Using a VHF Antenna splitter for AIS

Thank you. Thats what I thought. Doesn't seem to be a problem with AIS though according to this from the manufacturer earlier

Hello Mike,

just let me bring clearance into this topic:

The easySplit is, as the name tells, a passive antenna coupler/splitter.


The principle of operation for VHF during reception is the following:

· The incoming VHF signal from the antenna is divided in two paths:

o One path is feeding the VHF radio.

o The other path is feeding the AIS receiver.

o The two outputs are decoupled by more than 20 dB to avoid feedback between the two outputs in case one of the two devices is not connected.

As the easySplit is a passive device, the dividing of the incoming antenna signal into two paths (one half is feeding the VHF, one half is feeding the AIS) results theoretically in a path loss of 3dB per branch. Due to the fact, that the divider is built with real components (coils, capacitors, etc), which generate additional loss, the overall path loss per branch adds up to 3.8dB maximum.

In reality you will not recognize this. The overall performance of the easyAIS (sensitivity, capability of decode weak signals) is so good, that the influence of the splitter is getting lost in effects of the fluctuating propagation between the transmitter and the receiver. These effects are for instance multipath propagation due to sea surface reflections, heavy moving antennas (mast top installation), tropospheric phenomenas, etc.


The principle of operation for VHF during transmit of the VHF radio is the following:

Sensing a RF energy injected by the VHF radio, results in a direct connection between the antenna and the VHF radio.

The path loss between the antenna connector and the VHF radio connector in this case is 0.8 dB maximum.

All other branches (which are active during reception) are switched off. That means of course, that you will not receive AIS messages the time you are transmitting with the VHF radio.

Hope this helps?


Best regards

Wolfgang Werner

Weatherdock AG

Am Weichselgarten 7


If I therefore use the splitter in the emergency radio antenna line the worst that can happen is AIS goes on the blink if it fails. If it does then an antenna lead swap will solve it. In this mode therefore I don't expect any failure or degradation that will cause a serious problem and in the unlikely event that I lose the DSC radio for any reason I still have a back up. Best of both worlds I think?
 
To an extent, you're arguing for the sake of arguing. A masthead antenna will improve your reception range but, with this passive splitter, degrade the signal. Whether the receiver has sufficient sensitivity to overcome such a defficiency can only be found, if ever, when installed and you have yet to ascertain a) whether your existing VHF masthead aerial and cabling are in good condition and b) is able to receive signals at the top end of the VHF bandwidth - for which your VHF aerial may not be entirely suitable.

None of this matters with a pushpit mounted aerial; its a new installation and you can more carefully select an antenna. Its more work, of course, but you do get a ready-made spare antenna if your masthead fails.

In practice, and assuming your masthead setup is up to it, you won't see any difference between a masthead or pushpit mounted aerial.

Unless you are buying a twin channel receiver, only half of the available messages will be received. A masthead aerial ought to receive an AIS signal from much further away except that the signal degradation will reduce the number received. A pushpit aerial will receive signals from 16m away but at a rate of no better than 1 per minute - over 80% are failing the message checksum. As the target approaches to <10m the rate increases to better than every 10secs - approaching 100% success rate. It is only at that range that targets become of specific interest to a yacht and it matters not a jot whether the aerial is pushpit or masthead mounted.
 
Im not arguing for the sake of it. In fact I think you are as a very late entry to an already resolved thread. My original post had nothing to do with signal degradation at all. It was answering the fail safe question raised in the previous thread. Others then brought in the subject of signal degradation which we have now agreed upon. There is no argument unless you start one. I certainly dont intend to. As far as I am concerned the issue is closed.
While some posts have seemed a little aggressive in style I have not raised to the bait and thank all those who contributed for their time in particular pvb and dogwatch whos comments have helped clarify the issue in my mind.
 
I too have opted for the pushpit option for AIS, and was going to add my piece, but most has been said already,

However, no-one has commented on the signal degradation in the coax cable, which is of the same magnitude for RG58 cable as the losses in the splitter. In trying to find some data on this I came upon this site, which forum members may find of use:

coax losses
 
Yes I think the point the manufacturers e-mail to me seemed to be trying to make is that there are many things that degrade the signal and the length of coax is one of them for sure. Not being an electronic engineer (as I keep saying) I find it difficult to judge what is significant and what is not. Nevertheless, as I have to run a new antenna cable to my mast head to a brand new antenna with a new DSC radio to boot I will have as near perfect setup on that as I can get. I will, as I said earlier, leave my old non DSC radio in place and initially at least, install the splitter in that antenna to feed to the AIS. I think from what the manufacturers say the AIS should work OK. What will be interesting is to compare the actual performance of my old VHF set with a splitter, using the existing antenna mounted on the goalposts (about 4 1/2 to5 metres above W/L) to the new one. On one hand the splitter is in the circuit, on the other there is about 3 times the coax...
If I don't like the performance of the setup once in place I can deep six the old radio and splitter and just use the stern antenna for AIS but it seems a shame not to give it a fair trial as the old radio is a good back up that costs nothing if it works OK that way. We will see.... The main thing at the end of the day is that the VHF radio is not optional. It's an essential piece of kit. That's what persuades me to keep it seperate in the end as people have suggested. Everything else is good to have but not essential.
 
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