Reducing radio noise and interference


How often have you heard a voyager complain that every time they try to use their SSB, it kills their autopilot? Or those who say they can never receive weatherfaxes, though the boat next to them gets crystal-clear charts everyday?

These are common enough complaints and they often lead to disuse of radio equipment or the perceived need to rip everything out and start over. That’s an unfortunate reaction since the problems are solvable given a little understanding, some patience, and a bit of forensic work.

Any time radio signals get into places they are not intended for, we use the phrase radio frequency interference (RFI) to describe the situation. In the case of a voyaging boat, we’re on both sides of the RFI fence at once: it’s our autopilot that’s getting knocked offline and it’s our radio receiver that can’t get through the ship’s noise to hear the weatherfax.

An automatic antenna tuner in the aft lazarette. Maintaining tight connections on an antenna tuner will prevent it from leaking interference.

Reducing interference

Let’s deal with these two categories of RFI separately, starting with how our HF radio signals get into autopilots, stereos, and reading lights. There are two ways that your radio signal gets into other equipment: conduction through physically connected wires and transmission through unintended wires acting as antennae.

Here are three basic steps to cleaning up unwanted interference:

1. Keep it clean. Your radio is designed to transmit clean signals and, in a perfect world, it will. But the world is imperfect: that backstay antenna is a compromise in length, the ground system is a mishmash of copper foils and metal surfaces, and the wires connecting everything together are slightly lossy and tarnished from living in a salt-laden environment.

One step you can take immediately to clean up your radio’s output overall is to reduce your output power. Typical output power for a marine SSB is about 150 watts peak. If you try to wring more out of it than this, it will behave badly, going non-linear we say, and producing all sorts of unwanted spurious transmissions.

You can reduce or eliminate spurious transmissions by operating your radio at lower power. And with SSB transmissions, this is no big deal. The difference between a 50-watt transmission and a 150-watt transmission is almost indistinguishable at the receiving station. In fact, your likelihood of establishing a contact is much more dependent on how well a particular frequency propagates over the necessary distance than it does on the power involved.

So find the knob or programmatic setting that selects your output or transmit power and cut it to 50 percent. This will immediately reduce some of your interference problems.

If a boat near you is transmitting, talking on one channel, and you can hear splatter on adjacent channels — voice-like ‘splat’ sounds that decrease with increasing channel distance — then the radio on that boat is probably being over-driven, going non-linear, and causing all sorts of interference on their boat and yours. (They may not be aware of the problem and a polite conversation about output power could be beneficial.)

A second step to cleaning up your transmissions is to literally clean up the antenna and ground connections. A loose clamp on your backstay antenna or a loose terminal on your antenna tuner will result in unwanted transmissions on all sorts of frequency. If a connection is arcing and sparking, it is producing unintended signals all over the radio frequency spectrum.

Inspect all of your antenna and counterpoise connections. If they’re loose, tighten them. If they’re corroded, trim the ends and re-terminate them. Avoid sharp bends. If you suspect anything is arcing, do an inspection at night while someone transmits — the sparks will be obvious.

Finally, make sure your batteries are well-charged before you begin transmitting. An HF radio consumes a lot of power on transmit and is sensitive to low-input voltage conditions.

2. Choke it at the source. In our imperfect world, some of the signals inside your radio leak out on the power conductors that go to your batteries. If these signals are allowed to make it all the way back to the circuit breaker panel, then they can couple into the power inputs on other devices like stereos or sailing instruments.

A toroid wrapped with wire acts as effective choke to RFI.

So, how do you isolate the radio without putting it in the dinghy? It has to be tied to the house batteries, right?

The solution comes from understanding that radio frequencies — while still electrical voltages and currents — are inherently different from the direct current (DC) that powers things. There are devices that can choke RF signals without affecting DC at all. (Not surprisingly, they’re called chokes.) What we need to do is install a good quality choke on the power supply lines going to the HF radio.

A good choke can be easily installed in a few minutes using a ferrite toroid — a thing that looks like a small, metal donut. One recommended by the American Radio Relay League — a U.S.-based ham radio organization — is the Amidon FT-240-43 ( Take the two power wires from your HF radio and wrap them several times around/through the toroid/donut. Do this as close to the radio as you can. The choke will suppress any radio frequency signals from getting out of the radio onto the power wires without disturbing the DC connection that powers the radio itself. Try to get five full turns around the toroid; three is the bare minimum. This provides several times greater impedance to RF signals in the frequency range we’re concerned with (1-30 MHz).

3. Filter unintentional antennae. Any wire on your boat can act as an antenna, given the proper circumstances. Having a cursory understanding of radio waves will help in visualizing the nature of radio noise and where it comes from and goes to.

Radio waves are usually characterized by their frequency. For example, you can listen to Coast Guard Station NMN on 8,764 kHz (kilohertz) or to weather guru Herb Hilgenberg on 12,359 kHz or to WGBH Boston on 89.7 MHz (megahertz). In each instance, the number identifies the fundamental frequency (often called the carrier frequency) where you can find what it is you are listening for.

Radio signals are a form of electromagnetic radiation, like light. And just like light they move at a very fast speed: about 186 thousand miles per second or 300 million meters per second in a vacuum. They move a little slower through wires and through air, but we’ll ignore this difference.

Since the radio waves are moving at a constant (more or less) speed and since each wave has a fundamental frequency, we can determine how far apart adjacent peaks are in that signal — its wavelength.
Wavelength (meters) = Speed (million-meters per second) / Frequency (MHz) or,
Wavelength (meters) = 300 / Frequency (MHz)
This means that for every radio signal, there is a characteristic wavelength as well as frequency. In the earlier example, I could have as easily said NMN on 34.231 meters, Hilgenberg on 24.274 meters, and WGBH on 3.345 meters.

To prevent RFI make sure the lead from the tuner makes a good electrical connection to a backstay antenna.

Any piece of metal or wire can look like an antenna if it’s the right length. The right length? One-quarter of a wavelength. For example, on 12 MHz — with a 24-meter wavelength — any wire that is a quarter of that length (six meters or roughly 18 feet) acts as if it is an antenna. That means when you’re talking to Hilgenberg, getting the latest on cold fronts and cold eddies, there can be a lot of hot 18-foot-long wires in your boat. And the wire doesn’t have to be 18 feet long; it’s just that 18 feet is an ideal antenna length for that frequency.

An effective way to block RF from getting into unwanted places — like stereos and autopilots — is to add chokes to some of the wiring associated with these devices. You can make chokes from ferrite toroids as described earlier, or you can use the clamp-on variety which are much simpler to install.

Basically, a clamp-on ferrite is a pair of half-toroids held in a plastic shell. When you click the shell shut around a wire (or wires), the two halves form a whole toroid and begin to block RF signals. Good ferrites are produced by Fair-Rite ( and available from Mouser Electronics (; their Type 31 manganese-zinc ferrites are recommended for 1-30 MHz.

Clamp-on toroids are extremely simple to install, but they have some shortcomings: generally wires are not looped through them, but only make a single pass, and sometimes the plastic shells do not make a secure connection between the two halves. Still, working with pairs of half-toroids is much simpler than with a donut since you do not need to disconnect wires already in service.

You can improve the efficiency of a clamp-on toroid by looping the target wire through the center three times and by removing the toroid from the plastic shell, securely taping the two halves together instead once they are in place over the wire(s).

If you are having problems with RF noise getting into your stereo, add toroids to the power lines (positive and negative DC should run through the same toroid) into the stereo and to the speaker wires close to where they come out of the stereo. By the way, lamp cord (‘zip’ cord) is the worst thing to use for speaker wires — the long parallel runs pick up unwanted radio signals; use twisted pair instead. The power wires should be twisted too.

If you are having problems with your autopilot, again add a toroid to the power wires close to the autopilot electronics. You may also want to add smaller toroids to any long wires coming into the autopilot such as connections to a compass, remote display, GPS input, etc. Even shielded wires with a proper ground will benefit from being wound on a toroid.

For all other electronics, take the same approach — a clamp-on ferrite on the power wires and on any long connections (even antenna coax).

When equipped with a wind generator and other electronic gear, there is plenty of opportunity for interference on a voyaging boat.

Finally, recheck your RF ground system. A poor grounding system will result in radio signals using any metal they can find as reflectors and ground connections. If you haven’t installed a good RF ground, you will have more interference problems as a result.

Additionally, you can take steps to further improve your ground system: you can add copper screening inside your boat or Dynaplates outside; you can add DC isolation capacitors in your copper foils between the antenna tuner and your ship’s ground; and you can add line isolators between your HF radio and your antenna tuner. See the references in the accompanying sidebar for more elaborate solutions and ideas.

Reducing your own RFI

Your HF radio is an extremely sensitive receiver; it has to be to pick those weak signals out of the air. But that means that it is susceptible to interference from local equipment. All of the above suggestions have to do with keeping your radio signals out of other equipment. But what about keeping other equipment signals (intentional or not) out of your radio?

Noise picked up by your radio will come from one of two types of sources — rotary equipment or electronics — and it will get into your radio by one of two means — through the antenna or through the power leads.

The best way to diagnose where noise is coming from is to first turn off all other equipment on your boat: inverters, fluorescent lamps, instruments, radios, GPS, refrigeration, etc. Then tune your radio to a quiet channel and listen to the noise level. Now turn on one piece of equipment at a time, listening for increases in noise.

You will find that rotary sources — motors, wind generators, shaft grounding brushes — create wideband noise that sounds like an increase in static. This noise will show up pretty much on all frequencies that you tune to. Electronic equipment will produce noise that sounds intentional — it will generally have some pattern or rhythm to it and some characteristic tone or tones. This noise may only show up on certain frequencies.

When you find a source of noise, your simplest solution may be to turn that equipment off when using your radio. This might be a useful approach, say, for refrigeration, but not so practical for a wind generator half-way up the mizzen.

Motors can be quieted by the addition of capacitors across the power leads and from the power leads to ground. The BEAM robotics wiki in the references in the accompanying sidebar has a good explanation of the three-capacitor filter solution. Multilayer ceramic capacitors (0.1uF and 0.047uF) type X7R are inexpensive and good for filter applications. (Also available from Mouser Electronics.)

If you find noise coming from systems that must be operational simultaneously with your radio, you can add these filters to the motors, wind generators, and shaft grounding brushes. Your alternator should already have a noise suppression capacitor in it.

Electronics equipment often radiate unwanted noise on their power leads or on connections between equipment, remote displays, computers, etc. When you find a piece of equipment that produces unwanted radio noise, add clamp-on ferrites to the power leads and other wires close to the equipment case.

In extreme cases, you may find that you have to relocate long wires that run parallel to your radio’s power wires, control cable, or antenna coax. It may also be necessary to physically move a piece of equipment to reduce or eliminate noise. On our J/40, Gryphon, our remote instrument display causes interference if you lay it in a certain position on our chart table. The solution? Don’t put it there!

Also, if you haven’t done so already, make sure that the power leads into your HF radio are twisted and that you’ve run them several times through a toroid or clamp-on ferrite.

Radio frequency interference problems still involve a certain amount of smoke and mirrors, though the level of witchcraft involved has decreased in modern times. Armed with a little knowledge and a lot of toroids, you can conquer this devil inside your boat.

Jeff Williams trained as an electrical engineer in the days when a computer filled a room and witchcraft was prevalent in antenna theory and design. He now drives remotely operated vehicles in the deep ocean ( He lives in New Zealand.

By Ocean Navigator