AIS Antenna Height

[Pinnacle]

So I am about to purchase an AIS transponder plus a chart plotter ( good old LIBS chose it there)

if height of an aerial is a problem what would happen if i undid my backstay ( No the mast would not fall down) swung it in to the mast
stood on the cabin & taped the aerial to the backstay as high as possible ( stand on a box etc) then re rigged the backstay
I would get plenty of height, have little to snag on- lines sail etc
BUT-- here is the big question---would the backstay wire upset the aerial reception?

I also intend to fit an EChomax active radar reflector- could I do the same with that?

Our transponder aerial is part way up the backstay.

We have a single backstay which comes down to a block over which passes another ss wire which goes from/to the aft corners of the boat. one side is adjustable so we can adjust the tension in the rig.

The AIS aerial ( short black stubby type ) is mounted where the fixed part of the backstay ends at the turning block and the cable is run down the fixed part of the adjustable bridle. We get good reception and ( based on the experience of friends who were watching their own plotters ) good tx too. I would guess the height of the aerial above sea level to be about 4 to 5 metres.

Because we usually sail with a reasonable amount of tension in the backstay, the antenna is held pretty firmly in position and we have not had any issues with it "wafting around" as described by Salty John. I do appreciate if we were to loose the rig we will lose our AIS aerial......thats why I carry an emergency antenna on board - one each for VHF and AIS.

You can see it at 6min 15 secs here. Best watched in HD full screen!

 

[Daydream believer]

Slight thread drift. May I ask which transponder and plotter you have chosen? I am going through the same process, but still not 100% convinced about the need for the transponder?

Neil

I have chosen a Lorenz X7 plotter- expensive but good display & very user friendly - with full map of uk at the ridiculous price of £297-00.
The main point was that it has a detachable section so I can use it on deck & take below when not in use . I can also have another base unit & use it below for passage planning
To match this I chose the Echomax reflector & Amec Camino 101a transponder ( 459-00)- Mainly because I can get the complete package from Maintsbrite who are only 10 miles from me.
I have thought about just getting a receiver & they have offered me the Amec Cypho 150 dual channel receiver which would save £ 230-00
But I see from the spec for the plotter that Lorenz do an AIS receiver or transponder specific for that plotter so when I go to collect on Wed I am going to have a quick chat to see if it would be better

I was going to get a Vesper( think that is the name) transponder from Cactus but when I went on the stand the sales people were very off hand & one Polish salesman could not communicate easily. They put me off dealing with them very quickly. Plus I could not get an accurate price from them as the chap who I finally spoke to said it was a Polish computer & he could not use it properly!!!!!

The lady on Maintsbrite stand knew the product inside & out & gave me a very excellent demonstration & spent time with me & it gives me a lot more confidence in who I am dealing with

 

[Daydream believer]

Our transponder aerial is part way up the backstay.

We have a single backstay which comes down to a block over which passes another ss wire which goes from/to the aft corners of the boat. one side is adjustable so we can adjust the tension in the rig.

The AIS aerial ( short black stubby type ) is mounted where the fixed part of the backstay ends at the turning block and the cable is run down the fixed part of the adjustable bridle. We get good reception and ( based on the experience of friends who were watching their own plotters ) good tx too. I would guess the height of the aerial above sea level to be about 4 to 5 metres.

Because we usually sail with a reasonable amount of tension in the backstay, the antenna is held pretty firmly in position and we have not had any issues with it "wafting around" as described by Salty John. I do appreciate if we were to loose the rig we will lose our AIS aerial......thats why I carry an emergency antenna on board - one each for VHF and AIS.

I have exactly the same sort of backstay set up I did think about loosing the aerial if the backstay broke. But I think I will see if the aerial will fit in a plastic tube. I will fix the tube to the stay. Poke the aerial up it & hold it in place with a foam plug. Even if the mast went the backstay will still not let the wire stretch so the aerial will not be pulled. The mast will not go if the backstay broke as it will stay up with virtually no backstay tension & I often slacken it right off. I can also fit a thin dynema line along the wire to stop it overstretching the aerial wire.
The main point is that your experience shows it would work fitted to the backstay-- Thanks

 

[Pinnacle]

Even if the mast went the backstay will still not let the wire stretch so the aerial will not be pulled.

If the backstay fails on the static side of the bridle, you may well lose it!

 

[Daydream believer]

If the backstay fails on the static side of the bridle, you may well lose it!

Point taken-- but if the aerial pulled the foam plug out of the tube it would just flop down (& hit me on the head)

 

[Pinnacle]

Point taken-- but if the aerial pulled the foam plug out of the tube it would just flop down (& hit me on the head)

 

[Andy_H]

The backstay mounting looks intersting.

I originally had my AIS transponder fitted with a splitter, but the splitter interfered with VHF transmissions. I now have a dedicated AIS antenna fitted on the pushpit, but the range is greatly reduced. On a channel race last year others in the fleet lost our transmission at about 4 or 5 miles, mounting the antenna on the backstay looks like it might be a fair compromise. How did you fix the antenna to the backstay?

 

[PeterR]

Just to give some idea of range: On a small boat a pushpit mounted antenna might be 10' above sea level and will be presenting it's transmission up to 4 miles distance. If there is a similarly located receiving antenna on another small boat they will communicate at 8 miles. If the receiving antenna is on a larger boat, let's say 60' above sea level, then combined range will be about 13 miles. If it's on a ship at 100' then 16 miles is about the range. I think those ranges are OK for AIS given the likely speed of the vessels involved, others may disagree.

You can increase range by mounting the whip antenna on a pole - at 20' above sea level this would present the signal at 7 miles, combined range of 14 miles to a similarly fitted boat and 19 miles to the ship.

Two yachts with masthead antennas 40' above sea level will communicate at 16 miles. Masthead antenna to a big ship, about 20 miles.

These are theoretical figures, taking into account the slight gain through the curvature of the earth, but can be exceeded in certain atmospheric conditions - unfortunately you can't count on those conditions.

This is all true if you are fitting a Class A transponder but remember a Class B transponder is limited to 2 Watts. As far as your signal being received by a ship the maximum reliable range is about 7 to 10 miles – according to a friend who works on big ships – regardless of the siting of the antenna. He says he normally picks up yachts on radar long before he sees any AIS signal. Similarly you can expect a maximum of 7 to 8 mile reception between yachts with pushpit mounted antennae but you only gain an extra mile or two with masthead AIS.

A masthead antenna does pick up signals from ships transmitting on Class A equipment long before a pushpit mounted one receives anything. I normally expect about 25 miles range.

However, manufacturers recommend that if you are transmitting on AIS the antenna should have at least 1 metre vertical and horizontal separation from your VHF antenna.

I am sure someone will come on and say that they have seen and been seen at ranges much greater than this. My record has been tracking the Holyhead - Dun Laoghaire ferry whilst berthed in Bangor over 100 miles away. However, the fact remains that the strength of a Class B AIS signal is only about the same as a handheld radio on low power and you cannot expect line of sight transmission distances.

 

[William_H]

Anyone racing at ORC CAT2 (Fastnet/ round Ireland etc) the new regulations state a masthead antenna for the transponder. We have a full size whip for ours on the back - it's neatly out the way. But now facing the fact we need a masthead one!

I can not imagine how you could get decent spacing of 2 VHF antennae on one mast top. Unless one is on a sort of crane facing down ward from top. Or do they not require VHF Com antenna on top. seems absurd to me.
I have a stern rail mounted VHF com antenna and have always been bpleased with range and performance in my limited range of sailing but frequent use of VHF com. olewill

 

[onesea]

To me part of the thing that is being over rated here is the amount of time both AIS and VHF are transmitting at the same time.

If the ORC have requested mast head units they must have noticed a difference, I do not believe they would change the rules for no reason.

For me I am still thinking a splitter but then you take your choices...

 

[Pinnacle]

The backstay mounting looks intersting.

I originally had my AIS transponder fitted with a splitter, but the splitter interfered with VHF transmissions. I now have a dedicated AIS antenna fitted on the pushpit, but the range is greatly reduced. On a channel race last year others in the fleet lost our transmission at about 4 or 5 miles, mounting the antenna on the backstay looks like it might be a fair compromise. How did you fix the antenna to the backstay?

I am afraid i dont have any pics that clearly show how its done - I will try and remember to take a few when I am next on the boat.

So, here goes with a description;

Talking to a rigger at SIBS in 2011, he advised against using any sort of crimp or clamp onto the backstay stainless wire itself, as it is possible it could deform the wire and create a weak point. So instead, I have fixed it to the eye swage at the bottom of the fixed part of the backstay. To keep the aerial vertical, I made a small teak wedge that sits between the antenna bracket and the swage. The bracket and the wedge are fixed to the swage by bolts that pass through an eye strap similar to the one in this picture; http://www.ebay.co.uk/itm/2-X-4MM-WIRE-EYE-STRAPS-IN-STAINLESS-STEEL-316-/151123634605?pt=UK_CarsParts_Vehicles_BoatEquipment_Accessories_SM&hash=item232faba9ad#ht_2382wt_1388

Its not a great effort, but below is my attempt at a sketch to show the fixing position and method. Hope it helps.

View attachment 38850

 

[timmygobang]

I'm going to throw a curve ball here. My AIS aerial was fitted in a cabin locker next to the nav table (not my choice but what's done is done). I get very good range, and its using a VHF splitter. I know some people say its all the same technology and components, but maybe there is something in spending a little extra on your AIS

 

[lw395]

I am afraid i dont have any pics that clearly show how its done - I will try and remember to take a few when I am next on the boat.

So, here goes with a description;

Talking to a rigger at SIBS in 2011, he advised against using any sort of crimp or clamp onto the backstay stainless wire itself, as it is possible it could deform the wire and create a weak point. So instead, I have fixed it to the eye swage at the bottom of the fixed part of the backstay. To keep the aerial vertical, I made a small teak wedge that sits between the antenna bracket and the swage. The bracket and the wedge are fixed to the swage by bolts that pass through an eye strap similar to the one in this picture; http://www.ebay.co.uk/itm/2-X-4MM-WIRE-EYE-STRAPS-IN-STAINLESS-STEEL-316-/151123634605?pt=UK_CarsParts_Vehicles_BoatEquipment_Accessories_SM&hash=item232faba9ad#ht_2382wt_1388

Its not a great effort, but below is my attempt at a sketch to show the fixing position and method. Hope it helps.

View attachment 38850

I have sailed on a boat with something similar and it worked well.
IIRC the bracket was simply clamped to the backstay wire, after padding the wire with self amalgamating tape.
It did wobble a bit under engine TBH, but it gave good range.
I think it is key that the aerial is spaced away from the stay and at an angle to it.
Having it close and parallel as suggested in another post is bad news, the aerial would almost be shorted by the stay at RF, possible reducing its effectiveness grossly.

 

[Pinnacle]

I'm going to throw a curve ball here. My AIS aerial was fitted in a cabin locker next to the nav table (not my choice but what's done is done). I get very good range, and its using a VHF splitter. I know some people say its all the same technology and components, but maybe there is something in spending a little extra on your AIS

I find this rather "odd".

We all know VHF is basically limited by line of sight ( yes I know its more complicated than that, but it will suffice for this purpose ) and if you have your VHF aerial located this low down, I must say I am surprised when you say you get reception. Do you have a receiver or a transponder? I can see how you might get reasonable range receiving from Class A transmitters, but I do fail to see how you will get good TX from such a low position.

Forgive me for being cheeky but is it your VHF antenna thats in a locker and not the GPS mushroom that supplier the AIS with data? I ask this because our GPS mushroom is located in the boat, under the deck behind the plotter at the chart table. It works fine.

 

[timmygobang]

I find this rather "odd".

We all know VHF is basically limited by line of sight ( yes I know its more complicated than that, but it will suffice for this purpose ) and if you have your VHF aerial located this low down, I must say I am surprised when you say you get reception. Do you have a receiver or a transponder? I can see how you might get reasonable range receiving from Class A transmitters, but I do fail to see how you will get good TX from such a low position.

Forgive me for being cheeky but is it your VHF antenna thats in a locker and not the GPS mushroom that supplier the AIS with data? I ask this because our GPS mushroom is located in the boat, under the deck behind the plotter at the chart table. It works fine.

ahh, ok I'm being a complete noob, yes its the mushroom. ok ignore me! I'm slowly backing away from the conversation

 

[Salty John]

That looks a really good arrangement. The antenna is vertical and held away from the stay.

 

[Twister_Ken]

That looks a really good arrangement. The antenna is vertical and held away from the stay.

Well, yes, but only vertical when the boat is not heeling or pitching or rolling.

 

[Salty John]

Well, yes, but only vertical when the boat is not heeling or pitching or rolling.

True. Perhaps he could have gimballed it.

 

[Rivers & creeks]

Does an AIS transponder setup require two aerials, one for transmission and one for reception?

 

[haydude]

I changed over to try with the VHF mast head antenna and immediately appeared on Marine Traffic.
Should I now consider a splitter instead?

Yes. This is because your Class B transponder transmits at 2 or 3 watts only and to increase the chances that the low power signal is picked up you need an antenna that is clear of obstacles. The VHF mast antenna is the best you have on board and it is worth using a splitter like EasyOCB that I can recommend because it is the one I have been using for years.