Antennas too close together?

[pandos]

I fitted my vhf (stainless rod) and ais (stubby rubber yoke) on the top of my mast. I think they were about 300 mms apart. I never used the vhf except to receive, but the ais functioned brilliantly.

I am thinking of repeating this practice but I have read greater separation is need pr they will interfere with each other.

Is this just a theoretical issue or have others experience of problems.

There is one other boat in the yard that seems to have a similar set up..and subject to the answers received here my friend may do the same.
Many thanks..

 

[William_H]

Yes it is bad practice to have 2 antennae essentially using same frequency close to one another. Just how much iot matters is a variable question. it can cuase large voltages to appear in the receive circuits when the other is transmitting which could cause damage. But then I have never heard of that. The antenna nearby can distort the radiation or receive pattern possibly limiting range beyond in line with the other antenna. But then you may never notice it. The radio boffin purists would never do it.
I would suggest AIS on stern rail which seems to provide adequate range but also gives an alternative VHF antenna for mayday if you lose a mast. Worth arranging cables so AIS antenna can be connected to VHF com radio. ol'will

 

[lustyd]

Since the AIS won’t work when transmitting VHF you may as well fit a splitter and use a single antenna. That way you benefit from the range with height.
I had a second antenna on the pushpit and it was fine but fitted a splitter (for FM reasons) and the range improved on AIS quite a bit so my pushpit antenna is now just a spare.

 

[onesea]

Your going to get lots of responses for this one.

Spliters - reduce power out put, reduce the need for 2 Ariel's, mean your AIS transmission will frequency will not be tuned to the ariel reducing efficiency,

2 Ariel's -Mens problems getting separation, reduced height on one Ariel. However AIS Ariel can be tuned to the right frequency. If not on same mast gives an emergency Ariel,
Means more clutter around boat.

Some also question if your ais antenna needs to be at top of mast.

All compromises I went for splitter.

 

[lustyd]

Some also question if your ais antenna needs to be at top of mast.

I've not seen anyone question whether it's better higher up (and if they did, they'd want to re-do their radio training!). Some just ignore that and claim that good enough is good enough.
Personally I think the pushpit worked well enough when we used it. In hindsight though, the additional installation effort for a reduced range was not a sensible decision. A good splitter doesn't affect the signal much at all, it's easier to install and gives far better range so (again, in hindsight) seems a no brainer to me.

As you say, if I had two masts I might think differently, but then you'd lose the benefit of the emergency antenna on the pushpit should the masts come down.

 

[Alex_Blackwood]

Since the AIS won’t work when transmitting VHF you may as well fit a splitter and use a single antenna. That way you benefit from the range with height.
I had a second antenna on the pushpit and it was fine but fitted a splitter (for FM reasons) and the range improved on AIS quite a bit so my pushpit antenna is now just a spare.

Why not if they are on two separate antenna? If using a splitter yes the receive will be disabled for the AIS. If, as the OP says, he has two separate antennas then they need greater separation than he has at present. That was his question. the reason for the separation is to prevent damage to the AIS system, antenna included, when transmitting on VHF.

 

[ylop]

Since the AIS won’t work when transmitting VHF you may as well fit a splitter and use a single antenna. That way you benefit from the range with height.

But, adding a splitter will result in some loss (both in Tx and Rx on the main VHF). I guess if you boat in busy areas when AIS likely matters most then range to CG may not be a point of concern.

I've not seen anyone question whether it's better higher up (and if they did, they'd want to re-do their radio training!). Some just ignore that and claim that good enough is good enough.

Putting AIS really high will get you most AIS signals and stuff to plot on your plotter. On a yacht doing say 7 knots - its not obvious what advantage that might have (at least for collision avoidance on a boat with a good lookout - singlehanders trying to grab a powernap may be different) over an anatanae at a much lower height - probably giving 3+ miles of range?

Why not if they are on two separate antenna? If using a splitter yes the receive will be disabled for the AIS. If, as the OP says, he has two separate antennas then they need greater separation than he has at present.

My expectation is that when making voice comms on the VHF the signal from his main antenna just 30 cm from the AIS one will swamp the incoming AIS signals.

 

[geem]

We have twin vhf radios with an antenna on each mast. The separate AIS antenna also shares the top of the mizzen mast but with 300mm separation from one of the VHF antennas. We have good AIS transmit and receive signals. I think having two separate antenna for vhf and AIS is preferable to a single antenna and splitter.

 

[Refueler]

Loss on the splitter depends on whether its Passive or Powered ....

Most powered will bring the signals near enough to separate power equivalents.

Separation of the antenna - its bad practice to have them too close - the VHF when transmitting could cause serious induction to the AIS antena ..

 

[B27]

Two parallel aerials at the same height can theoretically become a directional array.
So you could create a null where you don't see ships on the AIS.
Whether that happens in practice with a stubby and a whip? Well you won't really know until you measure it?

Most people don't speak often on the VHF so voice Tx drowning AIS Rx is probably not a real concern.
AIS Tx drowning voice Rx can be more irritating IME.

At sea, AIS seems to need only a couple of metres of antenna height to show all the ships you care about, as the big fast ones tend to have higher antennas themselves. If you want to sit in a marina and watch the ships on AIS in the Solent, then you gain from a masthead AIS antenna. It could be handy to see what's around the next band in the river.

 

[MontyMariner]

If you can't get horizontal separation, mount the AIS stubby upsidedown to give it vertical separation.

 

[westernman]

Why not if they are on two separate antenna? If using a splitter yes the receive will be disabled for the AIS. If, as the OP says, he has two separate antennas then they need greater separation than he has at present. That was his question. the reason for the separation is to prevent damage to the AIS system, antenna included, when transmitting on VHF.

Why not simultaneous VHF transmission and AIS reception?

Because the antenna doing the reception for the AIS will be picking up mostly the signal from the transmission which can be 10^10 times more powerful (100mW) than the AIS signal it is trying to receive (1pW). A few cm's difference will reduce the tranmission power, but nothing like as much as 30km distance for the AIS.

So the poor AIS receiver is trying to filter out an extremely small signal from a very very large signal which is at almost the same frequency as the VHF transmission. That filtering won't be perfect - i.e. it won't be able to completely block out the VHF signal.

And that is assuming your VHF transmitter is good and is not leaking any signal out from its frequency into frequency for the AIS (which is very very close).
It will leak a bit (there is a specification which has to be met), but it won't be perfect.

Also the LNA in the AIS receiver may be saturated by the VHF signal, in which case it won't see any AIS signal at all.

 

[lustyd]

But, adding a splitter will result in some loss (both in Tx and Rx on the main VHF). I guess if you boat in busy areas when AIS likely matters most then range to CG may not be a point of concern.

Range to coastguard is rarely a concern, given their installation size and power, and a tiny loss in your signal won't make a real world difference there.

On a yacht doing say 7 knots - its not obvious what advantage that might have (at least for collision avoidance on a boat with a good lookout

The container ship might be doing 25kt, your speed is irrelevant.

 

[B27]

If you can't get horizontal separation, mount the AIS stubby upsidedown to give it vertical separation.

That tends to put it too close to the mast

In practice, you can get away with all kinds of non-ideal stuff, at least 90% of the time....

 

[Momac]

I have vhf and AIS antennas mounted perhaps 0.7m or thereabouts apart which is perhaps a bit less than recommended and have no issues as far as I am aware .
The AIS is not transmitting all the time . The transmission period depends on the speed of the boat.
Nor is the VHF transmitting all of the time .
I found twin antennas less cost than a splitter when I installed the equipment in about 2016.

 

[ylop]

Range to coastguard is rarely a concern, given their installation size and power, and a tiny loss in your signal won't make a real world difference there.

Like I said - I think that may depend on where you are. In the Solent/Channel you are probably right. In Belfast/Stornoway (and probably others) CG area of operation I often hear vessels which the CG are struggling to hear. Obviously I don't know why that is - but it strikes me a 3dB signal reduction isn't going to help in that scenario (ie. takes struggling to impossible or difficult to struggling?)

The container ship might be doing 25kt, your speed is irrelevant.

My heading is rarely consistent / accurate enough that getting 10 miles warning of a ship doing 25 kts on a putative collision course would be useful.

However, I was thinking only in terms of AIS Rx. If the OP it Tx too - then (a) he probably doesn't want it that close to his main aerial and (b) giving a supertanker time to spot your signal and react at a distance is probably a good idea. What happens when you use a splitter - lets say you have a serious problem and are transmitting a pan-pan, then all the inevitable back n forth with the CG for 5 minutes? Does that mean your AIS Tx is effectively muted because you are talking so much? Does that mean others who are close and might be able to help can no longer see you?

 

[lustyd]

I have vhf and AIS antennas mounted perhaps 0.7m or thereabouts apart which is perhaps a bit less than recommended and have no issues as far as I am aware .
The AIS is not transmitting all the time . The transmission period depends on the speed of the boat.
Nor is the VHF transmitting all of the time .
I found twin antennas less cost than a splitter when I installed the equipment in about 2016.

Probably works better on (I assume!) a motorboat where antennas can be separated and running a cable is simple

 

[MontyMariner]

That tends to put it too close to the mast

I don't know about yours, but my mast doesn't transmit
If you're thinking about shielding, I guess it depends on the mounting arm length more than the radial direction of antenna to mast.

 

[lustyd]

I think that may depend on where you are. In the Solent/Channel you are probably right. In Belfast/Stornoway (and probably others) CG area of operation I often hear vessels which the CG are struggling to hear

People often have issues in the Solent. A bad installation is a bad installation regardless of location. The difference between a splitter and not a splitter on a good installation might be measured in metres though, when talking to a coastguard.

it strikes me a 3dB signal reduction

My B&G is less than 1dB

My heading is rarely consistent / accurate enough that getting 10 miles warning of a ship doing 25 kts on a putative collision course would be useful

Not sure why I would be constantly changing heading in a location where this would matter. In local waters everyone will be slower anyway, but crossing a channel/sea you'll be holding course for the couple of minutes it takes for a container ship to close from horizon to your location quite often.

 

[pandos]

Thanks for the responses.

What I take away is that: in theory and from a purists point of view I should not do it, but it is unlikely to cause any harm to either of the appliances and the additional range will probably make up for any ill effects on the signals.

My friend is doing a solo transatlantic so the additional range on both devices may be useful.