In general, any marine VHF antenna is suitable for receiving AIS signals. If you try to select the one that suits you best, you will come across the Directional (Yagi) and the Omni-directional (Omni) terms quite often.
Let's have a quick look at both types:
- Yagi Antennas (aka Directional/Uni-directional)
As implied by their name, Directional antennas are able to pick-up signal transmissions coming from a specified direction (the direction the antenna is facing). Thus, directional antennas must be aimed in the direction of the potential signal transmitter(s) - in the AIS-receiving case, the vessels in the open sea!
Aiming the antenna efficiently (right on the transmitted signal's source) is essential for optimal reception (strong and high-quality signal reception).
Knowing the width of the antenna's reception cone is the best way to ensure optimal aim. If you are not sure about the actual specs of your antenna, an empirical way is to judge by its dBi (decibel isotropic) rating. The higher the dBi rating the narrower the reception cone. This can help your aiming towards the sea.
Note that: the higher the dBi rating = further distance covered (not area).
Important: In the case of AIS, the Yagi antenna must be vertically polarized (as shown in the picture above).
- Omni Antennas (aka Omni-directional)
Omni-directional antennas can pick-up signals coming from all directions. They are most commonly used for signal reception over a large area where the potential location of any incoming signal's transmitter varies. Thus, unlike Yagi antennas, Omni antennas do not need to be pointed to a particular direction since their reception cone is 360°.
Each specific antenna has a gain rating or dBi (decibel isotropic) number which coincides with the performance of the antenna.
Note that: the higher the dBi rating = larger area covered (not distance).
The location that you plan on installing your AIS-receiving Station may help you define the antenna type that would do the trick for you. In short, Yagi antennas will need some more tweaking but will reward you with long-range reception while Omni antennas are all-around units but more limited in terms of range!
As a rule of thumb, choose a Yagi antenna if you want to boost signal reception coming from a certain direction or select an Omni antenna if you are surrounded by sea and want to receive incoming signals from any direction.