Using a VHF Antenna splitter for AIS

boatmike

boatmike

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Several people may have been worried by claims that this could cause the VHF to blow the AIS or chartplotter on another thread. As I said there I didn't think this was possible and neither did the techie guys at Greenham Regis. I did however e-mail EasyAIS in germany who manufacture the things and this is their reply.

From: "Kotouczek-Zeise, Alfred" <akotouczek@weatherdock.de>

Sent: Tuesday, January 15, 2008 12:45 PM
Subject: AW: Email von www.easyais.de


Dear Mike,
thank you for your interesting question.

The "VHF" is the most important.
If the easySPLIT fails, or if the easySPLIT is without current the situation is as following.
Reception of VHF and transmission of VHF is always possible. This is a safety feature.

But, the transmission power of the VHF is neither going into the AIS, nor into the FM car radio if connected.

This means, you can be very calm.
A not powered or a not working easySPLIT won´t destroy anything.

WBR
Alfred

Incidentally, there are no "relays" internal to the splitter at all. It is a totally solid state device. ( unless the term "relay" means something other than I understand)
I hope this puts everyones mind at rest.
 
[ QUOTE ]
This means, you can be very calm.
A not powered or a not working easySPLIT won´t destroy anything.

[/ QUOTE ]

I personally would not give a flying fick about my car stereo or AIS engine when attempting to call out for help, but I would however be concerned about not getting the power up the mast!

I still don't get this need people have to buy splitters rather than lash up a £4 car aerial up the mast? or even on the pushpit etc.. Mine now feeds from my TV aerial and gives a marvellous signal, but that was a side effect of fitting a TV aerial.

I used to change TR switches regularly in RADAR, but these were radioactive (ionising) devices and I never took one apart (for obvious reasons). They switched far more regularly than anything in the subject we are talking about, but they did fail, with alarming regularity. Though a catastrophic fail was rare, the biggest fail was ramp times, that is, how quickly the switch changed from allowing the signal to pass to fully blocking, measured in micro seconds. Too slow and the receiver will fry.

Would I ever use one on a small boat - NO. Unless they have invented a 100% (100% chinese?? <span class="small">you're avin a laugh</span>) reliable product that does not add any interference or impedance, which I doubt. KISS applies to this set up if nothing else on board.

Would you consider a diverting adapter for a distress flare that would allow it be used as a lighter for the cooker, it shouldn't affect it, as the spark is diverted, so when you come to fire it as a flare, it will probably work?
 
Boatmike, I have my radio and AIS fitted with separate aerials; but not because I have a concern that one might 'blow' the other; but because I understood ( radiotech advice at the time of fitting ) that to have both radio and AIS simultaneously fed from the one aerial would degrade the strength of the signal to each. Assuming that to be so, does this splitter amplify the signals to each device so that one to one signal strength is maintained? /forums/images/graemlins/cool.gif
 
I am not a salesman for EasyAIS or an electrical engineer so I cannot answer your question with any authority. However I am told that using a splitter does not degrade the signal at all. As far as I am concerned it is a black box that does what it says on the tin. They have indeed sold literally thousands of them and I have not heard of any failures. All I was seeking to do was give some confidence to the people that have read the doom and gloom posts about the Tx on VHF blowing up everything else as an earlier post suggested. I think the comments from the manufacturer are quite clear and in addition to that the Regis Marine guys who I respect have also said that such claims are rubbish and there are no switching relays in the box at all. What you are thinking perhaps is that you can't connect two things to the same antenna without a splitter. It would degrade the signals and cause other problems especially when transmitting.
 
__________________________________________________________________
I personally would not give a flying fick about my car stereo or AIS engine when attempting to call out for help, but I would however be concerned about not getting the power up the mast!
___________________________________________________________________

Eloquently put, but I agree with the sentiment. That is why I am encouraged that it fails safe to VHF. As a matter of detail I intend to fit mine feeding a new DSC radio
as well as the AIS (the FM radio connection is welcome but immaterial) to the masthead antenna and leave the existing VHF in place (why throw it out?) connected to an antenna on my stern "goalposts". That way if the antenna gets damaged or I get dismasted I still have a radio. I also have 2 hand held ones on board! If the splitter does fail I can then connect the MH antenna direct to the AIS if necessary and still have a fixed VHF.
As far as the rest of your post is concerned if it makes you happy to rant on be my guest but it's really not pertinent to the subject. What is pertinent I think is that these splitters are not instruments of the devil and won't blow anything up as it was claimed so those who choose to use one can do so with confidence. Those that want to fit two antennae and find it convenient to do so... well that's fine too!
 
The fail safe setting looks right - power fails then you can still use the VHF (no power, very clever !! but I suppose a cable could break or fuse blow).
However, I understood that each connector introduced in the antenna cable reduced the resulting power (3Dbs rings a bell) - so a perfectly valid question is - does the EasyAIS splitter amplify the signal from the VHF when transmitting? (assumes it has power).
 
I can't answer that directly. What I can say is that I am told that the signal through the Splitter is NOT DEGRADED. As far as connections go there are two more coax connectors than you would otherwise have if you connected the radio direct to the antenna. I can't see that this would degrade the signal if the connections to the coax are properly made and soldered.
 
This splitter does look like it's fit for purpose with proper failsafe qualities.
The big problem with endorsing splitters as a general concept is that people might buy those passive coax splitters for around £10. They are bad news in this application for the reasons stated elsewhere.
The other issue with using an active splitter is, as others have mentioned, losses of up to 3.5db, which halves the signal power. This particular splitter may have much better performance but, when powered down due to a fault, it may degrade the signal significantly.
With a very good antenna system such a loss may not matter hugely but should be a consideration.
Remember that sharing your VHF antenna means you can't be receiving AIS data whilst talking to ships to discuss avoidance, but that might be of little significance.
 
The signal thorugh the splitter MUST be degraded Mike (Whilst on tx vhf) Also, a failsafe as they describe is certaibyl NOT fail 'safe'. the other issue is that transmitting into no load at all is not good for the output stage of the transceiver. The modern ones are far better protected, but it is not a good idea.. also, how do you KNOW its failed... ??.. you tx, so think its ok.. you call and no one replies, or you call and damage the output of the vhf.

It is one of those things that seem a good idea but is not. An AIS rx antenna could be a simple length of ss rod.. its just the length that important... for instance, for a quarter wave..
300 / freq in Mhz
Multiply result *.95 (Electrical wavelength as opposed to free space)
the divide by 4
Will work luvverly.. if you have an arch, make a simple dipole, 2 pieces as above (can be normal wire in this instance,) .. one connects to the inner coax and points up, the other connects to the outer coax and points down.. bit of perspex with 2 holes in for the centre... self amalgamating tape...
Bobs yer aunties huband.
 
Good points. From the handbook however the following:-

Coupling factor (Active VHF Transmitter ON )
VHF Radio (Transmit Mode) < 0.8 dB
Max transmit power 30W
Min transmit power (limit to activate splitter) >400mW
Activation delay after RF ramp up <10msec
This does not directly answer the question of signal degradation but indicates performace that would suggest that their claim of insignificant degradation is valid does it not?
 
[ QUOTE ]
This does not directly answer the question of signal degradation but indicates performace that would suggest that their claim of insignificant degradation is valid does it not?

[/ QUOTE ]According to the specifications I linked to, loss when receiving is <3.8dB. Whether this is insignificant or not depends on your viewpoint.
 
You're right, but it's probably the degradation in VHF reception which might concern people more. I notice that Raymarine's AIS250 combined AIS receiver and antenna splitter comes with the warning that "The on-board antenna splitter enables you to connect your existing VHF radio antenna to your AIS receiver and then connect from the AIS250 to your VHF. However in doing so you may experience some level of degradation to your VHF's receiving performance. A separate VHF antenna is recommended if you wish to maintain the same performance."
 
Boatmike, thanks for your measured reply to my question. This thread has been very useful for me as an amateur techie - particularly in the area of radio signals. My money is still on keeping my two aerials. Anyone coming to this anew at least has an abundance of advice to weigh in the balance. If we were in the Guinness bar, this would be the point where one of us is saying 'what are you having?' /forums/images/graemlins/cool.gif
 
There are two cases to consider.

1. VHF in transmit mode - loss is <0.8dB. i.e. with 25W in, you get 20W out worst case. IMO, not a lot of difference to the VHF transmit performance. However, the VHF will leak into the AIS receiver. The spec is >20dB. i.e. at 25W transmit power, 0.25W will leak directly into the AIS receiver. It won't damage it of course but depending one the receiver design, it will probably cause it to become deaf to all but the strongest AIS signals. i.e. while your transmitting VHF, the AIS won't receive.

2. VHF in receive mode. It will degrade both the VHF & the AIS signals by 3.8dB. i.e. a little over half. Compare a pushpit mounted AIS antenna with a masthead mounted one attenuated by 3.8dB & I'd take a guess that there will be little or no difference. In fact the mast head one may perform better. However, the VHF will be attenuated and so the receive performnace may be noticeably degraded.

IMO, a third point is far more important. If the splitter develops a fault for whatever reason, there is a chance of pushing far more than 0.25W up the AIS receiver & you kill it.

I have a second pulpit mounted antenna & can easily swap it from AIS to VHF if the mast falls down.
 
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