Re: [Amel Yacht Owners] Range of AIS

Herbert Lackner

Dear all,


I invested some time on my AIS and would like to share my findings, maybe that will answer Pat’s question and will add some info to Bob’s mail. (just for info: there are so many things regarding our Amels that I do not know and where I need info and help from the group, this area is finally something that I understand being an electrical engineer and computer scientist J )


general information about splitters and how they work:

·         If someone wants to use one antenna for multiple devices a splitter ha to be used.

·         Splitter will split up the received signal to multiple outputs. If the splitter has no amplifier the power for each output will be reduced, Bob did explain it already excellent

·         There are some splitters that – due to their manufacturer – will not make the signal weaker (~0dB). Examples are “Watchey S Splitter” or” Simrad NSPL 400”. I did not check the data sheets but I believe them if they tell it.

·         A splitter uses relais to make sure that, if one device (eg the VHF) is transmitting, the other  device (eg the AIS transponder) is disconnected. This is necessary that the other device will not suffer damage (- it is not good for the AIS if it receives the 25Watt from the VHF…). The relais  also makes sure that only one device is allowed to transmit and not both at the same time. The VHF has priority while transmitting.

·         If the splitter is not powered or if there is a defect  the relais should make sure that only the VHF is connected (at least for sending)


Range of AIS:

·         As already mentioned by Mark AIS (VHF) the propagation is moreless optical. Therefore the higher the antenna is the bigger is the distance that can be covered (Mark explained this in detail). More transmitting power may add only some additional  miles

·         Transmission Power for class B AIS is 2 W, for class A 12,5W.  class A transmits every 2-3 seconds, class B every 30 seconds.


where to mount the antenna ?

·         Main mast: best choice for VHF antenna, but a splitter has to be used for the AIS because antennas should have a minimum distance from 1 meter from each other, otherwise sending can become difficult.

·         Mizzen: best choice if no splitter will be used

·         Rail:  ok for AIS when distance is not so important. will reduce distance by some miles (~8 miles less), still absolute ok for AIS for most sailors.

·         Cabling: if you use Aircell 7 instead of  RG58 you will have about 2 dB less damping at 20 meter cable length (please think here about the 3dB splitter damping discussion!!)


implications on distance:

·         You will receive AIS signals from a bigger distance the higher the antenna is mounted, height here is more important than transmitting power. If the antenna is mounted on the main mast that is  ~3,5 meters higher than the mizzen you will have a bigger range of about  1 mile.   

·         Even if the splitter reduces the VHF signal by 50% for the AIS you will still get an AIS signal not later compared to have a second antenna mounted on the mizzen.

·         If you use an active splitter with low signal reduction (eg “Watchey S Splitter” or” Simrad NSPL 400”) and the antenna is on the main mast you will see other ships earlier compared to an antenna without splitter on the mizzen


So, answering Pat’s question if he would get a better, stronger signal with a dedicated antenna the answer is:

·         It depends. The signal will be stronger compared to a not amplified splitted signal (but with minimal effect on distance)

·         The signal will be approx.. the same strength with an amplified signal

·         But: the range is determined by the height , and here is the main mast with the splitter (amplified or not) better.


Implications on reliability and security:

·         Using one antenna on the main mast with a splitter is less reliable than using two antennas without a splitter,  one on the main and the other one on the mizzen

·         For me personally reliability is more important than an additional range of one or two miles. I will add an extra antenna at the mizzen for the AIS and change my existing installation.



I hope I could explain it clear enough and it helps when deciding where and how to mount.


Fair winds,







Von: amelyachtowners@... [mailto:amelyachtowners@...] Im Auftrag von divanz620@...
Gesendet: Montag, 24. März 2014 12:49
An: amelyachtowners@...
Betreff: Re: [Amel Yacht Owners] Range of AIS



True if it is really a passive splitter...

The ones with a relay for switching to transmit should have minimal loss, there maybe splitters with amplifiers to compensate for the  losses.....but they need to be powered...


We have a separate VHF antenna on the pushpit rail and it works fine..we see ships 10-12 miles away...and always someone is on watch ......with the CPA and TPCA alarms set...we haven't hit anyone yet !!




SV Elyse SM437

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