One of the most common questions we are asked by our AIS customers, is “what antenna should I use for my AIS ?”, so we thought we would post a short article to provide an answer.
Basically there are two options; fit a second dedicated VHF antenna a suitable distance from the vessels main VHF antenna or use the main antenna for both VHF and AIS by fitting a special device called a “Splitter”.
Generally our recommendation for yacht owners who want to fit an AIS, is to use a “Zero Loss” splitter like our SPL2000. By utilising the main VHF antenna at the top of the mast, you will definitely get maximum transmit range, plus the ease of installation often makes it a cheaper option than paying for a second antenna to be installed.
However, for power boaters who do not benefit from the height advantage of using the main antenna at the top of the mast and for AIS receiver owners where the splitter can often cost more than the AIS receiver itself, it is often desirable to fit a second VHF antenna, particularly if you intend to save money by doing the antenna installation yourself.
We have already covered the issue of AIS antenna separation in another post, so we will not repeat this information again, but we will just touch on the issue of “AIS Tuned” antennas versus normal VHF antennas.
AIS operates on two dedicated channels within the marine VHF frequency range – 156.0 to 162.025 MHz. The two AIS channels are at the top end of this range namely; 161.975 and 162.025 MHz (channels 87B and 88B). Most VHF antennas are designed to give maximum gain across the whole VHF frequency range centred on Channel 16 (156.8 MHz).
Pretty much all of the antenna manufacturers now produce “AIS Tuned” antennas, which have their centre frequency shifted from Channel 16 to 162Mhz (exactly half way between the two AIS frequencies). If you are going to mount your AIS antenna on the stern rail of a yacht or radar arch of a power boat, then using an “AIS tuned” antenna to get an extra bit of gain to compensate for the antenna being effectively at deck level, is a good idea.
The graph above shows how the tuning of the antenna to 162MHz gives it an extra boost in VSWR (gain) across the two AIS frequencies.
Which ever option you choose, having AIS on your boat will without doubt make your sailing experiences safer and less stressful in poor visibility or when crossing busy shipping lanes. Even a simple receiver with a small whip antenna at deck level will keep you informed of what ships are around you and which ones you need to keep an eye on.