AIS Transponder, dedicated antenna or antenna splitter ? [Update]

spl2000I relayed by an article published in 2014 on the blog Digital Yacht which provides an excellent discussion of the choice of the antenna to be installed for a transceiver AIS. You will find below a transcript freely adapted by my care.

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 "data only" channels in the VHF frequency range to transmit and receive its digital data packets. 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.

(Source: Digital Yacht)

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 :

  • Set up a dedicated antenna at the AIS in somewhere else on the boat (back porch, pulpit, material, mizzen on a ketch)
  • Use a "splitter" (separator in French) to enable the sharing of a single antenna between AIS and VHF radio.

Antenna splitters have been in use for 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

RA-111 AIS Glomex

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, still offering excellent reception of class transponders has larger vessels whose antennas are located several metres, 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 output. Even with an ideal antenna atop a mast installation, one can not really expect a range of about 10 to 15 nautical in emission. A gantry mounted rear only slightly reducing the scope (example opposite the antenna RA-111AIS de Glomex).

In addition, antennas dedicated to AIS frequencies provide maximum gain (VSWR) to 162 MHz, corresponding to the middle of the AIS frequency range, while the maximum gain of a VHF antenna is at the frequency of 157 MHz (²).

Another advantage of using a dedicated AIS antenna is that, with a little thought when installing, she can serve as a backup VHF antenna. Just provide the necessary adapter (usually a BNC connector PL259) to connect the VHF fixed antenna loss or worse Dismasting.

Antenna splitters

So, why install a Brand ? Unlike the first separators, the latest generation of splitters designed for AIS class B transponder, uses new "zero loss" technology that stimulates received signals before splitting them. 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 the maximum range of the AIS system and the installation simply consists of disconnecting the VHF antenna at the rear of the radio and inserting the splitter between it and the VHF. But these splitters are still quite expensive,often more than installing a dedicated antenna.

AIS Splitter scheme

Schema with a single antenna and splitter


A class B transponder requires a special type of splitter with two intelligent switches that can detect the transmission or VHF, soit AIS, and disconnect the other device within a few milliseconds while the transmission begins. The cheap separators are only provided one single switch and must only be used with simple AIS receivers.

(¹) AIS, the second maritime revolution after the GPS
(²) Which VHF antenna for an AIS transponder


5 Replies to “AIS Transponder, dedicated antenna or antenna splitter ? [Update]”

  1. Thank you for this item.
    I'd use a splitter on a plane, connected to the VHF antenna Aviation. The goal is to have a Marine AIS Recepter to locate boats, without installing a second antenna.
    Is it possible ?

  2. Thank you for this article useful. There are some days I was just looking at the AIS.
    What is there like other make / model with similar "split-intelligent" technology ?

    I am starting to think about an AIS which also makes "NMEA2000 concentrator & Wifi gateway ". I spotted the brand WEATHERDOCK, but as I doubt it includes a spliter-smart like the SPL2000.

    Finally I think that perhaps it would be better to have TWO devices : one for AIS, another for "NMEA xxx multiplexing + WiFi gateway " . (The aim is to visualize a maximum info NMEA xxxx on an iPad).

  3. Francis, Thank you for your comments today. I have a Class B transponder with a splitter and antenna mounted on top of our 54′ mast. We are able to receive Class A targets as far as 40 nm away! I am completely satisfied with this performance and would suggest this as a workable solution for any sailor. Even with the Victoria Express ferry traveling at 25 kts it still would give us more than one hour to change course to avoid collision. One would have to be seriously asleep to see that as a problem.

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