I relayed a recent article here on the blog Digital Yacht which provides an excellent discussion of the choice of antenna for an AIS transponder install. You will find below a translation freely adapted by me.
One of the most frequent questions asked by boaters who want to buy an AIS Class B Transponder : which antenna VHF use ?
AIS (¹), uses two special channels «data only» in the VHF frequency range for transmitting and receiving digital data packets its. Thus, any VHF antenna can be used to receive and transmit AIS data and generally the same selection criteria, advice Assembly and Installation, apply.
One or two antennas ?
In absolute, for optimum reception AIS, must mount the AIS antenna location highest boat, either at the top of the mast on a sailboat, or on the radar arch support or upper deck of a motorboat. However, antenna VHF radio is usually already in this exact spot and you can not have two antennas spaced less than 2 meters from each other, if not all of the power 25 Transmitting antenna VHF Watts will go directly into the receiver of the AIS and riquera irreparably damage.
On a motorboat, you can get away with placing the VHF antenna on one side of the boat and the AIS across (if the width of the vessel is sufficient) and a ketch or schooner you have the luxury of two masts and therefore can mount VHF and AIS antennas on each. However, for most sailboat owners, who already have their VHF radio antenna on top of mast, they must make a choice between two options :
- Establish a dedicated AIS antenna to another location on the boat (back porch, pulpit, material)
- Use « splitter » (separator in French) to enable the sharing of a single antenna between AIS and VHF radio.
Splitters are used for quite some time, well before the AIS, to enable / FM radio receiver share the AM antenna VHF. When the first AIS receivers have been launched, many boat owners have used a splitter to allow the AIS receiver and VHF antenna to share the same. While this makes installation easy, separators were originally quite rustic device simply making that split the signal from the antenna, half the power going to the VHF and the other half to the AIS. This has meant that many boaters, having worked hard to install a good quality VHF antenna at the top of their mast for maximum VHF range, found a significant reduction in their range VHF reception after the installation of the separator.
AIS antenna dedicated
Therefore, manufacturers such as Digital Yacht have long recommended the use of a dedicated antenna for connecting AIS transponders. Ideally mounted as high as possible, for example on an antenna support to aft. Failing mounted at the bridge on the back porch, which still provides a reasonable reception transponders large ships of Class A, whose antennas are located several meters, even tens of meters, above the sea level. It is also important to note that the Class B transponders will transmit with a power 2 Watts so that, even with an ideal antenna installed on top of a mast, one can not really expect a range of about 8 NM – mounting on a rear porch probably reducing the scope for 4 or 5 nautical miles.
Another advantage of using a dedicated AIS antenna is that, with a little thought when installing, and can provide relief VHF antenna. Just provide the necessary adapter (usually a BNC connector PL259) possible to connect the VHF in case of loss of the antenna or worse dismasted.
So, why install a separator ? Unlike the first separators, the latest generation of separators designed for AIS Class B Transponder, as SPL2000 from Digital Yacht, using new technology «zéro PERT» which stimulates the received signals before dividing. This results in no loss of reception or on the AIS or VHF, and it is now possible to have a simple and easy to install solution, without loss of performance. Using the VHF antenna on top of the main mast, this gives a maximum range of the AIS system and the installation is to simply disconnect the VHF antenna to the back of the radio and insert the separator between it and the VHF.
DISCLAIMER REMARQUE : A Class B transponder requires a special type of separator equipped with two smart switches can detect the issuance or VHF, soit AIS, and disconnect the other device within a few milliseconds while the transmission begins. Separators low prices are provided only one single switch and should be used with simple AIS receivers.