the Navtext and the Iridium antennas.
 
jdc said:
You also get an emergency VHF antenna should you lose the masthead one (or, heaven forfend, the mast). Our AIS antenna is on a comms post sticking above the aft rail. It shares this post with the GPS, the ordinary AM/FM radio, the Navtext and the Iridium antennas.
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That's my line of thinking too, if I manage to squeeze somewhere (high!) a second antenna, it should also work well at Channel 16. Thought about backstay, but that's not a very good idea if I want something in case of dismasting. Got a radar pole, about 4m above sea level, but that's not too much. Very good AIS transmission can also be helpful in distress, if I am lucky enough that the class B signal is picked up by the satellites, the whole world will know my position. However, that needs a very low loss system I suppose...
 
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I've just encountered a VSWR problem with my (em-trak) AIS.
I use a seperate whip antenna at the top of my mizzen, it was ok when I installed the Transceiver last year and on previous outings this year but on my trip back from Weymouth last w/e I noticed the error indicator came on when I switched on the Tx.
Reseated the PL259 plug to no effect so I am now in the process of making up an aerial extension cable to see if I get the same problem on my main VHF antenna.
 
GTom said:
That's my line of thinking too, if I manage to squeeze somewhere (high!) a second antenna, it should also work well at Channel 16. Thought about backstay, but that's not a very good idea if I want something in case of dismasting. Got a radar pole, about 4m above sea level, but that's not too much. Very good AIS transmission can also be helpful in distress, if I am lucky enough that the class B signal is picked up by the satellites, the whole world will know my position. However, that needs a very low loss system I suppose...
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You don't need an AIS antenna a mile above sea level, short pole on the pushpit works fine. How do you think mobos get on with VHF/AIS antennas and no masts ?
 
LadyInBed said:
I am now in the process of making up an aerial extension cable to see if I get the same problem on my main VHF antenna.
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I'd suggest fitting the extension cable and leaving it in place (if that's possible), so you could quickly use it in an emergency if the VHF antenna failed.
 
PaulRainbow said:
You don't need an AIS antenna a mile above sea level, short pole on the pushpit works fine. How do you think mobos get on with VHF/AIS antennas and no masts ?
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Big ships plan their maneuvers 6-8-...NM ahead, does a pushpit mounted anteanna transmit that far?

Meantime I found some measurements, apparently not all antennas made equally well: https://www.thehulltruth.com/8840429-post19.html , this one is a recommended brand tuned to 153MHz, SWR is quite far from the "nice" 1.1-1.5 values. Probably out-of-spec antennas are behind some of the VSWR woes. Another story here, with a Metz VHF-tuned antenna.
 
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The 'vswr errors' on transceivers will be gross vswr problems, not a slight departure from ideal tuning. Most VHF transmitters will work fine into a 3 or 4 :1 swr, even 4:1 is only a 2dB mismatch loss.
For VHF and AIS, a swr error indicates something shorted or broken, unless you've managed to connect it to completely the wrong antenna.
I don't like antennas mounted directly on the pushpit, I've seen too many get damaged.
A short pole putting it above boom height gives fine transmit coverage in my experience, yacht-to-yacht often 15 miles, the bigger the ship, the higher their AIS aerial tends to be, so the ships that most need to see your AIS early probably will. Although they'd prefer to see you early, the distance at which they need to see you to avoid you is not that great.
The nice graphs in the brochure are probably measured on a big flat groundplane.
You'll get a different answer when you mount the antenna on a metal pole a wavelength or two long above the water.
 
GTom said:
Having only one mast I am puzzled where to put a second AIS antenna. You have the advantage of a direct antenna-radio connection, without a ~1dB loss splitter and some loss on the additional connectors.:encouragement:.
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Advice from NMEA below
VHF ANTENNA & MOUNT PLACEMENT
VHF radio signals are line of sight. Range increases with antenna height. Select an antenna mount suited for the specific antenna and vessel type.
Recommended Antenna spacing:
3 feet between GPS & VHF antennas.
2 feet between Radar & VHF antennas.
4 feet separation between two VHF antennas.

We have AIS on one mast and vhf on the other. Gps at deck level. We have excellent radio range with nothing else other than navigation lights sharing the top of the mast. We see a number of yachts with two antennas at mast head and no standoff brackets to comply with NMEA recommendations.
 
geem said:
Advice from NMEA below
VHF ANTENNA & MOUNT PLACEMENT
VHF radio signals are line of sight. Range increases with antenna height. Select an antenna mount suited for the specific antenna and vessel type.
Recommended Antenna spacing:
3 feet between GPS & VHF antennas.
2 feet between Radar & VHF antennas.
4 feet separation between two VHF antennas.

We have AIS on one mast and vhf on the other. Gps at deck level. We have excellent radio range with nothing else other than navigation lights sharing the top of the mast. We see a number of yachts with two antennas at mast head and no standoff brackets to comply with NMEA recommendations.
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A 4 feet standoff bracket would look quite weird on the masthead.
 
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