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Old 21-09-2012, 06:25   #1
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AIS Antenna Placement

I,am unstepping the Mast.While down for other reasons I,wish to install a AIS antenna opposite the VHF antenna at the Mast Head.I,plan on installing a Class B AIS.I,will use Metz antennas for both AIS and VHF.Is this a Bad or Good idea.Is the distance between the antennas to close?
Thanks all for the input.
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Old 21-09-2012, 07:12   #2
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Re: AIS Antenna Placement

Hi, I have read several times in manufacturers literature to NOT put the AIS antenna next to the VHF on the mast. I took this following statement off the Vesper AIS site.


You should place it as high as reasonably possible but as far away from masts and other antennas, particularly other VHF antennas.

Jim
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Old 21-09-2012, 07:32   #3
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Re: AIS Antenna Placement

I don't see the need to put the AIS antenna up high. For reception you only need say a 5-15 mile range and for transmission maybe a 5 to 10 mile range. Getting targets from way from away isn't real useful.
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Old 21-09-2012, 10:10   #4
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Re: AIS Antenna Placement

Definitely a bad idea because the antenna's are too close together in a horizontal plane. If you are going to use a dedicated AIS antenna, you should get one tuned specifically for AIS frequencies which is around 162Mhz. General marine VHF antenna's are tuned for channel 16 which is 156.800Mhz. The Metz Manta-6 advertises that it is AIS compatible which simply means that it will work on the AIS frequencies but it won't be as efficient as one designed specifically for AIS. The Manta-6 antenna rod can be cut for better efficiency for AIS but you will need someone experienced with an antenna analyzer to do that for you.

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Old 21-09-2012, 10:37   #5
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Re: AIS Antenna Placement

The AIS signal that gets to your boat, or the one that is received by that VLCC bearing down on you, is the key. If it doesn't get received, it is useless. Therefore, do everything that will improve your chances for success. My own AIS and separate VHF antennas are located four feet apart, are TALL 6dB units, and are located on an "antenna farm" seven feet above the deck level. I also will have, after overhauling the stick, a small wire whip 3 dB at the masthead. I already have the VHF antenna selector switch wired in to choose which one I want for comms, though I may just use one for the second VHF I install in the future.
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Old 21-09-2012, 11:52   #6
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Re: AIS Antenna Placement

Thank you folks for the help.I,have read tha the higher the antenna the clearer the target beomes at any distance,a side benefit would be range.Not being a Teh wiz I,don't understand where mounting these two antennas at mast head willl cancel out each other.Seem's like a simple solution when you are space changled.If mounting the two side by side is a bad idea,what happens if you run the coax wires close to each other?Will the shielding protect to reception and transmission?
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Old 21-09-2012, 12:17   #7
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Re: AIS Antenna Placement

As Eric intimated, it's not a good idea to place the AIS antenna nearby the main VHF antenna.

Here's why:

1. Both devices use the same VHF marine band, and while the AIS signals are located a mHz or more away from the VHF voice signals, there is the possibility of interference.

2. You don't want to do ANYTHING to potentially compromise the VHF voice signal, since it is arguably the most important piece of communications gear on the boat.

3. You don't need to place the AIS antenna very high up in order to receive signals 20-30 miles away. Who cares? Rather, you want to know about boats which are fairly close in to you (5-10 miles) and which represent the possibility of a collision. An AIS antenna placed on the stern rail or arch is perfectly capable of doing this job, without the possibility of interfering with your VHF voice system.

Bill
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Old 21-09-2012, 13:45   #8
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Re: AIS Antenna Placement

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Originally Posted by btrayfors View Post
As Eric intimated, it's not a good idea to place the AIS antenna nearby the main VHF antenna.

Here's why:

1. Both devices use the same VHF marine band, and while the AIS signals are located a mHz or more away from the VHF voice signals, there is the possibility of interference.

2. You don't want to do ANYTHING to potentially compromise the VHF voice signal, since it is arguably the most important piece of communications gear on the boat.

3. You don't need to place the AIS antenna very high up in order to receive signals 20-30 miles away. Who cares? Rather, you want to know about boats which are fairly close in to you (5-10 miles) and which represent the possibility of a collision. An AIS antenna placed on the stern rail or arch is perfectly capable of doing this job, without the possibility of interfering with your VHF voice system.

Bill
Thank you Bill,how about I,drop the AIS antenna down to the spreaders probably 15 ft or so beneath mast head,it will be blocked to one side by Mast,will that make a difference?My pushpit is crowded,and I,am probably overthinking this.
Thank's Dave
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Old 21-09-2012, 14:10   #9
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I know some don't like them, but I,m a fan of splitters, the fact is your VHF antenna does nothing most of the time except act as a receive antenna.

Carry a spare antenna as well. In my opinion there is no particular benefit to a tuned specific AIS antenna, there are way more losses in the connectors and co-ax.

I use a plotter and a masthead VHF antenna, ships 20 miles away get my AIS beacon, very useful for those fast ferries. It's nonsense to suggest you only want to see vessels In The nearest 5 miles. For fast cats I want to see them In the next friggin galaxy , more importantly I want him to see my AIS beacon 100 miles away !!!!!

Dave
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Old 21-09-2012, 14:13   #10
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Re: AIS Antenna Placement

Dave,

That MIGHT work if:

1. the AIS antenna isn't too bendy; and
2. there's room so it won't hit anything as it whips around in a seaway.

I was going to install one on the spreaders of a 43' Beneteau recently but concluded that it just wouldn't work well because the antenna would flop around in a seaway. The Metz s/s whips are particularly "bendy".

Mounted it on the radar arch instead. A few weeks before that I put another AIS antenna on an arch; worked fine.

One caution: because it's such "neat" technology, and you can put AIS displays on radar screens, chartplotters, etc.....one tends to get lulled into the belief that it's "seeing" all the dangers out there. It's not! In fact, on a good day it may "see" 10-20% of the things out there which can hit you or you can hit....yachts, buoys, small boats, flotsom & jetsome, etc., etc.

Just after we finished the install of the first one on the arch, we were watching the chartplotter with the AIS overlaid. The owner was happy to note that there was an AIS target nearby....a boat which was berthed at a nearby marina. Just 200 feet away was a commercial passenger vessel...about an 80-footer...coming up the channel toward our boat. It was NOT on the AIS display....because it wasn't broadcasting an AIS signal! Duh! Keep your eyes open.

Bill
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Old 21-09-2012, 15:38   #11
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Re: AIS Antenna Placement

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Originally Posted by btrayfors View Post
Dave,

That MIGHT work if:

1. the AIS antenna isn't too bendy; and
2. there's room so it won't hit anything as it whips around in a seaway.

I was going to install one on the spreaders of a 43' Beneteau recently but concluded that it just wouldn't work well because the antenna would flop around in a seaway. The Metz s/s whips are particularly "bendy".

Mounted it on the radar arch instead. A few weeks before that I put another AIS antenna on an arch; worked fine.

One caution: because it's such "neat" technology, and you can put AIS displays on radar screens, chartplotters, etc.....one tends to get lulled into the belief that it's "seeing" all the dangers out there. It's not! In fact, on a good day it may "see" 10-20% of the things out there which can hit you or you can hit....yachts, buoys, small boats, flotsom & jetsome, etc., etc.

Just after we finished the install of the first one on the arch, we were watching the chartplotter with the AIS overlaid. The owner was happy to note that there was an AIS target nearby....a boat which was berthed at a nearby marina. Just 200 feet away was a commercial passenger vessel...about an 80-footer...coming up the channel toward our boat. It was NOT on the AIS display....because it wasn't broadcasting an AIS signal! Duh! Keep your eyes open.

Bill
Thank's again Bill,The whippy part I,would beleive that as long as the antenna did noy make contact with the mast or stay's all would be ok.I'll take some calculations while the mast is down.I,somewhat aggre with the quote I,want to see everything out there and them see me,have played DODGE em Frieghter too many times.
Thank's again Dave
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Old 21-09-2012, 17:28   #12
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Re: AIS Antenna Placement

I went to a ham store and bought a magnetic mount, we have a steel boat.

Stuck it on the coaming.

I get returns from up to 40 miles.

Now, I found myself in the middle of the One World race. None of those boats showed up at over two miles. I was told they stuck their antennas in the bilge.
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Old 21-09-2012, 18:41   #13
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Re: AIS Antenna Placement

Quote:
Originally Posted by Roy M View Post
............. My own AIS and separate VHF antennas are located four feet apart, are TALL 6dB units, and are located on an "antenna farm" seven feet above the deck level. I also will have, after overhauling the stick, a small wire whip 3 dB at the masthead. I already have the VHF antenna selector switch wired in to choose which one I want for comms, though I may just use one for the second VHF I install in the future.
While this probably works well for Roy M assuming he has it on the Trimaran, I am not an advocate for 6dB antennas on the average monohull. IMO, monos roll (or heel) too much, thus decreasing the antenna's potential range to that below a 3 dB unit with its broader field pattern.
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Old 22-09-2012, 10:17   #14
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Re: AIS Antenna Placement

Spot on, Wotname! I should have mentioned that in my comment. I get about a 12 degree snap roll, as opposed to a 30 plus degree slow roll experienced by "leaners" I chose the 3 dB whip for the masthead solely for weight and windage considerations.
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