Should I split my VHF and AIS aerial?

Yachting Monthly - - ANY QUESTIONS? -

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.

A foam core with a gen­er­ous layup on ei­ther side is a light and strong way to build a boat, as with the Koop­man Foxx 30

An ac­tive split­ter lets you use one aerial for your VHF ra­dio and AIS set

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