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

300 80
Aves Marine
by Alan Vick

AIS aerials/ antennas and AIS:

AIS (Automatic Identification System) is transmitted on two channels at the top of the Marine VHF band. Regular marine band VHF aerials are tuned to around the centre of the band at 156.8 MHz (channel 16)  and AIS aerials at 162 MHz (the AIS frequency). This means that 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:  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 dependant 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 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 expensive 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 buy in has been 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 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 undoubtable 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, 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 likely better than between two yachts 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 close up so no need for long distance transmission and reception to the horizon.

Third consideration are signal losses across the cable, as 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.

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 and AIS works for power boats……conclusion: an AIS aerial can be mounted low down and work well for the purposes of AIS.

 

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 it must be an electronic splitter which is an electronic switch that senses which transmitter is in use and disconnects the other. This is most important as if both transmitters are connected together with a passive splitter (used to connect two receivers together) and the VHF is keyed on high it will pump 25 watts of RF power into the AIS unit of the plotter which will not appreciate the experience and may result in at least a blown AIS board and possible damage to the VHF as well. An electronic splitter will prevent this by only allowing only one transmitter to connect to the aerial at any one time. However, electronic splitters are expensive – expect £200+ whereas an AIS aerial will be less than half that amount.  Another useful use for the AIS aerial might be that in the event of the loss of your VHF aerial, the AIS aerial could be used as an emergency VHF aerial.

 





On my own boat I have a 50 cm 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 is virtually impossible to damage these by bending and why we  sell 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

 

 

Glomex RA111AIS aerial designed for 162 mHz AIS channels

 

 

 

Bracket for Glomex RA111AIS

 

 

TECHNICAL SPECIFICATIONS for Glomex RA111AIS –

FREQUENCY RANGE 161,975 – 162,025 MHz
*GAIN AVERAGE 1 dB *
IMPEDANCE 50 ohms
POLARIZATION Vertical
SWR ≤ 1,2 at 162 MHz
MAX INPUT POWER 50 W
DC GROUND No
ANTENNA LENGHT 140mm/5”
ANTENNA WEIGHT 220g/7,76oz
TERMINATION SO239 for PL259 male connector

 

*A note regarding gain of an aerial.

An aerial cannot amplify the RF signal in the same way that an audio amplifier will make quiet sound much louder. 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 in that direction will be the best choice but it will be very much less efficient in other directions . Where a signal may come from any direction 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.

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 (antenna-theory.com)

 

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

Glomex RA111AIS aerial and bracket (as shown above)            £65.00

Low loss RG8X cable cut to your specified length, per metre   £01.35

Two high quality PL259 RF connectors                                         £08.00

+Post (for UK this is £5.50. If part  of a plotter sale there will be no post cost as we simply add the aerial to the plotter box.

 

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 etc easier, one end can be prepared but the connector not actually fitted. Without the PL259 installed you need only a 7mm hole to pass the cable through a bulkhead (with a PL259 installed you will need around 13mm). 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.

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 important.  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, generally non-compressible cable clips or tape is preferred to fix 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 be the case.

Call or mail me 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. Which leads me onto the 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 as both are used to describe the same thing 1.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 arthropods which could cause some confusion but actually doesn’t as we all understand what an antenna is in the context of radio transmissions.

© alanvick2021

Alan Vick.      07930 127512         alanvick@avesmarine.com