I recently added a Transmitting AIS antenna to an existing (working) 8 foot Shakespeare VHF antenna with alarming results. It was giving me high VSWR alarms (3.5:1) despite it testing good (1.5:1) on the VHF. Although the AIS frequencies are technically in the marine VHF band they are far enough away that some antennas will struggle with them. The voice channels are generally in the 156-157MHz range and AIS is up at 162MHz. If you look at some of the specs of common antennas you’ll see why. A SWR of 1:1 is perfect and under 3:1 can be acceptable. As it gets higher, performance degrades and equipment damage is possible.
Shakespeare 5225-xt 8 ft SWR 1.5:1 at 156.8MHz, Bandwidth 3MHz within 2.0:1
Shakespeare 5400-xt 4 ft SWR 1.5 at 156.8MHz, Bandwidth 5MHz within 2.0:1
Shakespeare 5225-xt-AIS 8 ft SWR 1.5 at 158.6MHz, Bandwidth 3Mhz within 2.0:1
Shakespeare 5396-AIS 4 ft SWR 1.5 at 158MHz, Bandwidth 5Mhz within 2.0:1
You’ll notice the top 2 VHF are made to work best on channel 16. They will then get worse SWR the further away from that center point. (see figure 1). The 8 foot is best working between 155.3 and 158.3MHz. The 4 footers have a wider working range from 154.3 to 159.3MHz however, both fall short of AIS channels. In my case the 8 foot was a bit over 3:1 and was alarming. A 4 foot might not have hit the 3:1 alarm point, in the case of the AIS antennas. What they have done is shifted the center point up which ends up being a bit of a compromise between voice and AIS. It’s now best at frequencies that are not even used. However both will handle voice and AIS within better specs. The 4 foot one a little better. Changing the 8 foot VHF antenna out for a 4 foot AIS antenna with a 4 foot extension pole now gives me a ~1:1 reading on the AIS, and will give the similar height and range as the 8 foot one did.
It’s much less of an issue with a receiving only AIS. And I wouldn’t bother to replace antennas. You will notice the marine weather channels are also around 162MHz and you can receive those just fine.