AIS or Radio antenna at top of mast?

[jamie N]

I've just been putting my Echomax XS on the masthead, alongside a VHF antenna base fitting. On the mast I've got 2, the other antenna fitting is about 4 metres lower on the mast, to avoid interference between AIS & VHF, which are entirely separate and unconnected.
Given that both can't be at the masthead, which one would be the preferred one?
The AIS will give an earlier alert to another vessel by being higher, but the VHF range will potentially be greater by being higher. I'm an electronics type, so it'd be easy to have a transfer switch to swap them over as circumstances change.
I should say, that it was a boring job in a cold wind, giving plenty of time for idle thought, and these were those thoughts.

 

[dom]

Why not just install a good AIS/VHF Splitter?

Like this:

Although if you have two aerials I'd default to VHF at the masthead. Then for a simple test, switch them around when say going offshore and check on MarineTraffic/VesseFinder for how good the AIS transmission is from both antennae.

 

[MoodySabre]

VHF at the top. VHF range will be greatly improved. You only need shorter reach for AIS.
I think that a transfer switch will involve a break in the shielding and therefore a much reduced signal quality, if indeed it works at all.

 

[Achosenman]

Why not just install a good AIS/VHF Splitter?

Like this:

View attachment 111087

Agreed I went the splitter route. Why make work for yourself!

 

[dom]

VHF at the top. VHF range will be greatly improved. You only need shorter reach for AIS.
I think that a transfer switch will involve a break in the shielding and therefore a much reduced signal quality, if indeed it works at all.

With cheapo splitters, sure. With a good active splitter and well made terminations the signal loss is miniscule.

Some of the better splitters -- like Vesper -- even claim a signal gain!

 

[dom]

Agreed I went the splitter route. Why make work for yourself!

Same, and I just banged an emergency antenna on the stern rail which only takes a second to switch over to.

 

[MoodySabre]

With cheapo splitters, sure. With a good active splitter and well made terminations the signal loss is miniscule.

Some of the better splitters -- like Vesper -- even claim a signal gain!

I had a Comar splitter and it worked fine and gave priority to VHF. The OP referred to a switch.

 

[Achosenman]

VHF at the top. VHF range will be greatly improved. You only need shorter reach for AIS.
I think that a transfer switch will involve a break in the shielding and therefore a much reduced signal quality, if indeed it works at all.

I'm berthed at the top of Portsmouth. I pick up contacts south of the IOW. If there is a loss, it's nothing meaningful.
Edit
PS, I've just seen the OP mentioned switch, not a splitter. Make life easy, fit a splitter.

 

[pvb]

VHF range is much more important than AIS range, so put the VHF at the top.

 

[jamie N]

Good points all. The coax and the hardware for them have been installed for about 5 years now without issue. It was an idle muse where my thought does lean towards VHF having primacy here.

 

[dom]

I've just been putting my Echomax XS on the masthead, alongside a VHF antenna base fitting. On the mast I've got 2, the other antenna fitting is about 4 metres lower on the mast, to avoid interference between AIS & VHF, which are entirely separate and unconnected.
Given that both can't be at the masthead, which one would be the preferred one?.....

One more point: antennas should - where possible - be mounted well away from other vertical metal objects, like other antennas (as you rightly say), but also masts, stays, basically any other lengths of metal in close proximity to and parallel to the antenna.

The existence of such objects can cause serious interference and may even block out a clean signal to and from the antenna at certain angles.

Where as a matter of interest is the lower antenna mounted and have you fully tested it?

 

[LittleSister]

I don't know about transceivers, but an AIS receiver antenna doesn't need to be that high, in my experience. 4m below the top of the mast would be plenty, in my view. Height of VHF much more important.

I have an AIS antenna/receiver on the pushpit rail. That worked absolutely fine on my previous boat - saw other craft as far as I wanted to 'see' them and beyond (any further would have just added clutter and false alarms). I've never felt a need to mount it up the mast.

I've noticed a couple of times on my current boat that there are sometimes craft visible by eye some distance directly ahead which don't immediately appear on the AIS. I have put this down to its wheelhouse which is higher than the AIS antenna and forward of it, and which has the galley, instruments and electrics etc. inside it, as well as the wheelhouse structure itself. (I have a radio with built-in AIS that I'm intending to fit, and that will receive AIS via the masthead VHF antenna, so I haven't bothered with re-mounting the antenna on the wheelhouse roof or some alternative position.)

 

[pvb]

I don't know about transceivers, but an AIS receiver antenna doesn't need to be that high, in my experience. 4m below the top of the mast would be plenty, in my view. Height of VHF much more important.

I have an AIS antenna/receiver on the pushpit rail. That worked absolutely fine on my previous boat - saw other craft as far as I wanted to 'see' them and beyond (any further would have just added clutter and false alarms). I've never felt a need to mount it up the mast.

I've noticed a couple of times on my current boat that there are sometimes craft visible by eye some distance directly ahead which don't immediately appear on the AIS. I have put this down to its wheelhouse which is higher than the AIS antenna and forward of it, and which has the galley, instruments and electrics etc. inside it, as well as the wheelhouse structure itself. (I have a radio with built-in AIS that I'm intending to fit, and that will receive AIS via the masthead VHF antenna, so I haven't bothered with re-mounting the antenna on the wheelhouse roof or some alternative position.)

This raises an interesting question, worthy of a bit of thread drift perhaps. Will an AIS antenna mounted 4m below the top of the mast suffer significant losses due to the shadow of the mast?

 

[jamie N]

In answer to a couple of replies here, the mast is a wooden item with the lower 'AIS antenna' above the hounds.
I've no idea about how much shielding the mast will give it, certainly Echomax were very specific about mounting away from anything that might shadow the reflector, thus it's now at the mast head, and not on the pushpit.
How much does 'wood' quash a VHF signal?

 

[dom]

In answer to a couple of replies here, the mast is a wooden item with the lower 'AIS antenna' above the hounds.
I've no idea about how much shielding the mast will give it, certainly Echomax were very specific about mounting away from anything that might shadow the reflector, thus it's now at the mast head, and not on the pushpit.
How much does 'wood' quash a VHF signal?

Wooden mast should be okay but remember stays and fittings aren’t wood but I guess they might be rope! ?

 

[Rappey]

With suggestions that 75ft of rg58 coax will only transmit around 7.5w from a 25w set due to cable loss I would not be wanting anything else connected along the cable.
I get 1 mile ais reception without an antenna.
Can get 15 miles with antenna mounted on pushpit.
Ais wavelength is 1.85m, vhf is 1.88 so a proper ais antenna is shorter therefore why spend a small fortune on a splitter only to not use the correct length of antenna by using the vhf one ?

 

[dom]

With suggestions that 75ft of rg58 coax will only transmit around 7.5w from a 25w set due to cable loss I would not be wanting anything else connected along the cable.
I get 1 mile ais reception without an antenna.
Can get 15 miles with antenna mounted on pushpit.
Ais wavelength is 1.85m, vhf is 1.88 so a proper ais antenna is shorter therefore why spend a small fortune on a splitter only to not use the correct length of antenna by using the vhf one ?

I wouldn’t worry too much about this: many AIS sets have an in-built SWR meter and a VSWR of anything below 1.5 is absolutely fine.

This is generally achievable with good low-loss, double-shielded coax, good terminations, and a good VHF antenna.

Just borrow or buy (pricey though) a good SWR meter and you can test the antenna across a range of frequencies. Usually not necessary though.

 

[jamie N]

Ais wavelength is 1.85m, vhf is 1.88 so a proper ais antenna is shorter therefore why spend a small fortune on a splitter only to not use the correct length of antenna by using the vhf one ?

Without any good reason, I just didn't trust a splitter, and I've no idea why or whether it's in any way justified.
Having an AIS antenna slightly longer, would that degrade the AIS signal?

 

[Rappey]

Having an AIS antenna slightly longer, would that degrade the AIS signal

In reality I would have thought the difference is barely noticeable but when it comes to transmitting via a vhf a good grade coax such as lmr 400 (21 watts@ 75 ft compared to 7.5w for rg58)
and the correct wavelength antenna all add up to a much better transmit/ receive distance .
In the days of cb and adjustable antenna tips just1mm difference made a big difference to the swr reading.

 

[Roberto]

If you put an AIS txr antenna that high, be sure to have/install a Class B SOTDMA ais transmitter, with a CSTDMA type it will receive carriers from all over the place and it might just wait ages until it finds a free spot to send your data.