AIS aerials – all you need to know for a boat install.

300 80
AVES Marine Ltd
by Alan Vick

AIS aerials/ antennas and AIS.

AIS and VHF aerials – what’s the difference? AIS (Automatic Identification System) is transmitted on two channels at the top of the Marine VHF band, channel 87B and 88B at 161.975 MHz and 162.025 receptively. An AIS aerial will be tuned to precisely operate on these frequencies whilst a standard marine band VHF aerials will be tuned to operate most effectively at 156.8 MHz (channel 16). A standard marine band aerial will work on AIS but not quite as well as one tuned specifically to the AIS channels. This is more so on transmit than on receive as the losses that may affect transmitting efficiency:  different tuning, aerial position and height, cable type, length and connector losses will have a greater effect than on reception only.  It follows that whilst a standard VHF aerial will work for AIS, a dedicated AIS aerial will be better.

Cable: Losses over long lengths of co-axial cable can be quite severe and this is dependent upon the thickness of the conductors and the design and quality of the cable. Most frequently used cable on boats is RG58 which very flexible, thin and easy to install and is not expensive to buy; however, it has considerable loss – particularly where the cable length exceeds 30 metres.  Best cable for R.F (radio frequency) might be RG213 – it’s the cable Radio Hams tend to use; it has considerable less loss than RG58 but it is very thick and inflexible, very hard to install  and costly too, so clearly a compromise is necessary. RG8X (or mini8) is a good compromise; it is only slightly thicker than RG58 (by around 1.5mm) but importantly, the inner conductor is a lot thicker and the outer braiding much denser. Another factor is that there are co-ax cables and there are co-ax cables and one length of very high quality RG58 is a different item to another that is a lot cheaper. This will be because it is very easy to manufacture a co-axial cable and call it RG58 but to use just a few strands of copper for the inner conductor and just a hint of braid for the outer. This may be true as well for RG8X but all the RG8X we supply is  excellent – just don’t buy by price alone as a cheaper cable is likely to be cheap for a reason.

Which AIS aerial to buy?

A quick search on the internet will leave you spoilt for choice – there are lots at all sorts of prices. Most are whip type aerials – stainless or fibre glass thin rods around 50 cm long .There are also stubby types, short – 140mm and pliable.

Mast mounted?

A frequently asked question that we receive is “should I mount my AIS aerial on the top of the mast?” based quite correctly on the fact that as VHF is line of sight – higher is better. Whilst this is undoubtedly true there are reasons why an AIS aerial can be mounted low down:

First, as leisure boat users the vessels we most want to know about and to avoid are the commercials. Now, not only do they use *Class A at 12.5 Watts, they have their own AIS aerial high up on their superstructure so already line of site between our small boat and the super tanker is going to be better than between two yachts on Class B with aerials mounted on 30+ foot masts.

Second, and this is a very important factor to keep in mind, AIS is a close up collision avoidance system so being able to see vessels 20 miles away isn’t necessary, although it does have some novelty and interest value. The vessels that you want to see and to see your own boat will be relatively close by, so no need for long distance transmission and reception to the horizon.

Third consideration are signal losses across the cable, any benefit of the aerial mounted on top of a 30 foot mast may be negated by losses across the  30+ feet of poor quality cable that has been degrading for many years.

A further thought regarding mast mounting as against low down: small to medium power vessels don’t have a 30 foot mast so are obliged to mount their aerials low down. AIS works well for professional fishing and work boats with an AIS aerial mounted on the wheelhouse roof or an A frame – conclusion: an AIS aerial can be mounted low down and still work well for the purposes of AIS.

*AIS Classes

  • A:  Mandatory on all vessels over 300 tons gross and transmits at 12.5 Watts, 
  • B:  2 watts power, reduced access to AIS system depending upon other traffic needs
  • B+ 5 watts power, access to AIS system as for Class A.  Note-all Onwa AIS products are Class B+

Can I use an aerial splitter and use my VHF aerial?

A second often asked question: “Can I use a splitter and share my VHF aerial?”  Yes, you can but usually this will be an electronic switch that senses which transmitter is in use and disconnects the other. However, electronic splitters are expensive – expect £200+ whereas an AIS aerial will be less than half that amount. Another down side to electronic splitters is that only one transmitter can operate at any one time and the other is switched off from the aerial with preference always to the VHF. This may present a serious problem in an emergency when the VHF is operating that the AIS is lost. Consider the scenario of a MOB with the casualty wearing an AIS MOB device. Whilst trying to follow the casualty’s weak AIS transmission, using the VHF on 16 may lose the casualty. One further thought in favour of a dedicated AIS aerial – in the event of the loss of your VHF aerial, the AIS aerial can be used as an emergency VHF aerial.

2023 update: Onwa have recently produced a splitter which is passive, there is a 3dB loss using it but, contrary to the commonly held view (one I confess that I held) that a non-powered splitter will damage both the AIS and the VHF, Onwa guarantees that it it won’t. However, my preference is still to have a separate aerial and I won’t be fitting a splitter on my own boat as I just prefer an aerial and should I ever lose the VHF at the top of the mast aerial I can use the AIS aerial as an emergency backup. 


On my own boat I have a stainless whip aerial mounted on the stern rail of my 28′ sailing boat. I can see Class A (commercial) vessels from 16 or more miles and I can transmit around 8 miles. My very expensive whip aerial also has a bend in it from being leant over whilst deploying a rope during mooring – which is why I recommend the short stubby type as it will be very difficult to damage these by bending and why we sell lots of Glomex RA111AIS stubby aerials. They come with a stainless bracket that can be fixed to a vertical surface or to pole or rail ( 7/8” (22 mm) to 1¼” 30 mm. We also have some AIS rod aerials 50 cm length specially made for AVES Marine by a specialist company in the EU but beware when mooring up!


August 2023 update: following demand for a mast mount version AIS we have chosen to stock Shakespeare 5215AIS stainless whip aerials; they are produced in the USA and are amongst the best available. As with our Glomex aerials, the 5215 aerials (actually, as they are American better call them antennas) are tuned specifically to the to the AIS channels. They are omni-directional i.e. they receive and transmit 360 degrees, and have an antenna gain of 3dB. See my note below regarding gain of an aerial/antenna and why 3dB omni-directional is good for a marine band VHF aerial and 30dB is very bad.






Glomex RA111AIS aerial designed for 162 MHz AIS channels




Bracket for Glomex RA111AIS




FREQUENCY RANGE 161,975 – 162,025 MHz
SWR ≤ 1,2 at 162 MHz
ANTENNA WEIGHT 220g/7,76oz
TERMINATION SO239 for PL259 male connector


*A note regarding gain of an aerial.

An aerial alone cannot amplify the RF signal in the same way that an audio amplifier will make quiet sound much louder (an RF amplifier is needed for that). There is sometimes confusion, even amongst some sellers, quite what gain means when describing an aerial and some will stick to an incorrect view that more gain must in all circumstances be better. Gain in an aerial refers to how directional it is and an aerial can be designed to be more directional (high gain) or less directional (low gain). If you know where your signal is coming from then a very high gain aerial pointed exactly in that direction will be the obvious choice but it will be very much less efficient if pointed in other directions. Where a signal may come from any direction, as will be the case for VHF reception on a boat, it follows that a low gain aerial (less directional) aerial will be better. As an example of how this works in reality: your TV aerial will be designed to be be very high gain as it points to a specific TV transmitter that doesn’t move whereas your mobile phone aerial must receive signals from all directions so will be low gain. As anyone who’s TV aerial moves will know, it becomes very much less efficient if it isn’t pointing at the transmitter.

There are lots of explanations for aerial/antenna gain on the internet and some get very complicated but the link below has quite a good explanation for anyone interested.

Click this link (


We offer an aerial kit customised to suite the buyer’s exact needs:

  • Glomex RA111AIS aerial and bracket (as shown above)                       £75.00
  • Shakespeare 5215 AIS                                                                                  £75.00
  • Low loss RG8X cable cut to your specified length, per metre             £01.45
  • Two high quality PL259 RF connectors with gold plated centre pin  £10.00

+ Post (for UK) this is £5.50. If part  of a plotter sale there will be no post cost for Glomex aerials as we add the aerial to the plotter box. Shakespeare aerials are 903 cm long and cannot go by Royal Mail and are a special cost by other freight carriers because of the length. Freight cost is still the same cost (£5.50) for Shakespeare but it will be a separate freight charge.

Making your install a little bit easier:

Cable can be made up with both PL259 connectors installed or, to make installation of the cable through bulkheads and deck fittings easier, one end can be prepared ready for fitting but the connector not actually fitted. Without the PL259 installed you need only a 7mm hole to pass the cable through a small space, with a PL259 installed you will need around 18mm. After running the cable simply screw on the PL259 and solder the tip – which you must do as otherwise very serious signal losses will be experienced; we include some low melting point solder in the kit for you to do this. Low melting point solder is preferable as heating the centre pin for too long may result in softening the thermo-plastic material that supports the centre pin and a consequential distortion of the centre pin alignment; even though the PL259 plugs that we supply are chosen specifically to reduce this risk. Also in the kit we include some heat shrink to seal the PL259 against water ingress. We actually include a choice, standard heat shrink that shrinks to fit with general heat, glue-lined heat shrink if you have a heat gun or self amalgamating tape if either of the other two cannot be used. The glue lined shrink is the best as it absolutely seals the cable and the PL259 entry but it does need a heat gun to work. Standard heat shrink works at a lower temperature so any local heat will do it. Self amalgamating tape is a silicon rubber tape that is non-tacky but when stretched sticks to itself forming a solid rubber seal. Wrap this around your seal stretching the tape at the same time, it forms a solid rubber wrap. If new to this product – do get a roll for your boat toolkit as it has so many uses.

An important installation note: When installing your cable, if you use cable ties to secure the cable, DO NOT tighten so that the co-axial cable becomes compressed. This is surprisingly important for the reason that when a co-axial cable is compressed, the soft inner insulation (dielectric) will be squeezed changing its characteristics as the inner and outer conductors move closer together. This has the effect of changing slightly the impedance of the cable at that point which will affect the R.F signal flowing through the cable by reflecting a part of it back to the source and consequential loss of performance. Pulling a cable tie so that it is snug but not tight and if using cable clips that they don’t compress the cable is very necessary to secure a co-axial cable. For the same reason any bend in the cable should be a gradual one so that the cable is not kinked and right angle bends around corners should never-ever be the case unless the bend is gradual. Many co-axial cable manufacturers state a preferred Curvature of Radius (the bend that the cable can accept without damage/compromising signal). This is generally minimum 5 x the diameter of the cable. For our 7mm RG8X this will be 35mm although I would be more comfortable with 40mm as a bend.

Call, mail or message me via via this site if you would like to buy one of these AIS aerials and advise the length of cable needed or if you just want some more advice on an AIS aerial or antenna. When I receive enquiries from outside of the UK I have to be careful to use the correct terminology for an aerial/antenna appropriate to the country that the caller is contacting me from, this leads me onto a question, which is correct?

Aerial or Antenna?

From Wiki the definitions of the two words are: ” antenna is a synonym of aerial. and aerial is a synonym of antenna” so no difference at all as both are used to describe the same thing i.e. a rod, wire, or other structure by which signals are transmitted or received as part of a radio or television transmission or receiving system. Aerial is in common use in Britain and antenna in the U.S.

An interesting aspect is that the noun antenna means a pair of long thin sensory appendages on the heads of insects, crustaceans, and some other anthropods, which could cause some confusion but actually doesn’t as we all understand what an antenna is in the context of radio transmissions.

© alanvick2021 (revised August 2023)

Alan Vick.      07930 127512



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