One of the most common questions we are asked by customers wanting to purchase an AIS Class B Transponder like our AIT2000, is what VHF antenna should they use ?
The AIS system, uses two special “Data Only” channels of the VHF frequency range to transmit and receive its packets of digital data. Therefore any VHF antenna can be used to receive and transmit AIS data and generally the same selection criteria, mounting advice and installation issues apply.
In an ideal world, to get maximum AIS reception, you would mount the AIS/VHF antenna at the highest point of the boat i.e. at the top of the mast on a sailing yacht or on the radar arch or flying bridge of a power boat. However, the VHF radio’s antenna is usually at this exact same location and you cannot have the two antennas within 2m of each other, otherwise all of the 25W transmit power of the VHF antenna will go straight in to the receiver of the AIS and damage it. See our previous post on antenna separation by clicking here.
On a power boat, you might be able to get away with having the VHF antenna on one side of the boat and the AIS antenna on the other side (depending upon the boat’s beam) and on a ketch/schooner you have the luxury of two masts and can mount the VHF antenna on one and the AIS antenna on the other. However, for the majority of yacht owners, who already have their VHF radio antenna at the top of the mast, they have to make a decision between;
1) Fitting a dedicated antenna for the AIS in another location on the boat
2) Use a “splitter” to allow both the AIS and Radio to share the VHF antenna
Splitters have been around for some time and prior to AIS, splitters would be used to allow an AM/FM radio to share the VHF antenna. When the first AIS receivers were launched, many yacht owners used a splitter to allow the AIS receiver and the VHF to share the same antenna. Although this made installation quite easy, the traditional splitter was a fairly crude device that simply split the signal from the antenna – half of the power going to the VHF and the other half to the AIS. This meant that many yachtsmen, who went to a lot of trouble to fit a good quality VHF antenna at the top of the mast to get maximum VHF range, now found that their VHF reception range was significantly reduced after fitting the splitter.
For this reason, Digital Yacht have traditionally recommended the use of a dedicated AIS/VHF antenna for connection to their AIS units. Ideally this would be mounted as high as possible, for instance on an antenna bracket at the stern of the boat or, space permitting, on the spreaders. Failing this, the dedicated antenna can be mounted at deck level on the stern rail and still provide reasonable reception of the large ship Class A transponders – typically 10-15 miles. It is important to also note that Class B transponders only transmit at 2W so even with a perfect antenna installation at the top of a mast you can only really expect about an 8 mile transmit range – mounting on the stern rail will probably reduce your transmit range to 4-5 miles.
Another consideration of using a dedicated VHF/AIS antenna, is that with a bit of thought during installation, it can double as an emergency VHF antenna. If you keep the necessary adaptor (usually PL259 to BNC) then in the event of your main VHF antenna failing or a demasting, you can quickly unplug the AIS antenna and plug it in to the VHF radio.
You have to weigh up the cost/time of installing a dedicated VHF/AIS antenna versus the cost of a splitter. Our GV30 combo GPS/AIS antenna (see image above) helps to make installation easier by having just the one antenna to find a location for and slim FME type connectors on the two cables, to make routing the cables through the boat easier. On smaller boats the GV30 is an ideal antenna for any Class B transponder and is becoming a popular option with our AIT2000 transponders.
So why would you fit a splitter ? Well unlike the traditional splitters, our latest SPL2000 splitter uses a new “Zero Loss” technology that boosts the received signals prior to splitting them. This results in no loss of reception on either the AIS or VHF and for the first time yachtsmen can have a simple easy to install splitter with no reduction in performance. By using the main VHF antenna at the top of the mast, you get maximum AIS performance and installation simply involves unplugging the VHF antenna from the back of the radio and connecting it to the SPL2000 along with the supplied cables that go to the radio and AIS. Mounted in the same matching enclosure as our AIT2000 transponder, the SPL2000 is the perfect solution for larger yachts and will work with any Class B transponder – not just Digital Yacht ones !
IMPORTANT NOTE – a Class B Transponder needs a special type of splitter that has two intelligent switches inside that can detect either the VHF or AIS transmitting and in less than a few milliseconds disconnect the other device while the transmission takes place. Traditional lower cost (£50-£70) splitters only have one simple switch and should never be used with a Class B transponder