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Old 20-11-2010, 07:24   #1
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AIS Antenna Location / Splitter

Trying to decide where to put my class b ais antenna or go with a splitter using the vhf antenna. Where is your antenna located or are you using a splitter. Thanks
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Old 20-11-2010, 10:52   #2
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My Class-B antenna is on the top spreader, and the VHF antenna is at the masthead. I originally had the AIS antenna on the stern rail, which gave decent performance and made for a reasonable emergency "mast-down" antenna, but I do get better range at the spreader.

A splitter is probably fine, I just prefer the simplicity of two antennas.
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Old 20-11-2010, 11:01   #3
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I went with a dedicated antenna. I'm very leery of seeing a box put out watts of power into a device that generally sees microwatts of power.
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Old 20-11-2010, 13:12   #4
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I had similar consideration a month or so ago.

I decided to have a separate AIS antenna on the stern pushpit because
(1) cheaper
(2) can function as emergency antenna when mast top one goes wrong
(3) will probably last longer than switching system/active splitter
(4) you do not need to send data long distance for AIS since you are more voncerned to nearby ships.


y.lee
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Old 20-11-2010, 14:42   #5
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I agree with Capt. Douglas (who, by the by, is also an Extra Class ham). Do not use a splitter. Why risk losing the most important communications device you have aboard, when there are other inexpensive and better alternatives which also offer some redundancy?

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Old 25-11-2010, 18:36   #6
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When I first started playing with AIS in 2007, we had all sorts of problems getting splitters to work right. Since then I've advocated going with a dedicated AIS antenna: Cheaper, good redundancy, less to go wrong. Remember to use good quality cabling (RG-8 or RG-213 or RG-8x for shorter runs). Ours is above our aft deck, 10' off the water, next to the solar panels (negligable shaddow from a SS whip). We get reliable reception out to ~25 miles, with some contacts at much longer ranges. We might get better performance out of a higher antenna, but remember that most AIS signals are only 2W. At some point, signal strength becomes more of a factor than Line Of Sight distance.
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Old 25-11-2010, 20:34   #7
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25 miles with an antenna 10' off the water is amazing. Your line of sight to the horizon is about 4 miles. The other ships antenna would need to be some 220' off the water. I have an AIS receiving station in my shop at work on the Severn River near Annapolis, MD and my antenna is about 75' feet off the water. My range to the horizon is 13 miles and my reliable reception is around 20 miles with commercial ships and less with vessels with class B transponders. My station runs 24 hours a day and logs everything. I check ranges every morning before purging all vessels. I do get reception to extreme ranges of up to several hundred miles when tropospheric ducting is in effect but this is not normal line of sight reception.

The information on your AIS web page is incorrect. You state; "When talking to an antenna that's 100' high (and most ships VHF antennas are even higher than that) then the theoretical difference between having your antenna at your masthead or only 10' (3m) above the water is only about 2 miles, hardly worth worrying about." The line of sight distance between an antenna at 100' and 10' is 18 miles. 100' and 60' is 25 miles. A difference of 7 miles. If your mast is only 20' high, then yes, the difference would be 2 miles. Antenna height is everything when it comes to marine VHF range.

Eric
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Old 25-11-2010, 21:45   #8
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Hi Eric - Thanks for this. Can you point me to the formulas you're using for LOS range? I thought I had them correct but I don't remember where I got the formulae. You're obviously much more into these sorts of calculations.

I only JUST installed my Camino-101 class B, but I've had an SR161 receiver for almost 4 years now (early 2007). Both used the same antenna.

My record AIS reception range that I've noticed (I have no logging capability) is over 300nm off the east coast of South Africa. As you point out, I'm sure that was ducting (cool - not many folks know about VHF ducting). But ALL VHF traffic seemed to go further there. We routinely communicated reliably by VHF voice with other small sailboats at 60nm, & once at 85nm. Way further than theoretical LOS would account for.
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Old 25-11-2010, 22:01   #9
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Ducting is very common, at least in some areas. It is somewhat seasonal. With my AIS antenna at the upper spreader (about 40'), we've often received AIS signals from hundreds of miles. For that matter, we were receiving California USCG VHF voice transmissions 1000 miles from land during last summer's trip from San Francisco to Hawaii and back. From my rooftop antenna at 1000 ft elevation, I've picked up AIS from well over 1500 miles distance.

However, the reliable range with the spreader antenna is much less, perhaps 10-25 miles. It's usually 25-50, but I wouldn't rely on that.
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Old 26-11-2010, 03:45   #10
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Being "leery" of spliters is no basis to make a decision. The fact is that the masthead site is the best place, expecially for transponders. many of the major companies including Garmin and Raymarine are including splitters into their AIS transponders.

Ive used to splitters types of AIS and there both giving excellant service. Remember most of the time your VHF antenna is doing nothing .

Dave
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Old 26-11-2010, 08:15   #11
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Quote:
Originally Posted by Jon Hacking View Post
Hi Eric - Thanks for this. Can you point me to the formulas you're using for LOS range?
I think Eric's distances may be in statute miles (1.6 km). The distance to horizon in nautical miles from a point h feet above the see level is approx 1.17*sqrt(h).
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Old 26-11-2010, 10:28   #12
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Hi Eric - Thanks for this. Can you point me to the formulas you're using for LOS range? I thought I had them correct but I don't remember where I got the formulae.
Your probably using the formula for the visible or geometric horizon and not the radio horizon which is a little bit further due to normal atmospheric refraction. The formula we techs use is d = 1.42 X (h to the power of 0.5) where d is in miles and h is in feet. You can also use the square root of 2 X height in feet.

Speaking of ducting, on October 28th in the early morning I had over 230 ships showing up with a max range of 212 miles. I'm in Annapolis MD and was seeing 30-40 ships in New York harbor.

I agree with goboatingnow on the splitters. They work well and have a failsafe mode so that in the event of failure or even power loss, your normal VHF will still be connected. In the rare event of a catastrophic failure, you could just disconnect the PL259 connectors and join them with a PL258 adapter to bypass the thing altogether.

Eric
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Old 26-11-2010, 10:37   #13
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I don't care for VHF splitters. Having a reliable VHF radio and a reliable AIS system is too important. Neither should be dependent on a splitter functioning correctly.

On boats it is a good engineering philosophy to set up systems so that the failure of one device minimizes the chance of failure of other systems.
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Old 26-11-2010, 11:02   #14
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Vesper Marine has a new splitter out that has low insertion loss and even has a pre-amp for the AIS receive port that provides 12db of gain.

Eric
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Old 26-11-2010, 21:04   #15
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AIS range vs antenna height

Quote:
Originally Posted by fairbank56 View Post
You're probably using the formula for the visible or geometric horizon and not the radio horizon which is a little bit further due to normal atmospheric refraction. The formula we techs use is d = 1.42 X (h to the power of 0.5) where d is in miles and h is in feet. Eric
Thanks Eric! So if a ship has its AIS antenna up 100' & mine is up 10' then I should be able to see them at
10': 1.42*(sqrt(100')+sqrt(10')) = 17nm.
If my antenna was up 60' then I should be able to see that same ship at
60': 1.42*(sqrt(100')+sqrt(60')) = 26nm.
Is this correct?

So, putting an additional AIS antenna on top of my mast would gain me 9nm or almost a 50% improvement. (I think my mistake originally was simply summing the antenna heights, not summing their square roots, so I got a gain of only 3nm or 20%)

But it's a lot of work & it removes the redundancy of having a VHF antenna if I lose the stick. And my personal experience with splitters (trying to get one working on a friends boat) has not been good. And I still submit that, even with my AIS antenna only 10' off the water, I see ships reliably at 20-25nm. Still, it's worth considering.
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