AIS

pandos said:
As a teenager I used to build directional antennas for my CB...my memory was that the elements had to be precisely cut to length and carefully positioned to have any positive effect.
Click to expand...
Yes, for the best effect they have to be precise. But there is an effect even if the lengths aren't ideal. Because both antennae are for the same frequency range (AIS is in the VHF range) they won't be far from the right length anyway, and they will have a wider bandwidth than the single frequency CB band.


pandos said:
yes but is the loss in those directions not mitigated by the additional height of using the masthead? Also if the two antennas are substantially different in size ( One is a stubby) surely that won't cut off transmitting and receiving totally in some directions..)

Many mast head lights have some vertical metal elements in them do these not cause the same issue..
Click to expand...
No, because in some directions there will be very little transmitted. And it's not just vertical elements, it's vertical elements that resonate at the frequencies concerned. As both VHF and AIS are at similar frequencies (AIS is within the VHF band) their antennae resonate at the same frequencies.
 
Refueler said:
You seem to forget the 25W VHF signal and what it could do to the AIS ....
Click to expand...
It’ll be 12 at best at the antenna, and half that again by the time it’s back down even if we pretend there won’t be losses between the antennas.

Either way…splitter!
 
Refueler said:
But its still way higher than the Receive section is designed to deal with.
Click to expand...
A splitter solves that, although I’d be surprised if there would be any damage. Either way a splitter is the answer, there’s no good reason not to use one.
 
lustyd said:
A splitter solves that, although I’d be surprised if there would be any damage. Either way a splitter is the answer, there’s no good reason not to use one.
Click to expand...
It probably won't cause damage, but it will overload the input and make it impossible to receive.
 
I have a splitter for the masthead antenna to do AIS and VHF. There is a spare antenna on the solar arch that is routed to the splitter but not connected.

If I lose the masthead antenna I can connect the spare to the splitter and still have VHF and AIS with reduced range.
If I lose the splitter I can conect the AIS to the spare antenne and still have both, but AIS with reduced range.
If I lose both masthead and splitter, then I need to choose between VHF and AIS or switch back and forth.

... at some point the backup solutions start to become ridiculous, so after much deliberation this seemed to be the best compromise for failure tolerance and functionality IMO.
 
Hmmm.
The theory and the practice.
Mine works and has worked for the last ten years without any known problems (apart from the current one resoldering the inner at the masthead plugs - good fun!).
I like redundancy (except when it happened to me!).
Twin everything takes a lot of worry out of sailing.
 
Baggywrinkle said:
I have a splitter for the masthead antenna to do AIS and VHF. There is a spare antenna on the solar arch that is routed to the splitter but not connected.
Click to expand...
We have the same, mainly because I originally didn’t want a splitter, mainly because of bad advice on this forum.
What was the AIS antenna on the pushpit is now the spare as you describe.

Even better, we now get excellent reception on the FM radio out of the same splitter
 
Graham_Wright said:
Hmmm.
The theory and the practice.
Mine works and has worked for the last ten years without any known problems (apart from the current one resoldering the inner at the masthead plugs - good fun!).
I like redundancy (except when it happened to me!).
Twin everything takes a lot of worry out of sailing.
Click to expand...
You keep saying this is theory. I can guarantee that in practice having 2 antennas within the 1st Fresnel zone of each other will change the radiation pattern to something with a null - this is a terrible idea for a sailing boat's primary distress comms.
(This is how you design steerable arrays of antennas - something I've done a lot of)

I would a) use a powered RF switch designed for this purpose (some people have accidentally installed a passive splitter - so I'll avoid calling it an active splitter) and b) add a second antenna at the pushpit as an emergency spare.
 
Baggywrinkle said:
I have a splitter for the masthead antenna to do AIS and VHF. There is a spare antenna on the solar arch that is routed to the splitter but not connected.

If I lose the masthead antenna I can connect the spare to the splitter and still have VHF and AIS with reduced range.
If I lose the splitter I can conect the AIS to the spare antenne and still have both, but AIS with reduced range.
If I lose both masthead and splitter, then I need to choose between VHF and AIS or switch back and forth.

... at some point the backup solutions start to become ridiculous, so after much deliberation this seemed to be the best compromise for failure tolerance and functionality IMO.
Click to expand...

I kept my pushpit mounted antenna as back up in case of losing mast ... I could then use for VHF or AIS .. or the splitter.
 
doug748 said:
Bin the splitter idea and put the emergency aerial on the pushpit. The standard alternative, with a lot to commend it.
Click to expand...

We shall agree to completely and totally disagree .....

The difference in performance of the AIS .. rail vs masthead is seriously significant.

I used to have my AIS via rail antenna ... but changed to splitter to Masthead. Even though my splitter is the Onwa Passive Splitter - the improvement was massive.

Maybe worth reading my test thread on the ONWA splitter I fitted ....

Refueler

Post in thread 'Follow on from my VHF - AIS Splitter thread recently >>'

Update :

Onwa later communication made plain that their Splitter would not damage any item and in fact if it did - they would replace any damaged item.

Cannot be fairer than that ....

Onto testing.

Note :
Masthead antenna is approx 8m above ground level.
Rail mounted standard VHF antenna. The boat sits in a channel with a bank of about 2 - 2,5m. This puts the rail antenna only just above ground level. The channel is upriver about 2nm as crow flies from Inner Port area - through housing estates and general town constructions as seen here >

OogotzJl.jpg
...
  • Like

Not only does it compare the two mounts - but also shows why you should be careful what splitter you buy .. I give comparison of a VHF frequency splitter from eBay vs the VHF - AIS splitter unit ...

Why did I change from rail antenna ... on a trip across baltic ... I was losing targets due to wave and swell height ... which in my mind - meant I was likely not a steady target to others ....

My later acquired boat has splitter and when I venture out from harbour - I often get called by my pal ... HI Nige - out sailing again !! With my previous rail setup - he never called me ... (he watches online as well as on his boat ..)
 
st599 said:
You keep saying this is theory. I can guarantee that in practice having 2 antennas within the 1st Fresnel zone of each other will change the radiation pattern to something with a null - this is a terrible idea for a sailing boat's primary distress comms.
Click to expand...
May or may not be a simple question...if the metz is located approximately 300mm from the stubby Ais, both mounted at practically the same height, how big would the null area of the vhf be? I presume it is like a piece of an apple...but is it a good big wedge of say 45 degrees or is it a thin wedge of a degree or so...
 
Graham_Wright said:
The above head lining cable is roughly three metres in length, dangling freely and obviously not terminated. Despite that, it was apparently receiving information quite happily and occasionally transmitting.
Click to expand...

Many years ago I recall futzing about with cables behind the TV, and had also been playing a VHS tape on the VCR. It took my brain some time to realize that the video was still playing even though the cable was not physically connected.
 
pandos said:
May or may not be a simple question...if the metz is located approximately 300mm from the stubby Ais, both mounted at practically the same height, how big would the null area of the vhf be? I presume it is like a piece of an apple...but is it a good big wedge of say 45 degrees or is it a thin wedge of a degree or so...
Click to expand...
I would need to model it, but I'd expect a cardioid pattern, the extent of the null all depends on the interaction.

You then also have the issue of the power from the VHF stopping reception on the AIS and vice versa.
 
st599 said:
I would need to model it, but I'd expect a cardioid pattern, the extent of the null all depends on the interaction.

You then also have the issue of the power from the VHF stopping reception on the AIS and vice versa.
Click to expand...
With a splitter doesn't receive and transmit from the other device also end when transmitting from the vhf..?

the vhf is a Metz manta 6 with messi and poloni cable and fittings ( it's about 10mm, )
.https://www.bluemarinestore.com/metz-manta-6-stainless-steel-vhf-whip-antenna/

The ais is one of these. Using the cable that came with it.
https://www.chmarine.com/v-tronix-stub-ais-antenna-ha156c-ais/

There are no joints or breaks in either of the cables. The mast is about 12m.
 
You must log in or register to reply here.

Share this page

Top