Two AIS recievers sharing an aerial - OK?

Rogershaw said:
You are missing my point having the AIS aerial at the top of the mast will give you better range than on the pushpit. It also gives the best range for VHF. So the best would be to have 2 aerial's at the mast head but we know that the required separation would not be easy to get without interference.
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Agree that two vhf antennas would be the best solution, but that isn't what the OP was asking.
Anyway, one of the reasons for a second antenna is demasting, so having both up the same mast isn't a great idea. Pushpit or gantry would be a reasonable option.
 
Rogershaw said:
You are missing my point having the AIS aerial at the top of the mast will give you better range than on the pushpit. It also gives the best range for VHF. So the best would be to have 2 aerial's at the mast head but we know that the required separation would not be easy to get without interference.

The use of this switch http://www.mfjenterprises.com/Produc...uctid=MFJ-1703 would allow the AIS to have the best reception range abd the VHF to still be connected to an aerial on the pushpit. It would also allow quick connection of the VHF to the mast head aerial when max range of transmission is required say in case of a may day call. It also allows you still to use AIS and VHF alternately without fiddling with connections behind equipment in the case of a lost mast.

My setup is 3 separate aerials 1 mast head 2 on taga at the stern (at least 2.5m separation) for 2 VHF radios and one AIS but IMHO the changeover coax switch would allow more flexibility with the use of aerials.
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Can't say I'd want to have to neuter AIS reception to receive a weather forecast.
Nor would I want an unsealed switch as a single point of failure in the system. There are better switches, which are sealed, got some on the bench but they probably cost more than a VHF set.
You'll only get max range at the masthead if you keep the loss low, which is best done by having a minimum number of interconnects and using good quality parts.
If you can get the AIS up 2 or 3 metres, the range should be pretty good for any ship with its AIS typically at least 10m up.
A high AIS antenna does make a difference when you're up river or otherwise obscured by terrain, so may be useful when bored in the marina playing with the electronics.

For those who need the AIS at masthead to keep RORC happy, a helical mounted below the VHF antenna might be a good option. but you'd need to be sure the isolation was sufficient.
 
What I was proposing is an aerial at the mast head and an aerial on the pushpit but to ass the ability to quickly to change them over to allow better range / loss of mast.

The OP was asking about 2 AIS with a single aerial which I would not agree with even if it did work. I was just presenting and alternative answering the posting of Salty John.
 
nigelmercier said:
In my experience, it takes a lot more motivation to replace a bodge than to do a proper job in the first place.
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Not so in this case, you must be lazier than me. Thanks to all - I didn't expect to create such a fuss. I can plug my aerial into either device or both via a T-piece. This is a path to a solution, not the final implementation. I accept all the superior knowledge about radio frequency signals - mine is scant. The question is whether the bodge makes RF reception bad enough to bugger up the digital signal. To the user this is all that matters. It is easy and cheap for me to test this. Just see what targets disappear when I unplug the "correct" method and plug in the experimental bodge. I'm a coastal sailor, so extreme range is probably not key, though I accept that the slower your boat, and the less accustomed you are to being at sea, the greater the warning you want about big fast ships on your occasional long passage. Once all the pieces arrive I'll experiment. There'll be a chance for the experts to say "I told you so!".
 
lw395 said:
A stubby helical on the pushpit will do the job, see the big metal objects at >10 miles, a handy fallback when your windoze won't boot.
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Having started the practical business of installing this, I have concluded that you and Nigel are wise. My attempt at money saving and simplification is actually complicating the issue. By the time I buy and install all the bits to do it the wrong way, I could have bought the one thing the required to do it the right way. I'm going to buy an AIS specific stubby which will come with all the cable and connectors and put it on the pushpit. It does seem a bit odd for an 8 meter boat to have an antenna at the mast head and three more (two AIS and one Navtex) on the pushpit, but there it goes.

A further question: the "AIS tuned" antennas I'm looking at come with miles of cable and a caution not to shorten it. Radio geeks have also told me not to coil or generally create corners in the cable. How do I do this? The not-to-be-shortened cable is three times as long as my boat!
 
I suspect the 'don't shorten it' bit comes from problems with amateur/boatyard bodgers not putting the new connector on properly.
Or using rubbish connectors which don't seal the coax against moisture.
So cutting the cable endeth the warranty.
Depending on the cable, the minimum bend radius can be found. So don't coil it too tight.
Coiling it won't generally create much issue at VHF, but avoid coiling it close to e.g. HF transmitter and noise sources like inverters, chargers, alternators.

Personally I'd cut it and put a decent connector on, and sod the warranty, but I have put hundreds of radio connectors on in my career and seen them past test on proper Hewlett Packard network analysers.
Not the right option for everyone IMHO.

The things you really don't want to cut or extend include depth transducers, where the cable length is (often) part of a tuned circuit.

HTH?
 
I HAVE SINNED.

Thanks for that lw395. The installation expects a BNC connector, and many are fiendishly difficult to fit on a cable.

I went to the boat today determined to do a proper job - buying the stubby from Fox's, whose web site boasted its availability. Nope, they didn't have the obvious antenna. I decided to be patient and go back next week when they have it.

Meanwhile I popped into Maplins and bought a BNC T-piece. I have a dedicated pushpit antenna driving the ugly NASA AIS "Radar" beast. I simply Tee-ed into it and took a lead to the AIS engine. It all seems to work reasonably well. I'm getting targets at ten miles on both instruments on one bodger's antenna. That's about as well as I do in the marina normally with one instrument. It seems to do better at sea (could the forest of metal masts be a factor?).

Now Nigel's "Who ever replaces a bodge?" factor comes in. If it works will I fix it?
 
Amulet said:
Meanwhile I popped into Maplins and bought a BNC T-piece. I have a dedicated pushpit antenna driving the ugly NASA AIS "Radar" beast. I simply Tee-ed into it and took a lead to the AIS engine. It all seems to work reasonably well.
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I have exactly this, and have not noticed any loss of range.
 
Pye_End said:
I have exactly this, and have not noticed any loss of range.
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It is one of those things that may or may not work adequately.
It's quite easy to lose 90% of the signal power using a simple BNC tee.
People don't appreciate the huge variations in signal power radio receivers have to cope with, a factor of ten is a small part of the dynamic range.
But when it stops working, the time and aggravation will make fitting a second antenna look good value.
Let alone the consequences of relying on a bodge for safety equipment.
 
lw395 said:
It is one of those things that may or may not work adequately.
It's quite easy to lose 90% of the signal power using a simple BNC tee.
People don't appreciate the huge variations in signal power radio receivers have to cope with, a factor of ten is a small part of the dynamic range.
But when it stops working, the time and aggravation will make fitting a second antenna look good value.
Let alone the consequences of relying on a bodge for safety equipment.
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I may well do the job properly soon. It's true that it's a bit of safety gear, but it's one I've managed without for 5 decades of sailing. Your message implies that it might appear to work OK and then suddenly stop working altogether. Is there a reason why that might happen?
 
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