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VHF Antenna: The most important factor for getting AIS to work well

Posted by Doug Miller on 6/4/2019 to Configuration Examples

Having sold thousands AIS devices over the years, you start to get a feel for the most important factors for ensuring that AIS works the way it should. In my experience, the most important factor is the VHF antenna connected to the AIS system. And when I say VHF antenna, I include the antenna itself, the cable, the connector and the location of the antenna.

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.

So, with that brief introduction, here are my top recommendations for making AIS work well with your VHF antenna:

  1. Antenna placement: In this setup, the antennas are too close to each otherYour VHF antenna attached to the AIS device needs to be as high as possible and as far away as possible from other vertical metal objects. Vertical metal objects include other vertical antennas, sailboat masts, radar masts, stays, bimini frames and any other lengths of metal parallel to the antenna. Any of these metal objects can cause multi-path interference or potentially block a clean signal getting to or being transmitted by the antenna. Think of the radio signals being light beams and the metal objects around it being mirrors and you get the idea.
    How close is too close for a VHF or AIS antenna placed near another antenna or metal object? NMEA recommends 6 feet of vertical and horizontal separation between the VHF and AIS antennas. Most manufacturers recommend more. For example, AMEC states in its CAMINO-108 manual, “… the VHF antenna should be placed as high as possible and at least 5 meters away from any constructions made of conductive materials.” Vesper recommends, “Mount the antenna as high as possible (at least 2-3 meters (6-10 ft) above the water surface) and as far as practical from any existing VHF, HF or radar antennas and any metal structures. It is not recommended to place the VHF antenna directly alongside a mast or another VHF antenna such as on a mast-head.” Note that we are talking about any metal, not just active antennas. In my personal experience, having two antennas even 4 feet apart can have a substantial impact on range. In one test I ran, I saw AIS targets 30 miles away with a single antenna. With two antennas 4 feet apart, the receive range dropped to 8 miles.
    Antenna separation from other metal objects is by far the biggest single issue we run into as a factor for degraded AIS performance.
    How do you resolve this if your antennas are too close to other antennas or metal?
    1. Use an extension pole to elevate your AIS antenna above other antennas or metal. Stacking antennas usually works fine. This means having one antenna at one height and the second antenna higher so that there is no vertical overlap.
    2. Locate the AIS antenna in another part of the boat further away from other antennas and metal but again, try to get the antenna as high as possible.
    3. Remove any antennas that aren’t being used. While antenna farms might look cool, they typically mean degraded performance for all antenna-based devices.
    4. Consider using an antenna splitter. The current models of AIS/VHF antenna splitters that we sell work really well. Each of our main manufacturers sell a splitter that is designed to work with their AIS transponders. Use the splitter with the highest VHF antenna that is furthest away from other metal structures. For example, on a sailboat, sharing your single VHF antenna on the top of the mast will usually work much better than a single dedicated AIS antenna installed on the stern rail.
  2. Antenna type: The type of VHF antenna you use with your AIS system can also have a big impact on AIS range. Most VHF antennas are optimally tuned around 155-156 MHz but AIS uses frequencies around 162 MHz. In some cases, a good VHF antenna might be substantially less tuned at even close adjacent frequencies.
    If you are using a dedicated VHF antenna for your AIS system, consider purchasing a VHF antenna that is tuned for AIS frequencies. Both Vesper and Shakespeare make AIS-tuned antennas.
    If you are planning on using an antenna splitter consider matching it with the Vesper VA159 antenna which is tuned at 159 MHz between the typical VHF radio frequencies and the AIS frequencies.
    If you are using a VHF antenna – either an existing antenna or a new one – see if you can get the specs for the antenna to see how much tuning drop off you might expect at 162 MHz. For example, the Shakespeare Galaxy 5225-XT VHF antenna is tuned to 156.8 MHz with a nominal VSWR (more on VSWR later) of 1.5:1 but bandwidth falls off to 2:1 VSWR just 3 MHz away from the optimal tuning. Therefore 162.025 MHz (AIS2) at over 5 MHz away, would likely see a substantial deterioration in signal probably in the 3:1 or more VSWR range for AIS. The Shakespeare 396-1-AIS Classic AIS Antenna would be a better choice as either a dedicated AIS antenna or an antenna used with a splitter given the optimal tuning at 158 MHz – a bit closer to the 162 MHz AIS sweet spot – and its broader bandwidth (6 MHz within 2.0:1 VSWR).
  3. 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. 
  4. Check your VSWR: VSWR or Voltage Standing Wave Ratio is a measurement of how much power is reflected back from the antenna setup. Ideally all the transmission power should go into creating the desired radio signal which would be represented as a perfect 1:1 VSWR reading. However, no antenna setup is perfect and there is likely to be some reflected energy along the connector / cable / antenna path. A VSWR of 10:1 would be very bad with about 2/3 of the transmission power being reflected back to the transmitting device. This not only means your signal is not going as far as it should – you also run the risk of the reflected energy damaging your transmitting device (i.e. your VHF radio or AIS transponder). A VSWR of 1.5:1 is excellent, 2:1 is considered very good and even 3:1 is ok although anything beyond that is not good and should be addressed. Again, the higher the VSWR is, the less range you will have with your AIS and VHF radio setup.
    How do you know what your VSWR is? There are at least two ways to measure your VSWR. Some of our AIS transponders (e.g. ones from Vesper and em-trak) include software programs for Windows, Mac or mobile devices to see the status of your AIS transponder including your VSWR reading. Another method is to purchase a VSWR meter (such as the Shakespeare ART-3 Antenna Tester) and use it between your VHF radio and your antenna cable to check reflected power of your antenna setup. Either method will allow you to get some idea whether your antenna setup is working ok or not. Note, we have heard from one manufacturer that some fiberglass antennas simply do not ever show a good VSWR reading even when they are working properly.
  5. Check your AIS receive and transmit range: Once you have your AIS system installed, a good general indicator of how well your antenna setup is working is looking at AIS target range. Do this on the water away from large metal items, such as boat houses or sailboat masts, which might cause interference or block the signal. If you are in a busy shipping area and are on the open water, you should be seeing Class A AIS targets at least 20 miles away and in many cases over 30 miles away if your AIS system and antenna are working properly. If you only “see” targets 5-10 miles away and know there are more targets in the 10-30 mile range, then you may have an issue with your VHF antenna setup.
    Likewise, another good test is to see how far you are transmitting your AIS signal. Do this on the open water. Find another AIS target on your display device – preferably one 5-7 miles away if you have a Class B 2-watt system or 10-15 miles away if you have a 5-watt Class B system. Select another recreational vessel and call them on your VHF radio (preferably using DSC) and ask them if they see you on their AIS system. Don’t call a busy commercial vessel such as a ferry and don’t call the coast guard. There could be lots of reasons why they might not see you but if they do see you then you have a confirmation that your system is working. Online AIS services such as MarineTraffic can be used but due to a number of factors this should not be considered a definitive test. Again, if they see you – great. If they don’t see you, it is not proof that you are not transmitting. See my article here for more information.
  6. Check cables and connectors: Antenna cables and PL259 antenna connectors can substantially impact the performance of your antenna setup.
    Check your cables for damage including aging or cracking which may cause corrosion and degradation of the antenna cable materials. Note that the longer the antenna cable run, the “thicker” your antenna cable needs to be. For example, if you use RG58U with an outside diameter of 3/16” for a 100-foot run you will experience about 5db of loss at 160 MHz. If you upgrade to LMR400 cable which is 13/32” thick, your loss for the same run would be a much more acceptable 1.6db.
    Antenna connectors can be another huge area of concern. Over time, connectors can get coated with green or gray corrosion which can block or reduce the connectivity between your device and the cable. The inside of connectors can also degrade of over time. I had one experience where I was seeing poor AIS performance. The issue ended up being the PL259 connector, which once it was replaced with a new connector, resolved my performance issue. If you have antenna cable “joiners” (SO239-SO239 connectors), for example at the base of a sailboat mast, check all the parts for corrosion and clean with steel wool or replace parts showing possible deterioration. Be sure to seal your antenna cable connections to prevent moisture intrusion.
  7. Check your antenna. It may seem obvious but do check your antenna for physical signs of damage or deterioration. The cable connector on the bottom of the antenna needs to be clean. If you have a fiberglass antenna that is delaminating, it might be time for a replacement. If you have a whip antenna that is bent, it will not work well. Of course, make sure your antenna is up (vertical) and you are not inside a metal boathouse when you are testing.
  8. Test with a spare antenna. Having a spare 3-foot whip antenna with an attached cable on your boat is a good idea in case your main radio antenna is damaged or lost. But this antenna can also come in handy for testing your AIS or VHF radio setup. If you’re not sure if your antenna or antenna cable is the reason for degraded performance, try your spare antenna, temporarily rigged outside the cabin as high and away from metal as possible. If VSWR and AIS target distance improves with the new antenna, then you probably have an issue with the main antenna (the antenna itself or the antenna location) or possibly the main antenna cable or even the cable connector.
  9. Antenna cable length phase mismatch. According to the support folks at em-trak, in some cases, the length of your antenna cable can impact your antenna performance. If you are seeing poor VSWR and / or poor AIS performance, try adding or subtracting a quarter wave length of cable to your main antenna cable. A full wave length for marine VHF frequencies is about 6 feet so a quarter wave length is about 1.5 feet. It is probably easiest to try temporarily adding 1.5 feet to your setup (use a short cable with a PL259 on one end and an SO239 on the other) before cutting cables and making permanent changes.
  10. LED lights and other sources of interference. For a couple of years, we have been aware of possible interference from LED lights – in particular masthead LED lights close to the main VHF antenna. Most of the LED manufacturers are aware of this now and have fixed the issue but if you see degraded VHF and AIS performance when you turn on your LED anchor light, then you might be impacted by this issue. The US Coast Guard have an advisory on this here.
    Old-style radars are also a potential interference source. Try to keep your VHF, AIS and GPS antennas out of the radar beam – which is generally about 10 degrees above and below the horizontal plane of the radar.
    High power transmitting radios (such as HF and SSB radios) and their antennas (such as a tuned backstay) can also be sources of interference. Make sure your AIS antenna is not on the same plane as the other radios.

Hopefully this guide will help you debug any issues with AIS transmit or receive performance. While there is a lot of material here, ultimately good performance boils down to having a decent quality antenna setup (antenna, cables, connectors) installed as high as possible and as far away from other metal or interference sources as possible.

For additional information on this subject, we recommend the following resources:

As always, if you have comments, questions or feedback, feel free to use the comment tool on this page.

Safe boating!

 

Doug Miller

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