Antenna dilemma

[Sea Change]

This is one of these subjects where I feel like I'm opening a whole can of worms.

I currently have the mast down and it's a good time to revisit my antennae for VHF and AIS.

Currently I have a grp 4ft antenna at the masthead, with a fairly skinny unmarked cable attached. That goes to the VHF. It seems to work fine.

I also had, as a strictly temporary measure, another rigid antenna for the AIS. It was cable tied to the wind gen pole, at the stern. I know it's not a great way of doing it, it was strictly temporary. I managed 2-5nm of range with it, which is better than no AIS at all.

The wind gen is gone, so I need to think about how to configure everything. I think I have a few options:

1- keep everything pretty much the way it was, but with the AIS antenna in a new mount on the solar arch. It will cast a small amount of shade of the panels but not as much as the wind gen did.

2- move the AIS antenna to the masthead and extend its cable. I've heard you shouldn't have both AIS and VHF at the masthead but my old boat was set up like that and it seemed to work very well. I liked the extra range on the AIS. We do a lot of offshore passages and I like to be able to pick up shipping 80nm away. It seemed to make it easier for family to track us as well as we didn't disappear off AIS websites as soon as we left harbour, like we currently do.

3- keep the existing masthead antenna but add a splitter to it. Keep the second antenna as a spare.

Any thoughts?

 

[geem]

I have a vhf antenna and AIS antenna on the same mast. Both work well with excellent range

 

[Sea Change]

I have a vhf antenna and AIS antenna on the same mast. Both work well with excellent range

That's my experience too

 

[jdc]

I have my AIS antenna on the 'comms post' at the stern and the VHF antenna at the masthead. I get 8 - 10 nautical miles or more range with the AIS, which is more than adequate - if the ship is below the horizon why worry about it? Too big a range and it's just noise.

I favour two separate antennas just for redundancy - I can swap them over and use the comms post one for the VHF should I lose the masthead antenna (or, worse, the mast). I don't favour a combiner except on the smallest boats as I don't judge the increase in AIS range useful - actually the converse - and I'm not drawn towards increased electronics for reduced fault tolerance.

 

[Supertramp]

I have one antenna and a splitter for AIS and VHF. Works fine, excellent range. Vesper AIS and splitter.

Two antennae makes sense if they don't interfere with each other although it adds wiring and another deck gland.

 

[PabloPicasso]

Deck glands, ugh.

I replaced mine with one of these on my last boat. Excellent bit of kit. Takes all cables below through a sealed dorade. All electrical connections are the in the dry of the cabin.

https://www.svb24.com/en/p/deck-seal-stainless-steel.html

I've ordered one for my current boat and will fit it once it arrives.

 

[Sea Change]

So that's one vote for each option

I'm leaning towards the second masthead antenna, because that's what I had before and it worked well for me.

I have a swan neck at the base of the mast, so no extra gland required.

The existing AIS antenna has about 20ft of cable attached. I presume the correct thing to do is to cut this off short, and join it in the junction box that's already at the masthead. Rather than have the join part way down the mast.

Next question, the thorny topic of which grade of cable to buy...

 

[jdc]

Next question, the thorny topic of which grade of cable to buy...

RG-8X every time!

 

[RunAgroundHard]

I followed Salty Johns advice a long time ago and have had a very reliable VHF/AIS system. See his comments, second post, here:-

Thread 'What kind of coax for VHF antenna?'

1 Jul 2012

Hi all,

Just investigating running a fixed VHF rather than handheld. What kind of coax do I need to run down the mast?

Also, are these the connectors I need for either end?

Cheers

• Reptile Smile
• Replies: 5
• Forum: Practical Boat Owner's Reader to Reader

 

[geem]

I have one antenna and a splitter for AIS and VHF. Works fine, excellent range. Vesper AIS and splitter.

Two antennae makes sense if they don't interfere with each other although it adds wiring and another deck gland.

We have everything on the masts coming through swan necks. Far easier to live with than deck glands. All connections are made in the celling void below the masts

 

[Boathook]

Just fitted an aerial from seas the change. Salty John used to sell them.

Used M&P ultraflex 7 cable with Messi and Paoloni fittings.

Tested the radio yesterday and heard Cowes inshore lifeboat and what seemed to be Joberg ? traffic control all from nearly the north end of Hayling Island.

 

[st599]

2 antennas at the Masthead will interfere and give gain and some directions and nulls in others.

You can't use a passive splitter, you'd need something that switches the transmitter to the antenna.

 

[Channel Sailor]

If proposing two vhf antenna at the mast top, then what he said (st599).

Personally I prefer to have longer range AIS if possible.

 

[Sea Change]

2 antennas at the Masthead will interfere and give gain and some directions and nulls in others.

You can't use a passive splitter, you'd need something that switches the transmitter to the antenna.

I had two masthead antennae for years on my old boat. I never noticed any poor performance from either the VHF or the AIS. We generally saw larger vessels at 50-80nm distance, and occasionally hundreds of miles, which I presume was some sort of atmospheric effect at work.

 

[Channel Sailor]

Just fitted an aerial from seas the change. Salty John used to sell them.

Used M&P ultraflex 7 cable with Messi and Paoloni fittings.

Tested the radio yesterday and heard Cowes inshore lifeboat and what seemed to be Joberg ? traffic control all from nearly the north end of Hayling Island.

I too have occasionally heard Jobourg Traffic from just outside the Solent. I recall VHF radio waves can sometimes tunnel longer distances beyond the normal line of sight.

 

[oldgit]

RG-8X every time!

Decent RG 58, not cheap rubbish is perfectly acceptable for the job and lots easier to work with.
Have in the past used amounts far in excess of anything likely to be found on any yacht unless its the "Cutty Sark" of course with no noticeable loss of signal.

 

[dunedin]

Well I am happy with the splitter route. Great AIS range and less clutter on the mast. Carry an emergency VHF aerial as backup.

 

[st599]

I had two masthead antennae for years on my old boat. I never noticed any poor performance from either the VHF or the AIS. We generally saw larger vessels at 50-80nm distance, and occasionally hundreds of miles, which I presume was some sort of atmospheric effect at work.

The antenna radiation's pattern is affected by any metalwork within a couple of wavelengths - which is about 2m at marine VHF. It's a parasitic director (this is how Yagi antennas typically used fro TV reception work)

It doesn't stop them working, but 2 antennas will interfere with each other and add gain in some directions and attenuation in others.

 

[st599]

I too have occasionally heard Jobourg Traffic from just outside the Solent. I recall VHF radio waves can sometimes tunnel longer distances beyond the normal line of sight.

Line of sight depends on the refractive index of the air, but that's just one propogation mechanism at VHF, you can get refraction higher in the atmosphere that steers the beam back down to earth well beyond the horizon and can get a duct forming. There have been reported exchanges of well over 1000km at VHF.