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Old 06-11-2017, 12:50   #1
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VHF- AIS antenna placement. mast head or arch?

I currently have a 6' antenna mounted on my radar arch 12' from the water.
I was going to replace the antenna with a 8' version, but my vesper AIS notes to not put the antenna in the path of the radar. I have a solid state garmin radar.

Do I need to worry about the path of the radar?

What is better mast head 3' and longer coax cable or 8' (6 or 9db) antenna on 12' high radar arch with shorter coax cable.
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Old 06-11-2017, 17:03   #2
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Re: VHF- AIS antenna placement. mast head or arch?

I have a dedicated 4í AIS antennae mounted on the front of the flybridge. I mounted it on a small level area that was perfect for the mount I used, itís below the beam of the radar mounted on a 10Ē stand on the arch. On the upper Mississippi I pick up tows at 15 miles which is plenty. It would probably be farther if it wasnít for the bluffs on the Missouri side. What are you looking for on your setup to be better?
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Old 06-11-2017, 17:22   #3
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Re: VHF- AIS antenna placement. mast head or arch?

An antenna placed higher gives you the longer range.

A longer antenna is fine on a boat that does not heel much (a cat, e.g) but a soft mono may opt for a shorter antenna.

Cheers,
b.
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Old 06-11-2017, 17:41   #4
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Re: VHF- AIS antenna placement. mast head or arch?

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Originally Posted by River Cruiser View Post
I have a dedicated 4í AIS antennae mounted on the front of the flybridge. I mounted it on a small level area that was perfect for the mount I used, itís below the beam of the radar mounted on a 10Ē stand on the arch. On the upper Mississippi I pick up tows at 15 miles which is plenty. It would probably be farther if it wasnít for the bluffs on the Missouri side. What are you looking for on your setup to be better?
The note of not having the antenna in the radar path got my attention as this is how the boat is now.

My forum name mentions cat, but I ended up buying mono so I do heel. Maybe that is the deciding factor. I should go mast head and that eliminates the radar issue.
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Old 07-11-2017, 01:01   #5
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Re: VHF- AIS antenna placement. mast head or arch?

FWIW: We have antennas at the masthead and on the arch. Haven't done any VHF comms comparison, but the masthead antenna gives consistent 35-50 mile reception of class A AIS while the arch mounted antenna peters out at around 20-25. No idea about changes in the received range of our AIS transmissions. The arch antenna is direct with a pretty short cable. The masthead antenna goes via a Vesper splitter and a long (~30 meter) RG8 cable. By c hoice we use the masthead antenna, keeping the other as a backup.

Jim

PS They are both the usual short whip with nominal 3 db gain.
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Old 07-11-2017, 09:48   #6
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Re: VHF- AIS antenna placement. mast head or arch?

I have a 3 foot, 3Dba antenna at the mast head. Near shore I don't think it makes much difference however, Sailing, short-handed, in the ocean, I take comfort in the fact that it reaches well over the horizon (out to a good 20nm) and the TCP alarm, set to 2nm, gives me plenty of time to observe other vessel's movements, and decide what to do next.
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Old 07-11-2017, 10:39   #7
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Re: VHF- AIS antenna placement. mast head or arch?

I also have both a mast-head antenna and one on the radar mast.
I have not done any serious comparison between the two, but anecdotally I do not notice a dramatic difference in range (nor I get quite the range that others have reported). I thought it was due to the connecting cable, which was carefully routed and minimized for the one to the radar mast, but is "out of the box" for the one to the mast. I am very interested in hearing some other experiences on this.

The antenna should be certainly carefully placed to stay out of the radar path. In my case (I have a 4G radar), we placed a "shelf" of antennas just above/behind the radar dome, and it can barely be "seen". We did try different positions for the shelf, and some positions are much more noticeable than others. It seems to be fairly sensitive, so it is probably worth the effort. The best position was found empirically, within a few tries.
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Old 07-11-2017, 13:01   #8
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Re: VHF- AIS antenna placement. mast head or arch?

Quote:
Originally Posted by Jim Cate View Post
FWIW: We have antennas at the masthead and on the arch. Haven't done any VHF comms comparison, but the masthead antenna gives consistent 35-50 mile reception of class A AIS while the arch mounted antenna peters out at around 20-25. No idea about changes in the received range of our AIS transmissions. The arch antenna is direct with a pretty short cable. The masthead antenna goes via a Vesper splitter and a long (~30 meter) RG8 cable. By c hoice we use the masthead antenna, keeping the other as a backup.

Jim

PS They are both the usual short whip with nominal 3 db gain.


Jim's post is quite important, but at the same time do not let it fool you. It is important due to two factors:
1. He gets the best coverage or distance from his masthead antenna (height makes a massive difference), and
2. He has a good quality co-ax cable (not the high loss RG58 normally used in masts).

The "don't let it fool you" part is he says he has an AIS Class "A" system, which is normally found only on large commercial vessels and transmits a 12W signal, whilst the units normally found on sailing vessels is the AIS Class "B" system, which only transmits a maximum of 2.5W.

So, if you are using your masthead antenna with a Class "B" AIS, make sure you have the lowest loss co-ax cable you can afford, all the way from your AIS transceiver to your antenna. If you have standard RG58 co-ax cable, you may find you are actually getting less than a watt out of your antenna at the top of your mast and other vessels will not see you until you are pretty much on top of them.

John.
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Old 07-11-2017, 13:23   #9
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Re: VHF- AIS antenna placement. mast head or arch?

Hi
Do you really want or need to know what is going on 50nm away? Surely too much information.
John
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Old 07-11-2017, 13:40   #10
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Re: VHF- AIS antenna placement. mast head or arch?

Quote:
Originally Posted by JohnT View Post
Jim's post is quite important, but at the same time do not let it fool you. It is important due to two factors:
1. He gets the best coverage or distance from his masthead antenna (height makes a massive difference), and
2. He has a good quality co-ax cable (not the high loss RG58 normally used in masts).

The "don't let it fool you" part is he says he has an AIS Class "A" system, which is normally found only on large commercial vessels and transmits a 12W signal, whilst the units normally found on sailing vessels is the AIS Class "B" system, which only transmits a maximum of 2.5W.

So, if you are using your masthead antenna with a Class "B" AIS, make sure you have the lowest loss co-ax cable you can afford, all the way from your AIS transceiver to your antenna. If you have standard RG58 co-ax cable, you may find you are actually getting less than a watt out of your antenna at the top of your mast and other vessels will not see you until you are pretty much on top of them.

John.
John, read it again, mate! I don't have a class A AIS, mine is the standard spec class B (Vesper Vision). I was describing RECEPTION of class A from ships far away.

Jim
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Old 07-11-2017, 14:13   #11
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Re: VHF- AIS antenna placement. mast head or arch?

The reason for keeping the AIS antenna away from the radar is that traditional radars have high peak power output and there is potential to damage the AIS receiver. (low average power though) If the AIS or VHF antenna is, say at least 8-10 feet above or below the radar it should not be a problem.

As far as VHF losses go, with 60 feet of coax cable RG8 has about 1 dB loss, and rg-58 has about 3 db of loss. 2 db difference is not that much. Height is everything, and the major differences in range will be due to line-of-sight differences, not a small difference in coax or line efficiency. However, if you don't care about extra weight aloft, marine grade RG-8 will be slightly better.
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Old 07-11-2017, 14:14   #12
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Re: VHF- AIS antenna placement. mast head or arch?

Quote:
Originally Posted by Jim Cate View Post
John, read it again, mate! I don't have a class A AIS, mine is the standard spec class B (Vesper Vision). I was describing RECEPTION of class A from ships far away.



Jim


Jim, my error, sorry. But it still highlights the points I made regarding masthead antenna and a good low-loss cable, which is such an important factor in small craft AIS Class "B" installations. Use the common RG58 in a long mast and your losses are going to be so bad that both your reception and transmission is going to be mostly lost through cable loss. Your use of a low-loss RG8 is such an important factor in both voice VHF and a low power output AIS.
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Old 07-11-2017, 21:33   #13
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Re: VHF- AIS antenna placement. mast head or arch?

Ahoy canadian cat - You asked first about problems related to the radar. That depends entirely on how good a job the receiver (the AIS) does at rejecting out-of-band signals (from the radar). If Vesper says maintain separation, then I guess you should. Itís an easy experiment to do, though. Look at your AIS targets with the radar off and then turn on the radar and see if you lose targets. Since itís a function of the design of the particular AIS receiver, the best way to know is to do the experiment.

Your second question about AIS range is interesting. Except for more exotic propagation modes that arenít important for AIS, practical VHF service range is determined by the height of the two stations' antennas. So for this type of line-of-sight propagation, the extra 3 dB of gain you get from the 8í antenna at the same height means very little. But getting your AIS antenna up higher will extend your range.

Antenna gain is sometimes important when your signal is competing with other simultaneous FM signals or overcoming a lot of noise but, again, those are not generally important for AIS. HF (SSB) is an entirely different story.

Therefore the loss at VHF from a long coax run (even if itís RG-58) up a tall mast is made up for by the increased ďradio horizonĒ which is much more a function of the height of the two stationís antennas than either stationís radiated power (wattage). So whether itís RG-58 or RG-8X or RG-8, as long as you have good connectors, the ends are well-sealed so water doesnít wick into the coax, and the coax isnít crimped or damaged, then the higher antenna will have a longer range.

Aboard Liberty, my installation is a lot like yours: a VHF antenna on an arch and one at the masthead. I actually switch my AIS and VHF radio between them, depending on which one I want the greatest range on. (I also like redundancy!) Most of the time at sea, I want maximum AIS range. A little over 20 miles is quite reliable from my masthead antenna to a big container ship, mainly because my antenna is ~50í high and his is ~200í high!

And in terms of situational awareness, knowing early that guy is out there is fine with me.

Bottom line: as long as your installation is clean, height is the single most important factor in determining your AIS range.

Seth
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Old 08-11-2017, 01:48   #14
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Re: VHF- AIS antenna placement. mast head or arch?

At 160 MHz, RG58 loss is about 6 dB per 100 ft, say 3 dB to the top of your mast. That is a 50% power loss. Range is a square-law function of power, so the 2 dB increased loss over a 'good' co-ax might result in a decreased range of about 20 percent, 25% should your good coax be lossless (Ha!).

If range is important to you, height wins by a large margin. Masthead is best for range.

For collision avoidance, 4 miles range on my radar is enough, and I don't see the need for much more on AIS. I am sailing, and a faster boat might need more range, but 15 minuted is plenty of warning time, so do you really need 50 miles range?

So, I have my AIS antenna on the pushpit rail, about 6 ft above sea level, and my 'voice' VHF antenna at the masthead.

The AIS antenna is a backup for the masthead one, should the latter fail. Using the masthead antenna for AIS I can see targets at 30 miles range, so it is a good check that your antenna is working properly.

Do not site your AIS antenna closer than a couple of feet from the other one, or you can get lethally high (for the AIS receiver) voltages induced in the AIS antenna from the 25 Watt transmitter.

Alan
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Old 08-11-2017, 02:02   #15
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Re: VHF- AIS antenna placement. mast head or arch?

Alan, it may not be important to you to see AIS targets more than 4 miles away, but IMO it is quite important that the ship be able to detect your presence at much greater ranges. Your puny 2 watt signal from a low mount antenna may not meet that criteria very well.

AS for me, seeing ships further away when there is any sort of multiple ship traffic situation is important. I like to see how the pros are setting up the passes, and to be sure that they are including little me in the plan. This all happens at distances greater than 4 miles, often more than 10 miles.

so, those long range targets are not trivial in my opinion. Sure, seeing them at 50 miles, which we often do is not particularly useful, but I do find it interesting anyhow.

Jim
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