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AIS Transponder & transom antenna


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What disadvantages are there to using a dedicated antenna on the transom for an AIS transponder verses using a splitter and masthead antenna?

The boat will be used for coastal cruising and short offshore races.

Obviously the transmission and reception range will be less, that's not such a big deal unless you are mid ocean. There are no runners or anything else that could catch on the antenna. 

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Big ships have antenna placed high up which should compensate somewhat for your low antenna. However, shipping traffic can approach quickly. I have noticed that reception range is often not as great as one might expect. Depends if you will be traveling in shipping lanes. My worst encounters have been detecting tugs with tows in big seas. They are low, hard to see, and can approach fast.

It is a safety item. I’d say splurge on the splitter. Less clutter too.

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FWIW, I went with the splitter and a Vesper combo antenna at the masthead. But what made the biggest difference was replacing the 25 year old RG-8 cabling in the mast with proper LMR-400UF and good connectors. Both my VHF and AIS range increased 4X in one fell swoop even with the "compromise" antenna.

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7 hours ago, The Dark Knight said:

Obviously the transmission and reception range will be less, that's not such a big deal unless you are mid ocean.

I disagree. It's a bigger deal with coastal obstructions and islands that interfere with your signal reception the lower you put the antenna. The higher your antenna the better the visibility around the corners. and what IStream said, too.

Given that the range of vhf is approximately 1.23 * SqrRoot(antenna height in feet) + 1.23* Sqrt(other guy's antenna height in feet), I think getting the antenna high is important for the usefulness of AIS.

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Plus one for the splitter and getting it high up.  Have had two different splitter setups, never had a issue with either.  I think the separate splitter is preferable in the event you get zapped, small chance in not blowing up your whole n2k network.

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3 hours ago, ryley said:

I disagree. It's a bigger deal with coastal obstructions and islands that interfere with your signal reception the lower you put the antenna. The higher your antenna the better the visibility around the corners. and what IStream said, too.

Given that the range of vhf is approximately 1.23 * SqrRoot(antenna height in feet) + 1.23* Sqrt(other guy's antenna height in feet), I think getting the antenna high is important for the usefulness of AIS.

Using this equation, this difference works out to 3.9nm vs. 9.5nm on my boat with a 60' mast vs. 10' on a transom mount (as per news regs).  At 4nm visible range and a target moving 30kts (say a Ferry) and you moving at ~6kts , it would only give you 6 minutes 40 seconds between first becoming visible to a hypothetical collision.  Even with an alarm, that's not a lot of time.

I have the Digital Yachts AIT3000 with built in splitter.  I can see ships on AIS that would take me a long while to get to, even with a stubby race antenna and the thinnest cable I thought I could get away with. 

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4 hours ago, IStream said:

FWIW, I went with the splitter and a Vesper combo antenna at the masthead. But what made the biggest difference was replacing the 25 year old RG-8 cabling in the mast with proper LMR-400UF and good connectors. Both my VHF and AIS range increased 4X in one fell swoop even with the "compromise" antenna.

I believe LMR-400UF 0.375lb/ft  which would be 22.5lbs for my boat with a 60' rig vs. 6.3lbs for RG-8 (0.105lb/ft * 60').  I know I looked into it in January when I had the rig out and it seemed pretty massive to be putting into a carbon stick.  That and much more expensive.  My set up seems to work well for now, but I might have to replace it.

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8 minutes ago, Ultraman said:

I believe LMR-400UF 0.375lb/ft  which would be 22.5lbs for my boat with a 60' rig vs. 6.3lbs for RG-8 (0.105lb/ft * 60').  I know I looked into it in January when I had the rig out and it seemed pretty massive to be putting into a carbon stick.  That and much more expensive.  My set up seems to work well for now, but I might have to replace it.

Times Microwave (the manufacturer) quotes LMR-400UF as 0.088 lb/ft (131 g/m). Their LMR-240UF is 0.034 lb/ft (50 g/m).

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3 minutes ago, IStream said:

Times Microwave (the manufacturer) quotes LMR-400UF as 0.088 lb/ft (131 g/m). Their LMR-240UF is 0.034 lb/ft (50 g/m).

I'm sure I might be incorrect.  Just got the numbers off of https://www.gpscentral.ca/accessories/lmr-400-uf-ultraflex-coax-cable.html.

These numbers would work out to 2.62kg vs. ~1kg for my 20m mast.

What I do remember from my visit to Lee's Electronics in February was thinking: "Fuck me, I can't be putting that thing in my mast" and then scoffing at the price before walking out the door.

 

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4 hours ago, ctutmark said:

if the races you are sailing use the Offshore Special regs you will want to be sure your install meets the rule requirements  Rule 3.29.13

https://www.sailing.org/tools/documents/WSOffshoreSpecialRegulations20202021updated11December2020-[26824].pdf

 

The Australian SR's make no mention about the AIS antenna. VHF must be masthead and you must have a spare for emergencies.

 

Anyway, I will advise the owner to go for the splitter.

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43 minutes ago, The Dark Knight said:

The Australian SR's make no mention about the AIS antenna. VHF must be masthead and you must have a spare for emergencies.

 

Anyway, I will advise the owner to go for the splitter.

If the MH antenna is already required then going the splitter route makes good sense. 

 

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  • 2 weeks later...
On 12/21/2020 at 12:17 PM, Ultraman said:

Using this equation, this difference works out to 3.9nm vs. 9.5nm on my boat with a 60' mast vs. 10' on a transom mount (as per news regs).  At 4nm visible range and a target moving 30kts (say a Ferry) and you moving at ~6kts , it would only give you 6 minutes 40 seconds between first becoming visible to a hypothetical collision.  Even with an alarm, that's not a lot of time.

The 3.9 nm range is you have 10' on the transom - and the other guy's antenna is at zero.

If you are worried about big fast ships or ferries, their antenna is likely ~50' off the water. So the combined height range is ~13 miles. 13 miles/(30+6 knots) = 22 minutes warning.

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Went for the B&B/Simrad offerings to go with the Vulcan display. Also picked up a spare antenna that will be stored as a spare. It allowed me to bench test it from the comfort of home.

I did learn that the AIS/MFD won't talk to each other through the NMEA2000 backbone unless the NMEA2000 had power. I had assumed that the power was only needed for instruments and sensors that did not have a seperate power source.

 

 

 

IMG_0780.jpg

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For those with less expensive tastes I have been running one of these for a couple of years and it works great. They come with or without GPS receiver. I got the one without the receiver which I regret as I had to mess around wiring in from my Garmin for location. Cost me around $100 at the time plus a $30 splitter from glomex. Edit: woops after posting I see that the focus is on transponders. Still might be interesting for someone.

 

image.thumb.png.da21f3aae0a234dd6b350965ed850daa.png

 

Edited by 23feet
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On 12/21/2020 at 12:31 PM, Ultraman said:

I believe LMR-400UF 0.375lb/ft  which would be 22.5lbs for my boat with a 60' rig vs. 6.3lbs for RG-8 (0.105lb/ft * 60').  I know I looked into it in January when I had the rig out and it seemed pretty massive to be putting into a carbon stick.  That and much more expensive.  My set up seems to work well for now, but I might have to replace it.

I went with lmr200-db when I redid the coax. I felt it was a good compromise between the 400 and rg-8 or 58. 

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On 1/4/2021 at 1:31 PM, Bruno said:

The Vesper does look great but it's almost twice as much as the B& G and the Simrad offerings. Any word on which of those two are better?

I think the Panbo article makes it abundantly clear which AIS is best. Yes, the price is prohibitive if you already have a VHF radio you don't plan to replace.

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Because VHF at max power is limited by the horizon, antenna gain and cabling losses are not as important as height and placement issues. The few dB saved with a fat, heavy and expensive coax are not worth it in my professional opinion.

One of the AIS displays I had, cannot recall which, had a too short persistence of target display. Not very helpful to the busy sailor. The repetitive nature of AIS causes distant targets to be received sporadically at great distances. The designer must have thought it normal for huge ships to flash in and out of actual physical existence.....

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I use a spitter to my masthead antenna

We do several overnight races on Chesapeake Bay right at the point where the commercial channel goes from the eastern shore to the western shore.

On a rising tide there are LOTS of big tankers and 1300 foot container ships barrelling up the bay to be in Baltimore with the tide while lots of others are coming out of Baltimore.  And usually being a weekend, there are the big cruise ships to nowhere vessels lurking around.  So we have boats coming up and down the bay at 22 knots or so trying to avoid each other at a dogleg where we are usually ghosting along against some foul tide late in the evening.

I have a raymarine ais650 transponder.  When racing, I get LOTS of radio calls from the big boys asking intentions.  Having that transponder up high gets their attention long before I see them.  For them to avoid you, they first need to know you are out there.  someone said you have 22 minutes. What if a chore took their concentration away for several minutes?  Here is where your blip showing up on their screen earlier is good.  Ditto for you receiving their signal.

No weight penalty if you have a vhf, so why not use the masthead?  The splitter is smart enough not to transmit if you are using the radio.

Also, think of the extra range should things go pear shaped. You or a crew member has a serious injury and help is trying to get to you.  That extra couple miles range may save a life because they can find you faster.

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That last sentence is quite relevant - if you are searching for an AIS MOB signal, the masthead ant will be looking down at the water surface, rather than across the wave tops. Should increase range & signal acquisition a lot.

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