We are often asked about the best antenna to use with our AISNet Base Station Receiver, which is used by many customers to send AIS data to online AIS websites like Marine Traffic, Pinkfroot, AIS Live, Boat Beacon, Shipfinder, etc. We usually suggest a good quality, AIS tuned antenna, with at least 3dB gain and in most cases this gives a good reception range – up to 20-25miles or more. You can get greater range by going to a more expensive directional Yagi antenna, but these often have a limited beam width and may only receive strong signals from vessels within a 120deg arc of the antenna.
Generally speaking a 1/2 dipole, omni-directional whip antenna is probably the best choice for most installations, as they are generally not too expensive, give a good 3dB of gain and will receive targets through a full 360 degrees. In our opinion, it is better to spend time and money on getting the antenna mounted as high as possible than to spend more money on a higher gain directional antenna.
This week, however, we were contacted by a customer who had bought a well known brand of AIS whip antenna but was only seeing targets out to about 12-15 miles. Looking at the spec of the antenna, everything seemed OK but then we noticed that it came with 20m of RG58 coax cable which, according to the attenuation table below, would lose over -3.6dB negating the benefits of using a 3dB antenna.
| Attenuation figures of different types/lengths of Coax Cable | |||||
| Coax Type | 20′ (6m) | 40′ (12m) | 60′ (18m) | 80′ (24m) | 100′ (30m) |
| RG-58 | -1.2dB | -2.4dB | -3.6dB | -4.9dB | -6.1dB |
| RG-8X | -0.9dB | -1.8dB | -2.7dB | -3.6dB | -4.5dB |
| RG-213 | -0.5dB | -1.0dB | -1.6dB | -2.1dB | -2.7dB |
When selecting antennas, the coax cable is often overlooked and, as we discovered, this can have a significant impact on reception range. We suggested that they looked at an alternative whip antenna that came with no cable but had an N-Type or PL259 type connector on the base of the antenna. This would then allow them to make up their own coax cable, using the minimum possible length of low loss RG-213 coax cable.
Even with RG-213, you can lose approximately 1dB for every 10m of cable that you use and as the AISNet is fairly portable, it is always better to move the AISNet closer to the antenna and reduce the coax cable length. You can always run longer lengths of RJ45 network cable inside the building (which does not suffer the same attenuation as coax cable), or better still use a wireless access point or repeater to wirelessly connect the AISNet to the main router in the building.
For more information on our AISNet Base Station receiver, click here.
