Features February 2019 Issue

Special Report: How to Prevent AIS and VHF Antenna Malfunction

Power loss can plague poorly installed systems.

Allowing sailors to track nearby shipping traffic and to continuously broadcast their own boat’s position, the automatic identification system (AIS) transceivers available today have been a breakthrough for collision avoidance, communication, and distress signaling. PS tester Joe Minick’s field trials carried out in the Dardanelles Strait provides a useful overview of the pros and cons of AIS (see PS August 2010 “Marine Electronics: AIS Gets Ocean Tested Near Dardanelles Straits”). More recently, editor-at-large Ralph Naranjo examined how AIS fit into the safety-at-sea picture in his special report, “Trends in Safety Gear,” PS February 2018).

It’s important to keep in mind that an overwhelming number of small boats and many ships are still not equipped with AIS, so maintaining a vigilant watch with all available means remains an important aspect of seamanship, especially in coastal waters.

It’s also important to be aware of some common technical problems that can interfere with AIS performance. During one recent offshore race, some boats reported that the AIS signals of other competitors reportedly “disappeared” at night. These and other similar incidents prompted the U.S. Coast Guard to publish a safety alert after it identified that some (usually older, non-standard) LED navigation lights can interfere with AIS and VHF (see “Coast Guard Seeks LED Input,” PS October 2018).

A recent review of AIS systems aboard Volvo Ocean racing boats turned up a different problem. Even at this high level of racing, AIS equipment on many boats had less than optimal performance, primarily due to installation issues.

Fortunately, both of the above problems can be resolved with proper attention to detail when installing and checking equipment. In this special report, written by renowned offshore navigators Stan Honey and Dan Jowett and supplemented by articles from our PS testers, we take a closer look at ways to ensure your boat’s AIS/VHF transceiver installation is functioning as it should.

Choosing an Antenna

Most marine VHF masthead antennas are mechanically mounted using the coaxial connector. Common VHF whips use a female SO239 socket that mates with a PL259 plug on the coax cable. PL259/SO239 coax connectors (aka “UHF”) are common, mechanically rugged, and installable without tools, but they are not weather resistant. If you use an antenna with a SO239 mount and so use a PL259 connector on the coax, be sure to follow the notes below in this article to add water resistance.

Half-wave, end-fed antennas are about 36 inches (1 meter) long with a matching transformer at the base. These are the best choice overall for use on sailboats. These antennas provide reasonable gain but are not so directional as to be ineffective when heeled (see adjacent article, “Antenna Gain and VHF Transmission Range”). They do not require a ground at their base.

Half-wave antennas that mount with a PL259/SO239 include the common Shakespeare 5215, and the GAM SS-2 among many other options. Again, if you use an antenna such as these with a PL259/SO239 connection used as a mount, waterproof the connection carefully as described below because the PL259 is not weatherproof.

The V-Tronix MD20N is a half wave antenna made by Shakespeare which uses an N-type connector at its base. This antenna is the best choice for a half-wave antenna because the N-type connector is both mechanically rugged and inherently weatherproof, and so is ideally suited to the challenging environment at the masthead of a sailboat.

Crimping N-type connectors should be done with a special crimper, widely available from radio and telecom supply houses for about $30. Any electronics technician will have the proper tool but it is a tool worth owning.

Quarter wave whip antennas of 15 to 18 inches in length are widely used on racing boats and meet the OSR requirements. Farallon makes an Ultrawhip which mounts using a weatherproof TC connector as its base. The widely used Shakespeare model #5216 uses a PL259/SO239 at its base that needs to be carefully weatherproofed.

All quarter-wave antennas require that the base of the antenna, specifically the shield of the coax, be grounded to the mast at the base of the antenna. Aluminum or carbon masts work fine as a ground/counterpoise.

DC ground warning: You should have only one DC connection between the DC electrical system ground and the hull/keel and the ocean. Generally this connection is at the engine negative terminal or its bus. If the shield of the VHF coax is connected to the aluminum or carbon mast at the masthead (which is essential for a quarter-wave antenna) you will create a second and undesirable DC connection between the electrical system and the ocean via the keel. This is because the mast is usually connected to the keel for a lightning ground. 

To avoid the undesirable additional DC ground connection put an inner-outer DC block in the antenna coax. This and other important DC grounding details are covered in our report on grounding, "How to Wire Your Boat's Grounding System." Proper grounding and related topics are also addressed in "Systems Installation," the first volume in our five-volume ebook "Marine Electrical Systems."   

Safety of Life at Sea (SOLAS) ships—including tankers, passenger vessels and cargo ships over 300 gross tonnage—must be equipped with Automatic Identification Systems (AIS). In the U.S., carriage requirements also includes vessels over 65-feet. AIS can allow you to “see” a distant ship, but is not a substitute for radar.

VHF AIS Antenna Location

The Offshore Safety Regulations (OSR) for the offshore racing require that the VHF whip be mounted at the masthead. This makes sense because daytime VHF range, when there is unlikely to be ducting, is limited to line of sight. For you to have a reasonable range (e.g. 7 to 8 miles) to receive a call for help from somebody using a handheld VHF at deck level, sailboats should take advantage of the height of their mast.

The OSR’s permit the AIS to either share the primary antenna via an AIS antenna splitter, or to use a dedicated antenna that is mounted at least 3 meters above the waterline. Given that one of the most important uses of AIS is to locate a person in the water wearing an AIS MOB beacon, the masthead location is important if you want to receive AIS MOB beacon signal from the maximum possible range (about 1-3 miles depending on mast height).

Thus, it makes sense to use an AIS antenna splitter and share the masthead VHF antenna for both VHF and AIS. The AIS will not receive while the VHF is transmitting on voice, but AIS MOB beacons repeat their transmissions frequently, and MOB’s don’t move very fast, so the shared antenna at the masthead makes the most sense.

VHF antennas should never be mounted closer to one-another than one wavelength, which in the case of marine VHF is nearly 2 meters. Thus it isn’t possible to mount two VHF antennas on the same masthead.

The PL259/SO239 connection at the base of this 36-inch, 3dB VHF antenna needs to be carefully sealed against water intrusion.

RF Cable selection

The OSRs require that the coax from the VHF radio and AIS have a maximum loss of 2.2 dB. Add up the loss per meter of the coax and make sure that the total loss of your coax is less than 2.2 dB. Include all segments of coax from the VHF to the AIS antenna splitter, and then to the masthead antenna. Stan Honey has created a handy calculator at his webpage: http://honeynav.com/VHF-coax-loss-calculator/

Losses for some common types of coax (at 156.8 MHz), in dB per meter, are between 0.0324 dB/m for LMR600 and 0.1889 dB/m for RG58

Coax connectors have essentially zero loss at marine VHF frequencies, and modern AIS antenna splitters have essentially zero loss on receive. If your coax has less than 2.2 dB loss and your VHF whip is at least 15 inches long, your installation will meet the OSR’s and will work well. The most common performance problem is water intrusion into the coax at the masthead or at the base of the mast which can also be a wet environment.

To calculate loss, use the online calculator referred to in the article. Here is shown the amount of loss in 33 meters of cable. The goal is to keep power loss below 2.2 dB. In the above example, one would have to upgrade to LMR-LW400 to meet that standard. At left are two common types of marine coaxial, RG-58, and RG-8/URG-213, and the thicker LMR-LW400.

RF Cable Installation

If possible use water-resistant coax connectors such as Type N or TNC. PL259-SO239 and BNC connectors are not weatherproof and so are less dependable in a marine environment, particularly at the masthead.

Be cautious of “quick-connect” connectors such as BNC where the bayonet shell is held by springs. These connectors are made for rapid connection and disconnection. They do not provide strong and vibration-resistant mechanical support for the contacts.

If you do use a PL259 connector, unless you are an experienced professional, it is preferred to use a connector with a crimp for the shield rather than the kind that requires that the shield be soldered. A crimp tool only costs about $30, and can also be used on N-Type connectors as well. The PL259 connectors that require that the shield be soldered require significant skill and practice to attach properly, and are nearly impossible to install correctly when sitting in a bosun’s chair at the masthead on a windy day. There is a “how-to” reference at the end of this article.

Don’t skimp on the connectors themselves. Amphenol is a good source of quality connectors. Avoid quick-connect piercing or screw-on connectors for permanent installations.

If you use LMR-LW coax (e.g. lightweight) you must use a connector with a crimp for the shield because the shield braid is aluminum and cannot be soldered with conventional flux and solders.

All coax connectors (including weather resistant ones such as N-type or TNC) should be carefully sealed as described below, but careful sealing is particularly important for PL259 connectors which have no water resistance by themselves.

If a coax connector is used in a wet environment (e.g. the masthead or mast base), fill the interior of the coax connector with silicone dielectric grease (e.g. Dow Corning 4, but there are many equivalents). With no air cavities in the coax connector, there is no place for water to go. The silicone grease adds zero RF loss at any frequency below 1 GHz but adds enormous immunity to water intrusion and corrosion.

Wrap the connector tightly with self-amalgamating tape (e.g. Scotch 2228) for water protection, and then wrap again with vinyl electrical tape (e.g. Scotch 33+) for mechanical protection.

Test the coax loss periodically to detect water intrusion.

LED Light Interference

A few older design masthead LED lights (e.g. running or anchor lights) emit noise in the marine VHF band from their switching circuitry. A quick way to test for this is to tune your VHF radio to an unused channel, turn the squelch all the way down so that you can hear the receiver noise, and then turn the masthead lights on and off to see if you can hear them.

Another test that works in a coastal area with lots of AIS contacts is to run your AIS transponder for 10 minutes, and develop an impression of how many AIS contacts there are, and how far the most distant ones are. Then turn on the masthead LED lights, restart the AIS transponder, run it for the same period of time, and compare the results.

Most currently available masthead LED lights are sufficiently well-designed to emit no interference, or just barely noticeable interference, in the marine VHF band. It is worth testing however. Some early and poorly designed masthead LED lights, or LED “bulbs” used to replace the original halogen bulbs, would nearly deafen a VHF or AIS due to emitting strong RF interference.


Comments (7)

When Chris Oxenbould, Stan Honey, and I investigated the issues involved in high speed sailing yachts operating in congested waters, we found that several of the boats in the Volvo Ocean Race had compromised VHF antenna installations, leading to reduced AIS (and likely VHF radio) communications. This paper is available online at Stan Honey's website, however I can't embed a link so you'll have to search for it. Try VOR High Traffic Density Report.

A very brief summary of the report is that the use of non-waterproof VHF connectors at the masthead, leading to soaked foam dielectric insulation, causes enormous power and sensitivity losses, as the above article states.


Chuck Hawley

Posted by: ChuckHawley | October 15, 2019 11:23 AM    Report this comment

I know this article focused on the Radio side of AIS antennas but what about the GPS side?? A friend and I have agreed to disagree on this topic. I purchased an app for my phone that tells me GPS signal strength. Where I sail in thE Pacific Northwest I get basically the same strength below deck where I mounted my antenna as I get above deck - with lots of strong signals being received. He contends it needs to have a open view of the sky (he mounted it under the dodger next to a support tube). He contends my install us not safe.

Care to weigh in.


Posted by: LeslieTroyer | October 15, 2019 10:20 AM    Report this comment

Neither AIS nor Radar complete the requirements for a proper lookout. "All available means..." includes a visual and audible watch. Eyes and ears and a 360 degree scan several times an hour. I believe anyone on watch needs to know how to take a set of bearings to a vessel and interpret the drift direction (left,; right; steady). Pretty simple and no hand bearing compass required.

From my observation, there's way too much reliance on electronics-only for most navigation tasks today. Great stuff and I navigated ships for years plotting multiple contacts and collision avoidance maneuvers on a radar scope with a grease pencil and straight edge (before AARPA), but it's not a replacement for eyes, ears, or non-electronic brain matter. Simple still works. Big vessels or small.

Posted by: Lakota44 | February 21, 2019 9:49 AM    Report this comment

The Shakespeare V-Tronix MD20N antenna mentioned in the Article is an EU product. Does not appear to be sold in the US.

What antennas using the N type connector are sold in the US?

Posted by: J. MacKay | February 2, 2019 10:30 PM    Report this comment

The Standard Horizon Quantum GX6500 combines Class B AIS and VHF transceivers into one device, but is been tied up in FCC approval for a couple of years now.

Posted by: Caxton Rhodes | January 27, 2019 11:23 AM    Report this comment

A side benefit of installing an AIS transceiver is that it that it will check the integrity of your VHF cable.

I had my mast down in 2018 to replace the standing rigging. While I was at it, I had the boatyard pull an ethernet cable for a Bad Boy Wi-Fi extender and new wind transducer cable through my mast cable tray. Other cables in the cable tray were the VHF cable and the anchor light duplex cable. During the cable pull the VHF cable was scraped and shorted inside the cable tray. The mast was stepped without any check on the VHF cable (which is RG213 marine grade) or new wind transducer cable, I did check the ethernet and the anchor light cables before the mast went up.

After the mast went back up I got an VSWR alarm on my AIS system. I had a friend with a SWR meter and time domain reflectometer come look. He found that my in-mast VHF cable was shorted (it should measure as an open circuit) and using the time domain reflectometer determined that the short was 6.2M from the base of the mast just above the lower spreaders.

The VHF cable could not be pulled with the mast up, so I had the yard take down the mast again and pull the cable. To no surprise found the gouge and short exactly 6.2M from the VHF connector at the mast step.

Microscope pictures of the VHF cable break clearly show it was damaged recently (as opposed to 10 years ago when installed). The pictures and assessment were by a materials engineer. The likely cause was during the cable pull the tag line connection jammed and gouged the VHF cable resulting in a short. The yard agreed picked up the tab for unstepping-new cable replacement-stepping.

The scary thing is that if I had not had an AIS transponder (Raymarine AIS650) that sent the VSWR warning to my chart plotter, I never would have known that my VHF antenna was dead. I could still receive and transmit VHF but with extremely small range. The AIS transponder could still receive AIS but would not transmit due to the antenna fault. Since I am in the San Francisco Bay area and AIS and VHF signal are very close and strong I may not have noticed anything wrong until going offshore.

So bottom line, in the future when buying a boat or after anything is done with my mast, I will check the cable connection at my VHF radio for continuity and SWR. It's an easy cheap check to make sure you have a working radio!

Posted by: Caxton Rhodes | January 27, 2019 11:13 AM    Report this comment

I am one of those people with tricolor LED interference. The light is 2016 vintage. The minute I turn on my tricolor I loose AIS receive from class B sources. Fortunately I can still see ships with their more powerful class A transponders. When I pull my mast in the next few months one of my projects will be to remove a post below the tricolor which puts the light about one foot above the masthead and, coincidentally, in line with the whip of the VHF antenna. My hope is that getting the tricolor low and below the most sensitive lobes of the whip will improve the situation...maybe solve it altogether.

With regard to splitters. I have the Matrix AIS/GPS (GX2200) installed with splitter for my AMEC Camino transponder all plumbed with LMR400. This gives me a backup receiver by virtue of the radio AIS. What I don't understand is why the radio couldn't incorporate a full fledged AIS receive/transmit function. I assume there is some sort of a technical or high cost issue but it would sure be a nice package, GPS, DSC, full AIS transponder and radio all in one unit with appropriate NMEA 2000 interface to facilitate data distribution. The current GX2200 is funky in this regard, if I place the GX2200 AIS data online I find my own boat AIS signal is not discriminated and causes constant alarms as a result. So I just leave the radio stand alone. One good thing, it has an AIS alarm which will wake the dead..or give you a heart attack and make you dead.

Posted by: Mike Cunningham | January 25, 2019 5:22 PM    Report this comment

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