AIS Antenna on Backstay??

Dave100456

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I'd appreciate advice from those knowledgeable with VHF antenna with this solution to mounting my AIS antenna.

Background: My backstay clears the leech of the main and I can fabricate the mountings.

Plan: Attach a 900mm whip pivoted at its top using a flexible joint to the backstay so that it hangs like a plumb line. The antenna's base would be on a rigid rod that triangulates to the backstay and allows the antenna to pivot a limited amount left and right and hence gimbal. Coax would run along rod and down backstay.

I guess I need to know:
1 Would the antenna's performance suffer if its top is so close to the backstay?
2 Would the receive/transmit cope with the movement of the antenna if in a swell?
3 Does the antenna need to be insulated from the backstay.

Thanks
Dave
 

30boat

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It all sounds overcomplicated to me.I fitted a splitter and use the main VHF aerial. Works very well.
 

Pinnacle

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When I did ours - see pic

AIS antenna mounting.jpg

the advice I received was;

1 Don't use a splitter
2 The aerial must not be electrically connected to the rigging
3 The areal must not be parallel to any rigging wire
4 5 metres above the waterline was probably high enough ( but if you can go up to first spreader height, that would be good).
5 The aerial shold not be placed horizontally less than 50 cms away from any other aerial.
6 Don't clamp any fixing onto the rigging wire itself - only to the solid parts or terminals or turnbuckles - as you could weaken the wire if you over tighten the clamp.

Our installation met all of these criteria and worked well.
 

Dave100456

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When I did ours - see pic

View attachment 52977

the advice I received was;

1 Don't use a splitter
2 The aerial must not be electrically connected to the rigging
3 The areal must not be parallel to any rigging wire
4 5 metres above the waterline was probably high enough ( but if you can go up to first spreader height, that would be good).
5 The aerial shold not be placed horizontally less than 50 cms away from any other aerial.
6 Don't clamp any fixing onto the rigging wire itself - only to the solid parts or terminals or turnbuckles - as you could weaken the wire if you over tighten the clamp.

Our installation met all of these criteria and worked well.


Thanks for posting the drawing. Can I ask if you're satisfied with the performance of the stubby antenna and how high it is above W/L. Is your AIS a transponder or just receiver?
Thanks
 

30boat

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Whilst a splitter was your choice mine is to have a second antenna for a number of reasons including cost, desire to have second antenna and performance.
Thanks
Welcome.But was there any technical reason behind your choice?
 

Pinnacle

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Thanks for posting the drawing. Can I ask if you're satisfied with the performance of the stubby antenna and how high it is above W/L. Is your AIS a transponder or just receiver?
Thanks

It was probably about 5m above the w/l. We had the advantage of an adjustable backstay which offered a suitable mounting point.

Ref the other point, we had good performance from the stubby aerial and at no time did I feel it had "blind spots". It was wired to a transponder and seemed to have good range. Remember, the aerials on the large metal lumps are going to be well above the w/l - perhaps 25m above - so you will have good line of sight to/from the two aerials.
 

bbg

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FWIW when I crossed the pond my AIS antenna was a stubby rubber one that I fitted at the transom, a few inches away from a pushpit stanchion (to give a bit of protection to the antenna). I was picking up freighters at more than 15 miles.

If you wanted better performance just fit a larger whip antenna to the transom.

Do you really need the antenna that high? Is it important to pick up traffic at 40 miles instead of picking it up at 20 miles or even 12 miles?
 

Dave100456

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It was probably about 5m above the w/l. We had the advantage of an adjustable backstay which offered a suitable mounting point.

Ref the other point, we had good performance from the stubby aerial and at no time did I feel it had "blind spots". It was wired to a transponder and seemed to have good range. Remember, the aerials on the large metal lumps are going to be well above the w/l - perhaps 25m above - so you will have good line of sight to/from the two aerials.

Thanks for relating your practical experience. I'm keen to have a good transmission of my AIS data as well as see others. I have no desire to see/transmit to targets 20+ miles away and your positive experience with the stubby on backstay is reassuring.
 

bbg

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Thanks for relating your practical experience. I'm keen to have a good transmission of my AIS data as well as see others. I have no desire to see/transmit to targets 20+ miles away and your positive experience with the stubby on backstay is reassuring.

As I noted, I had pretty good experience of a stubby on the deck at the transom. Perfect is the enemy of good.
 

Daydream believer

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I was advised by the supplier not to mount parallel to the backstay due to interference.. I had queried whether i could put it inside a plastic tube just taped to the wire.
I did not put it vertical as suggested because I was worried it might rotate & instead of pointing vertical it would point just above horizontal & foul the sail.
Of course mounting it to the rigging screw would stop rotation. However if one wanted an emergency VHF aerial in the event of dismasting it would not be a good idea as it could get broken when the rigging screw went slack
in the end I mounted it on an extending pole ( a sailboard boom section) on the transom. It is about 7 ft above waterline & I can get an extra 2 feet above that if I extend it.
range is well over 15 miles in most situations. Less with smaller boats such as fishing boats. Presumably because their aerials are lower than large ships
As for ship seeing me - yes a good point- but so long as I can see them then I should be able to contact them or get out of their way OK
 

RivalRedwing

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Happy to see things a good few miles off but also keen to ensure my transmissions are seen by them and not ignored by their software if I'm in and out of troughs.

Are ships actively monitoring AIS from small boats? I thought Radar was their preferred medium. Fwiw I've also noticed the gradual proliferation of small boat AIS signals and find them more of a hinderance than an asset, I wish I could turn their signals off so I only see the big ships.
 

Koeketiene

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Are ships actively monitoring AIS from small boats? I thought Radar was their preferred medium. Fwiw I've also noticed the gradual proliferation of small boat AIS signals and find them more of a hinderance than an asset, I wish I could turn their signals off so I only see the big ships.

Inshore, I doubt it.

However, in all the Channel crossings we did since installing our AIS transponder (including night time crossings), I have always observed merchantmen taking avoiding action (if required) when we crossed the TSS.
The only ones who don't seem to take a blind bit of notice are fishermen. In my experience, not only don't the look at it, more often than not they don't transmit AIS - even when, because of their size, they are required to do so. Possibly because they are fishing where they are not supposed to.
 
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