Should I split my VHF and AIS aerial?
QI am planning to fit an AIS transceiver to my 44ft yacht, and was reading your review of the sets available ( YM May 2016). The one thing I can’t decide is whether to fit a splitter or not, so that my AIS can share the same aerial as my VHF radio. I can see the advantage of less cabling and equipment on board, but does it unnecessarily introduce another component that could fail? Or does the splitter significantly reduce the power and range of either the VHF or AIS? I'm unclear about the difference between and active and a passive splitter.
Additionally, I think there might be an error in your article. It states the ICOM MA-500TR has a built-in active splitter, but this is not mentioned at all by ICOM. Enrique Kariger
ANick Heyes, chief executive of Digital Yacht, replies: Any VHF antenna can be used to receive and transmit AIS data and the same selection criteria, mounting advice and installation issues apply for AIS as for VHF, except that you cannot have two of these aerials within two metres of each other. We have traditionally recommended using separate aerials, as you retain maxiumum transmit power. You also then have a spare antenna, should one be damaged. With a bit of thought during installation, this could be a simple as unplugging
one lead and replacing it with a lead to the other aerial. The only issue is that a lower mounting position will slightly limit the range of the AIS. Modern splitters, however, like our SPL2000, are zero-loss, so you don’t lose any power with them, and they are also failsafe for VHF. Virgil Parker, marine sales manager
at Icom replies: There is indeed no active splitter built into the MA-500TR AIS transceiver. You would need to fit it an external active splitter – It is important that it is an active splitter. A passive splitter would be fine with an AIS receiver only, because when the VHF radio is transmitting, it simply shuts off the AIS’s ability to receive.
With an AIS tranceiver, however, the VHF radio would still take priority over the AIS. The problem is that the AIS still transmits a signal and that energy needs somewhere to go, so it doesn’t bounce back into and damage the transmitter. An active splitter is therefore used, as it puts a ‘dummy load’ on the AIS transmitter, absorbing the signal’s energy until the antenna is again free for it to transmit.
Graham Snook adds: If the splitter were to fail, it can be removed from the system and you would have a working VHF as usual since the connections are the same. If you do opt to split, you should just ensure that the VHF areal lead still fits to the rear of the VHF radio. Personally, I chose to install a second aerial on a 2m pole on the push pit. I still have in excess of 20 miles AIS range and an emergency second aerial, should I require one.