VHF aerial pushpit mounted

[yotter]

We have an issue with AIS reception/transmission on our new to us boat, we do have more investigation to do when we can find the time. I know that the best option on a sailing boat is for a masthead VHF/AIS aerial. However we have an issue with the masthead aerial, the feeder cable and cable duct apparently need some attention (I have been informed), the duct can only be accessed with the mast down and that is not planned until next winter. So I wonder what VHF/AIS range I could expect with a pushpit mounted aerial, I guess that is what motor boats have, I would be looking for low loss feeder cable and a good quality aerial. And advice appreciated, I do plan to reinstate the masthead aerial ASAP. I do like to have a pushpit mounted aerial as a backup in any case, so I may fit 2 for redundancy, initially for VHF and AIS.

 

[stav]

I have a stubby dedicated (as in perhaps made for that frequency, not sure from memory) and I made a little block that clamps to the shrouds that the antenna fixes to and is about 8 feet above deck. Gives pretty good reception with targets from ships when crossing the channel quite a few miles away (12 or more) and small targets around the coast several miles away (5 or so). All from memory so don’t hold me to it! The block was a bit of black nylon bar 30 mm in diameter, hole drill at an angle to suit the shroud, cut flat on one side and cut in half following the hole. Small angle bracket and all bolted together on the shroud. Hope that makes sense. Mine is on the mizzen mast so have antenna redundancy as well. Good luck sorting it out.

 

[Plum]

I have a standard vhf aerial with the base 1.5m above water that I use for AIS reception and reliability identify ships at 8+ miles and small craft at 5+ miles

 

[oldmanofthehills]

Theoretically VHF range optimum is literal line of sight. Thus from tip of aerial till shoreside receiver just dips below horizon. Of course its a bit better than than that

To quote Practical Sailer: 'To determine communication range expressed in miles, you take the square root of the height of the VHF antenna above the waterline and multiply it by a factor of (1.42). Do this same calculation for the shore station or other boat that you are trying to talk to and add the two numbers together to arrive at the maximum distance that the two radios can communicate.' (Height in feet)

For the boat the distance to the dipping point for a 7ft height is 2.64*1.42=3.749 miles, and for 30ft mast its 5.477*1.42=7.77 miles plus whatever range from shore aerial to dipping point. So you lose about 4.2 miles on stern rail compared to mast head, but of course the CG shore station for that all important call may be 300 ft up thus adding 17.32*1.42=24.594 miles which may make the 4.2 mile loss less important

 

[petibonvm]

I installed a GLOMEX Easy FME RA300 modular AIS antenna - 1.2m - 3dB with a Glomex FME RG8X cable. The antenna is approximately 2m above the water on the solar panel's gantry.

 

[yotter]

Many thanks for your responses, I like stavs idea of mounting an antenna on a rigging shroud, I could get that fairly high up using the bosuns chair, perhaps up above the lower spreaders, 6 or 7 meters up.

 

[st599]

Theoretically VHF range optimum is literal line of sight.

That's only true in a vacuum. The radio horizon at VHF at ground level on earth is quite a bit beyond the visible horizon.

 

[oldmanofthehills]

That's only true in a vacuum. The radio horizon at VHF at ground level on earth is quite a bit beyond the visible horizon.

which is why i said "Of course its a bit better than than that"

Having cruised the east coast of Ireland there seemed many places where one could hear the voices of tractor drivers on the cliffs above but no VHF CG weather or other traffic. I guess their shore stations are far apart and not that high. I like maximising my 'reach' and mount my main aerial as high as possible but have backup on pushpit

 

[James_Calvert]

I've got an AIS antenna mounted on the pushpit. For the most part reception is at least as good as others say. And we are picked up by Marine Traffic pretty reliably by their various shore stations. Much better than the obviously dodgy masthead aerial which had us lost on the French coast somewhere.

But one surprising exception. If the pushpit aerial was in line with our own mast, we could lose sight of another small yacht's transmissions. They could literally disappear off the plot, even though they were in easy visual range. It tended to happen when we were both headed in the same direction, so at a constant relative bearing.

 

[William_H]

I think from comments from posters over the yeas an AIS antenna on push pit should be fine. (and will always provide an emergency antenna in case of dismasting) So I suggest perhaps a problem with antenna or cable if OP is not satisfied with performance. ol'will I have my main Com antenna on pushpit seems to work very adequately forr me.

 

[lustyd]

We have one on the pushpit and while I agree with others that it picks up ships 12 miles away that is far from the whole story.
We swapped ours to a masthead and splitter, and have now replaced the masthead antenna and cable too.

Thankfully we have Starlink, so also have continuous access to Marinetraffic.com. The difference between what’s available from the pushpit and what’s actually out there is frightening. The problem with AIS is you don’t know what’s missing. The masthead and splitter vastly improved this but not entirely, and we would often see ourselves drop before others in deadzones on marinetraffic (such as between the Skerries and Carlingford Lough). Changing to a new antenna and cable with better connections has fixed this too.

TL;DR don’t assume it’s working based on the most distant target.

 

[yotter]

We plan to go for a masthead aerial ASAP but not likely until the mast comes off at the end of 2026, this is because the internal cable conduit is loose and has damaged the cable (or so I am told). I will likely for the best feeder cable and antenna for the pushpit (and possibly up a cap shroud for more height), so any recommendations appreciated, I know that Metz are good but no longer available in the UK. Would a 2 metre aerial improve things over 1 metre.

 

[lustyd]

I’d get a good antenna and splitter ready to move to the mast. If the AIS is bad then VHF will be too so move everything together. Based on what you said above, if VHF is already OK then just split that

 

[johnalison]

Any VHF/AIS is going to be better than none, and a decent pushpit arrangement should be quite adequate until the masthead is available. As said, there are plenty of small motor boats going around with the above plus radar and they have to put up with the limitations.

 

[yotter]

Many thanks for the comments thus far, I probably should have added that we are planning a delivery from West country to the Clyde shortly after Easter 2026, and the primary concern is safely we will very likely plan to carry an EPIRB, so keen on best possible VHF range mainly for safely.

 

[RunAgroundHard]

Something to consider is the location of antenna and distance to people. Risks can be managed so worth knowing about.

https://www.theca.org.uk/system/files/CA RATS - RF Safety Assessment - Revised 20240208.pdf