To split or not to split your VHF antenna
If an antenna works well with a VHF/DSC voice radio, this is no guarantee that it will also work well with an AIS transceiver. This is because Class B AIS only operates at 2W output, whereas a VHF DSC voice radio operates at 25W output and so will still radiate a signal of some type. AIS Class B devices like the EM-TRAK B100 do not have the power to “push through” and may not work on such an antenna (even if a VHF Voice radio can use the same antenna).
Use of narrow band AIS ‘tuned’ antennas are not recommended as they are optimised at the standard AIS1 and AIS2 frequencies and may not give optimum performance when regional channel settings are used. **
AIS optimised ‘broadband’ antennas are suitable for use; these antennas typically have a wider bandwidth to give improved performance at the top end of the VHF band where AIS usually operates.
Siting of an antenna is crucial as even a well suited antenna may not radiate a good signal if installed in a poor location. Put simply, keep the antenna upright, clear of anything that could touch it (rigging/other antennas etc), keep a reasonable distance from any other devices/antenna arrays like Radar/HF/VHF radio that may be operating at high power and also keep clear of any other metallic obstructions.
Antenna separation from other metal objects is by far the biggest single issue we run into as a factor for degraded AIS performance.
If you’re still unsure of which antenna is right for you, please refer to the list below for some advice:
Typical VHF antenna specification:
- Band: Marine VHF band (nominally 156MHz to 162MHz)
- Impedance: 50 Ohms
- Gain: 3dBi
- VSWR: <1.5:1 at resonant frequency
- Max power: Typically >25W
- Polarisation: Vertical
- Bandwidth: Typically 6MHz
- Cable: RG58 or better. Note the IMO recommend RG214 or better for SOLAS installations
- Connector: PL-259 (sometimes called ‘UHF’)
Regardless of whether you use a dedicated VHF antenna attached to the AIS receiver or transponder or if you use an AIS-capable antenna splitter, there are some basic things to keep mind that substantially impact the ability of your AIS system to send and receive radio signals.
Before going into some recommendations, it is worth reviewing which frequencies are used by AIS and VHF radios.
While most AIS users are aware that AIS uses two frequencies to send and receive AIS messages (161.975 & 162.025 MHz), there are actually two additional transmit-only channels (156.775 & 156.825 MHz) used by Class B SOTDMA and Class A transponders for long range message 27 AIS broadcasts intended for reception by AIS satellites and other long range receiving stations.
In addition, AIS transponders listen for digital management messages on the DSC channel 70 (156.525 MHz).
Standard VHF radio channels range from 156.050 MHz to 157.425 MHz. For example, VHF channel 16 uses 156.800 MHz.
Knowing the range of frequencies used is important as you select your VHF antenna, especially if you plan to use an AIS/VHF antenna splitter.
Using a splitter: As mentioned above, antenna splitters designed for use with a VHF radio and an AIS transponder work really well. Splitters substantially simplify the installation of AIS and will often resolve the issue of having no good place for a second VHF antenna dedicated for AIS. While some splitters may show very slight signal loss (despite having built-in amplifiers), in most cases using an optimally-placed, high, single VHF antenna with a splitter will work much better than using a dedicated AIS antenna in the middle of an “antenna farm”. But even with a splitter, observe the recommendations in points 1 and 2 above. Keep the antenna as high and free of other metal as possible and use a VHF antenna that has enough spectrum bandwidth to adequately cover both VHF and AIS frequencies.
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