How To Reduce Interference On A Marine VHF Radio

With a good VHF radio, you will be able to hear other people’s transmissions loud, and clearly. Occasionally, however, you may find that there is interference, making their transmission difficult to understand. In these situations, it is important to understand some techniques you can implement to reduce interference and improve signal reception on a marine VHF radio.

To reduce interference on a marine VHF radio, you should first ensure that your squelch is properly adjusted. After that, confirm all potential sources of interference on your boat are minimised by grounding all conductors or installing electromagnetic shielding.

With radio equipment, interference just means that there are other electromagnetic signals fighting with the signals you actually want.

When you are listening out for transmissions from other people with a VHF radio, you may notice interference is present. You might hear either as some extra background static, crackles, or even other voices.

The easiest and quickest way to reduce this interference is to make sure the squelch is properly adjusted. The squelch acts a bit like a noise gate, blocking out weak signals so that only strong ones from real transmissions can get through.

There is a process for properly adjusting your squelch, which you can read in detail in this article: How To Properly Adjust The Squelch On A VHF Radio

Once your squelch is adjusted, your only ways to reduce interference further will involve some detective work. You need to track down the source of the interference and eliminate it.

The simplest case is if you hear other voices. This could be that someone else is transmitting on an adjacent channel and it is cutting across you. You should check the channels closest in frequency to your own.

With VHF, channels closest to you are not actually numbered sequentially. For example, if you have interference on Ch 9, the closest frequencies are on Ch 68 and Ch69.

Read more about VHF channels and frequencies in this article: Marine VHF Channels And Frequencies

Finally, the last things you will need to check are local sources of interference. This could be any conductor on your boat. If there is a current passing through, it could be acting as an antenna and transmitting a signal competing with your own.

If you locate a local source of interference, the easiest way to reduce it is to make sure it is properly grounded. This will take the current generating the stray signal and send it to earth instead.

Of course, locating a source of interference is not necessarily as easy as it sounds.

Locating local sources of interference on a marine VHF radio

We know that local interference is caused by conductors on your boat acting like transmitters. Either a current is passing through them, static electricity is being generated, or the conductor itself is generating an electric current.

Locating VHF interference caused by your boat’s electronics

It is fairly common for electronics onboard to cause interference with your VHF.

By its very nature, there is current passing through electronic devices. As the current passes through wires, it has the potential to generate electromagnetic waves that could be picked up by your VHF.

The easiest way to locate electronic devices that are causing VHF interference is to switch off all your vessel’s electronics and see if the interference is still present.

If the interference is still present, it is probably being caused by something else.

If the interference has gone, it is probably being caused by one of your pieces of electronic equipment. You should go around your equipment piece by piece, switching it on. When the interference reappears, you have found the device that is the likely cause.

Locating VHF interference caused by your boat’s machinery

Interference on your VHF can be caused by mechanical parts of your boat. The most common machinery to cause interference is your engine, but it could also be things like your propeller shaft or anchors.

With machinery, there are two possible causes of VHF interference. Either the machinery could be connected to an electrical supply, or the machinery could be generating its own static electricity.

To locate the source of interference caused by your boat’s machinery, you should follow a similar process of locating electronic interference.

You need to use each piece of machinery in turn, continuously checking your VHF radio to see whether interference is present.

Ensure you use the machinery in all its configurations as well. For example, you may find that the static electricity in your propeller shaft causes interference when running ahead, but not when running astern.

Locating VHF interference caused by your boat’s rigging

Interference caused by your boat’s rigging is perhaps my favourite source. It is the most scientifically unusual one because there is no obvious reason for electricity to be present in the rigging.

Consider, however, that your rigging is nothing more than a long, straight conductor. As your vessel moves, you move this long conductor through the earth’s magnetic field.

Moving a conductor through a magnetic field generates a current. As the current passes through your rigging, it can act as a transmitter causing interference.

Additionally, static electricity can be generated in your rigging by the movement of the wind, creating similar interference without your vessel moving.

To determine if your rigging is the cause of your interference, you can see what happens when you manually vibrate the rigging. If the interference changes when you shake your rigging, it is likely that it is causing interference in your VHF.

Reducing VHF interference caused by your boat’s electronics

Once you have located the offending piece of electronic equipment causing interference, you can begin reducing it.

Firstly, you should check out exactly how everything has been installed. During your inspection, you can take steps to reduce interference by:

  • Relocate electronic equipment that is close to your VHF antenna cables.
  • Ensure all grounding connections are intact and the boat’s ground is good.
  • Consider fitting filters or chokes to reduce noise from power cables.
  • “Twisting” wires together instead of running them parallel to minimise “transmission”.
  • Consider shielded wiring and cables.

Sometimes, it is too difficult you replace runs of cable and change your boat’s electronics. In this case, the easiest solution is to just switch off the offending equipment when you are using your VHF.

Reducing VHF interference caused by your boat’s engine

When mechanical equipment is causing interference on your boat’s VHF, the best solution is to enhance the grounding that is fitted.

This gives a “path of least resistance” for any current to escape to earth.

Effectively, the energy goes to earth instead of being transmitted.

The other consideration is to determine how close your boat’s wiring is running to your machinery. If it is close, moving it further away should help to reduce VHF interference.

The reason for this is that a metallic object can have a magnetic field. When the magnetic field moves close to a cable, a current can be generated in the cable. This current can turn the cable into an antenna, transmitting noise.

Reducing VHF interference caused by your boat’s rigging

The best way of reducing VHF interference caused by your boat’s rigging is to ensure it is properly connected to ground.

You cannot relocate the rigging because it is needed to hold up your mast. 

Instead, grounding it will give any current generated within the rigging a path to flow safely to earth.

Sending it to earth means that there is less energy left to turn into electromagnetic waves and cause interference on your radio.

Reducing interference caused by other transmissions

The best way to reduce interference from other transmissions is to make sure your squelch is properly adjusted.

How To Properly Adjust The Squelch On A VHF Radio

If that does not solve the issue, then you should consider switching radio channels so that the frequency you are working on is further away from the other transmission.

This should never become an issue on Ch 16 because the frequencies immediately either side are blocked as “guard channels”. The next two frequencies are also suppressed, restricted to low power transmissions only.

The result is that it is only working channels that should suffer from this sort of interference. You can easily to switch to another working channel if necessary.