Class B Transponder – Dedicated VHF Antenna or Splitter ?


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 ?. Do you choose a dedicated VHF Antenna or Splitter, this article will cover just that.

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. 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. For example, at the top of the mast on a sailing yacht. Or alternatively on the radar arch or flying bridge of a power boat. However, the VHF radio’s antenna is usually at this exact same location. Consequently, you cannot have the two antennas within 2m of each other. As a result, 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). On a ketch/schooner you have the luxury of two masts. Therefore you 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. 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. This allows 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. Many yachtsmen, now found that their VHF reception range was significantly reduced after fitting the splitter. This was not ideal for those 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.

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. This will 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. Therefore 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. It Achieves this by having just the one antenna to find a location for. Slim FME type connectors on the two cables also make routing the cables through the boat easier. On smaller boats the GV30 is an ideal antenna for any Class B transponder. It 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. Therefore 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. Installation simply involves unplugging the VHF antenna from the back of the radio. You then connect 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. Working 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 you should never use with a Class B transponder.

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Comments 10
  1. I am not sure the statement “all 25w of transmitted power will go into the AIS antenna is correct”. The transmission antenna radiates 360deg and the AIS antenna occupies maybe 5deg of that, so the power received would be 5/360 * 25w = 0.3w maximum. Is this really going to damage the AIS receiver?

  2. You are correct, my comment was a pretty vague generalisation of what happens, so that it was easier for everyone to understand. I agree that not all 25W will actually come out of one antenna and in to the other, but when two antennas are in close proximity you get a “coupling” that is unpredictable and you cannot just apply your angle formula either.

    Our AIS receivers are designed to withstand up to a 2.5W input signal (not sure about other makes and models) before damage might occur. With a couple of typical half wave whip antennas and a VHF transmitting at 25W, we generally regard >300mm as being a safe separation in our lab. However, allowing for different types and gains of antenna, other makes of Transponder (with unknown receiver sensitivity) and building in a good safety factor, I stand by the separation distances I recommended, but accept that in real life it maybe possible to have them closer together with no problems.

    I think it is better that I am conservative in my figures than encourage people to fit the antennas closer together and suffer equipment damage.

    1. On some ships the UHF/VHF antennas are mounted with one of a pair upside down to achieve the separation. Is this an option to place both at the top of the mast on the same bracket?

      1. Hi Iain, yes this should work and is a neat way to get around the separation problem. Whether you can this implement at the top of the mast on a typical modern yacht, will depend a lot on the sail layout and rigging, so it may not be a solution for everyone.

  3. Please tell me if I have understood correctly what exactly your “splitter” consists of. Is it what in radar terms one would call a double T/R switch? I imagine that it contains a low noise preamp for reception whose output is divided between the two receivers. But when either transmitter comes on, it switches rather quickly, surely in microseconds, not milliseconds, so that the active antenna has exclusive access to the antenna. When neither transmitter is on then the antenna is connected to the preamp. But what happens if both transmitters come on simultaneously? There must be some sort of interlock to prevent that happening. Do you use PIN diodes for switching the transmitter power?

    1. Hi John,

      Thanks for your interesting question about our splitter. Some low cost splitters, are only designed to allow an FM radio or AIS receiver to use the VHF radio’s antenna. These splitters generally, just consist of a single automatic switch (relay) inside that detects the VHF transmit and disconnects the receiver whilst the VHF transmits. With a Class B AIS Transponder and a VHF radio, you have two transmitters so the splitter needs to have two intelligent switches that can disconnect which ever of the two devices is transmitting. Also because the AIS transmission has to synchronise with a time slot that is only 26 milli-seconds (0.026sec) in length, you need a very fast detection and switching circuit. That is why, you must use one of these more expensive splitters such as our SPL2000 that is specifically designed for Class B transponders, rather than the lower cost (<$100 USD) splitters.

      Our SPL2000 always gives priority to the VHF radio transmissions and also has a "fail safe" feature where it is designed to protect/maintain the VHF connection in the event of the splitter circuit failing.

      Finally we also include a pre-amplifier in our SPL2000 that boosts the received VHF signals by 3dB prior to splitting them, so that there is "zeroloss" of the VHF and AIS reception. This is a fairly new development, with the last 2 years, and splitters prior to this (even our original SPL250) always reduced the VHF reception, which is not what you want after going to all of the trouble of mounting the antenna at the top of the mast to get maximum range !

      Hope this answers your questions.

      Best regards PAUL

  4. The suggestion of placing the AIS transmitting aerial on one of the stern or pushpit rails is not a good one. For safety reasons these aerials should not be placed horizontally within 3 metres of a person or vertically within 2 metres above head height. (Icom) This presumably so as not to fry any of the delicate parts !! As most small boats have cockpits at the aft end of the boat it means that positioning the aerial there as suggested would be too close to the persons in the cockpit.

    1. Hi Robert, I would agree with your comment, if we were talking about a VHF radio as all fixed mount VHF radios can transmit at up to 25 watts and I would not want to get too close to an aerial radiating this amount of RF. However, a Class B AIS transponder is only transmitting at 2 watts for 26 milliseconds every 30 seconds or less and this is not a dangerous level of RF exposure. When you consider that a handheld VHF (that you hold very close to your head) is transmitting at 6 watts and no one thinks twice about using these devices, my suggestion of a deck mounted AIS antenna is not a cause for concern.

    1. Hi Ricardo,

      If the VHF cable of the GV30 is connected to nothing, then yes you can directly connect it to a VHF radio.

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