Author Topic: RF Interference in Trolling Motors - AAARGH! Sonar Noise! by Adam Broughton  (Read 15060 times)

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Offline RGecy

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This is an article Written by Adam Broughton at Big Georgia Spots on Sonar Noise from Trolling Motors

AAARGH! sonar Noise!


Remember that science experiment you probably did when you were a kid, where you take a wire, wrap it around a nail, put a voltage to it, and magnetize the nail? Its not the coils that create the magnetic field. A magnetic field is present whenever electrical current passes through a wire. When you coil the wire around the nail, you concentrate the magnetic field onto the nail.

What does this have to do with sonar noise? Magnetic fields and electro-magnetic radiation are going to be present around ANY wire carrying current, and will cause small perturbations in the voltage of nearby wires. The little nail-wire experiment is evidence of this effect. Your sonar is designed to detect extremely small signals, and the very nature of its operation makes it susceptible to interference.

The worst offender is your trolling motor system, but it is not the only source. Noise can come from many sources. The question isn’t “how do you eliminate electro-magnetic radiation”, its “how do you deal with it?” Unfortunately there isn’t a lot of great information out there on how to deal with sonar noise, so we’ve put together this guide to help.

There are many reasons why you will get a messy display, but no matter what the source, most of us just simply call it all just “noise”. But before you can troubleshoot and remedy the noise, you have to understand the source of the noise. Here is a series of questions that I have learned to ask people when they say they have a sonar noise problem:

"Are the boat systems and trolling motor on separate batteries?"

This is the first question I always ask. Always. As far as an electronic device goes, electric motors are horribly noisy, and that noise travels all along the conductors of the trolling motor system. Connect your fish finder to it, and you’re guaranteed to have noise---not necessarily when the motor is off, but whenever it is on, you will have noise.

You should put your sonar fish finder on the same system as your cranking battery and boat systems (lights, live wells, etc…). The basic layout should be like this (click image to enlarge):


If you are guilty of having both on one system, the obvious solution is to add a battery and separate the two systems. Electrical isolation of the two systems is essential. BUT you don’t necessarily have to have a $150, 60 lb deep cycle marine battery for your sonar. If your sonar has a 5” or smaller screen, try a canister-type, gel cell, or motorcycle battery. Twelve volts is twelve volts---it doesn’t really matter what form it comes in. Smaller units draw less current and you can often get through a fishing day on a smaller battery.

Word of CAUTION: Not only is it a bad idea to run a trolling motor and sonar on the same battery for sonar-noise reasons, it also can leave you stranded should you be using that same trolling motor for primary transportation. With the sonar running all day, you run a safety risk of running down your propulsion battery and getting stranded. Always have at least two batteries (and uh,…an oar)

"Is the interference due to a 2nd transducer?"

If you’re only running one transducer, then obviously this isn’t your problem. However, you WILL GET interference whenever two transducers are firing on the same frequency in the same vicinity. How close is “the same vicinity”? Other boats within 100yds are well within range, and sometimes several hundred yards is close enough. This being the case, you can imagine that if you have a trolling-motor mounted transducer and a transom-mounted transducer, the two transducers will interfere with each other.

As you might have guessed, the “noise” that you see when two transducers are interfering with each other is not the same kind of noise you get from your trolling motor. Electrical noise is introduced in the conductors (wires) on your boat. Sonar noise as described here is due to reflecting sound waves under the boat; emitted from one transducer, radiating and reflecting off targets, and being detected by the 2nd transducer.

So you can’t just get rid of the 2nd sonar unit, right?

There are many advantages to having multiple units, SO, what do you do about it?

Split Frequencies

Most transducers are capable of firing at multiple frequencies; typically 83 and 200 kHz. These frequencies are dictated by the physical shape of the transducer crystal inside the transducer housing. Inside the sonar unit, the electronics can process either 83 kHz, 200 kHz, or both 83 and 200 kHz. If you look in your menus, you will find options to set the sonar to one, the other, or both.

By setting one unit on 83 kHz, and the other on 200 kHz, you can effectively eliminate a great deal of interference. The trade-off being that you are subsequently changing the sonar performance as an unfortunate side-effect of a trouble-shooting measure.  For more on sonar frequencies and sonar performance, please click here.


Another potential solution for two interfering transducers….just deal with it. The noise filtering algorithms of modern are so good, that interfering sonar units rarely result in significant deterioration of digital depth, bottom prints, or other targets. Most of the time you get streaking lines that appear like rain, but you won't loose targets. The interference may be annoying, but 99% of the time it doesn’t affect the ability of the sonar to resolve and display targets.

Below is an example of sonar display where the interfering transducer has been turned off. The interference is the vertical dashed lines on the left.  To the right, the display is clear (after the other unit was turned off).


NOTE: (before I get hate mail on this picture) Although it would appear that the brush pile sitting in 20ft shows a stronger return after the 2nd transducer is turned off, as indicated by the stronger red return, it actually shows the boat being more positioned over the pile rather than on the fringe of it—the boat was drifting slowly over the pile. You normally won’t see much difference in the strength of sonar returns when exposed to interference and not exposed to interference. However, this is not always the case and some (rare) installations may be completely blanketed by a 2nd transducer.

It Happens A LOT

Sonar interference can occur at any frequency and any type of display. The tell-tale signs of 2nd sonar interference can be clearly seen as similar dashed lines in this Side Imaging view at 455 kHz:


NOTE: trolling motor noise may appear similar, but will come and go as you engage and disengage the motor.

"Is the noise only present when other systems are running?"

As you’re out fishing and grumbling about interference on your display, you might not notice that the interference comes and goes as other stuff is turned on and off. For example, your livewell pumps are electric motors, and just like your trolling motor, are a HUGE source of noise.  As your pumps come on and off, you might see noise come and go. You might also see noise when your big (gas) motor is running, as the electrical systems controlling the motor emit electrical noise.

Paying attention to this behavior will help you locate the source of the noise. Once you locate the source, you can better target the offensive source and remedy the problem. For example, you might only see noise when…

The trolling motor is under power – This is by far, the most common type of noise that fishermen see on their sonar display. Assuming you have your sonar and trolling motor on separate battery systems, you are not out of options for fixing the problem. Read on to “RF Noise” and we’ll address how to fix it.

The big motor is running – the flywheel on the top of the motor works like a huge electric motor (principle of an electric motor run “backwards” is a generator). And as you may know, electric motors are extremely “noisy” and radiate noise like a blasting radio antenna. Each of the distributor wires carries tens of thousands of volts to the spark plugs, potentially blasting RF noise on each pulse. If this is you, read on to “RF Noise” and we’ll address how to fix it.

When the boat is on plane – for transom mounted in in-hull mounted transducers, you might see “noise” when the boat is running quickly over the water. This type of noise is caused by bubbles and disturbed water passing just under the transducer. Read on to “Physical Signal Disruption” to see how you might fix this type of noise.

You might have noticed that condition-specific noise can be split into two groups: electromagnetic noise (RF interference) and physical sonar signal disruption (bubbles). These two types of noise are so important, we’ll spend a little time on each one. First, RF noise…

RF Noise

In general, most guys know the basics of how to set up a fishing boat: keep the trolling motor and sonar systems separated, keep the transducer wet and clear of obstructions, use of 2nd transducers will interfere so turn one off or change frequencies, etc, etc…. But when we follow the rules and still get problems, it leaves us scratching our heads.

I Still Don't Get It

Here’s the voodoo that is going on: You can think of any wire carrying current as a broadcasting radio antenna, and your sonar system as the unfortunate receiver. The higher the current in the wire, the “louder” the broadcast.  Engineers refer to this broadcast as radio frequency (RF) noise, because the offending waves are typically in the radio range of electro-magnetic radiation.  The goal is to block and/or suppress the reception of RF noise wherever possible.

In order to better isolate sonar system from the noise, you want to

1. Physically locate EACH COMPONENT of the sonar system as far away from offending sources (eh-hem…trolling motor) as possible, and

2. Shield and ground any entry points.


A common mistake that many anglers make is to zip-tie a transducer cable to the steering cables of the trolling motor. The steering cable bundle includes the power cables going to the electric motor in the lower unit. When you tie the transducer cable to the steering cable bundle, you’re aligning the transducer cable to the trolling motor power cables.  Naturally, your ultra-sensitive sonar picks up trolling motor noise here and you subsequently have noise problems.

Try to keep the two apart as much as possible.

When routing your transducer cable, meet the trolling motor frame at the point where the trolling motor pivots, then secure the cable to the trolling motor the remaining way down the shaft. Don't worry about all the wires being aligned down the shaft --Routing the transducer cable down the trolling motor shaft is a necessity, and typically not a problem because the power cables are inside the conductive shaft. And be sure to see our tip on grounding trolling motor noise later in this article.

And keep the two systems apart.

Did I mention that?


Keep them apart.

NOTE: if your sonar system is working fine, then suddenly gets noisy, check your trolling motor head for a blown fuse. Consult your trolling motor manual for more details.

Points of Entry

We have already discussed some ways in which noise can be introduced into your sonar system. Here are a few points of interest of which you should be aware in your RF noise troubleshooting:

Transducer and data cables are insulated, but their connectors are not. Power cables are NOT insulated, which is not generally a problem because the input power is conditioned and filtered before going to the internal circuitry.

The control head housing is NOT insulated, and you CAN introduce noise directly into the circuit board if you have a strong RF noise source near the control head.

Should you identify a weak point in your electrical system, you need to properly insulate it from the noise. The job of how to properly insulate is a mystery to most people (even the experts, sometimes), and it can be frustrating if you don’t understand guiding principles for how noise enters your electrical system.

Here are the guiding principles...

Lets say you have a noisy component such as an electric motor. The radiating RF noise can pass through any insulator, such as plastic housings and rubber wire insulation, but will resonate and travel along metals. In a troublesome system, the noise will permeate your sonar system and show up as noise on your graph, like this:


To block the noise, you want to put a conducting metal between the source and the entry point, then ground this shroud to the negative side of the sonar’s battery.  Wire mesh  or foil will make a suitable shroud, but you must be sure to ground it to the (negative) return path to the battery. Simply wrapping an entry point, such as a connector, in foil will NOT sufficiently suppress the entering noise---you must ground it, and give the noise a path to go away.


Its in Your Cables Too

If, for some reason, you ever cut the transducer or data cables that come with your sonar system, you will see mesh or foil wrapping each of the wires inside the outer sheath. This wire mesh or foil is then connected to one of the pins in the connector. This pin is connected to the ground line, leading back to the battery. The principle in the design of these cables is the same as what we have described in your troubleshooting measure---shroud the conductors, then dump the noise into the negative node of the battery.

Principle Recap:

-Offensive noise passes through non conducting materials, such as rubber and plastic

-Offensive noise will resonate along metals, but still cause signal interference if not grounded

-Noise captured by a shroud and grounded to the battery will be effectively lost in the battery

The #1 Trolling Motor Noise Fix

If you have a particularly noisy component (eh hem…trolling motor) and suspect it is the culprit of noise problems, you can capture the noise near the source and dump it in the same manner described previously. Normaly, we try to capture noise near the victim component then dump the noise, but this works too...

Try grounding the trolling motor frame to the negative side of your sonar battery. The recommended method is to drill a hole in your trolling motor skeg, put a bolt through it, and connect a grounding wire. Then tidy up the installation with zip ties.


How that works...

Conceptually, the frame of the trolling motor is conductive and “shrouds” the noisy motor inside. All you have to do is ground the frame to the sonar’s battery.

If you are one of those disgruntled fisherpersons that have trolling motor noise problems, you might try this remedy first.

NOTE: drilling or modifying your trolling motor may void the manufacturer’s warranty. Do such modifications at your own risk.

Physical Signal Disruption

Physical Signal Disruption is just what it sounds like: disruption of the sonar sound wave below the transducer. You get this type of interference with transom-mounted or in-hull-mounted transducer and only when the boat is moving forward.

The disruption can be disturbed (swirling) water, bubbles, or an actual air gap. The first of these, disturbed water, can be the most puzzling to diagnose, because your transducer is fully wet, and the apparent “noise” comes and goes with small changes in boat speed. The setup looks like this:


Water slides along the bottom of the boat hull, then spills over the rear edge of the transom. The swirling water is detected by the super-sensitive transducer and may appear as clouds of haze on your display.

Under certain conditions, this swirling of water can cause serious performance issues for your sonar (see Compounding Complications later in this article).

At higher speeds, as water shears off the back of the boat, your transducer may become completely out of the water if it is mounted above the lowest part of the hull. The sonar signal cannot penetrate the air, and so you will lose sonar signal. The header image of this article (on the first page) is an example of what your sonar looks like when this happens---complete black-out. Below is an illustration of this case:


If you want high-speed depth, you need to mount your transducer lower than the bottom of the boat. Typically, about 1/4” is sufficiently low, but if the bottom of your hull is not smooth (has rivets, lateral steps, etc..) then you may need to go lower.


The risk of going lower than the hull is that you put the transducer at more risk for striking a floating object, such as a piece of wood or floating vegetation. Small objects floating in the water, struck at high speed, can put tremendous load on the transducer. So much so that they can complete rip the transducer from the hull. So you want to be just deep enough to get a good reading and no more.

Be aware also that your transducer should be level or slightly tilted forward when running ---not necessarily whatever angle it is at on your trailer. If the transducer is tilted backwards, you will get cavitation under the transducer and loose sonar signal, as illustrated here:


Having said all this…

Most experienced anglers don’t mess with trying to get running depth from a transom-mounted transducer. The trial and error tweaking to get it right can be a royal pain. An alternative solution to obtaining high-speed running depth is to mount another transducer inside the hull and shoot the sonar signal through the fiberglass. You can switch between the two transducers with a transducer switch (see your local dealer).

The idea behind this mounting location is that the very back of the boat is typically always in the water, even at 80 mph. This setup doesn’t work in all cases, and there are some things to watch out for. Read our guide to in-hull transducer mounting for more information.

NOTE: virtually no practical solution will give a perfect image all the time at high speed. High speed boats, especially high speed bass boats often trap pockets of air under the boat as they skip over waves, but more or less let you know if your current depth matches up with your chart. Slower boats, like deep-V center console boats, are more likely to get a good sonar picture at “high speed”.

Signal to Noise Ratio

It may be of interest to you to know what it is exactly that you’re fighting. In signal processing, the concept of signal-to-noise ratio is an important characterization of the information handled by the sonar circuitry. Your sonar sends a sound wave into the water, and coming back is a random, jumbled mess of strong and weak pulses. Which of these pulses are sonar targets? Which are noise?  The theories and practices that are used to make sense of it all have their roots in radar signal processing and were largely developed during World War II and fundamentally haven’t changed since.

However, the actual signal processing algorithms that make sense of the return signal are exceptionally complicated, constantly evolving, and proprietary---so we couldn’t tell you what they were even if we knew. However, the common principle among most of them is that signal strength over a threshold is interpreted as “signal” and below the threshold as “noise”. This concept is illustrated below (data is simulated):


The signal processing software in your control head will process both “noise” in the returning pulse signal and “noise” picked up in your boat’s electrical system. What is left over is the signal, and is what ends up being shown on your display. You can watch real-time signals by turning on real-time-sonar (RTS) on in your settings menu.

Sometimes the true signal is below the threshold, and sometimes the true noise is above the threshold---causing errors in both cases. The task of robust interpretation is non-trivial, and when you dig into the details of signal processing, it’s a wonder we are ever able to paint a fish on the graph at all!


You can often improve the signal to noise ratio with a ring magnet. Here’s how it works…

 When you wrap a data wire around a magnet, you create an impedance that effectively suppresses high frequency interference.  You might notice bulbous cylinders on your computer cables. These are ferrite rings that are intended to choke out unwanted noise. ---note that they are ALWAYS near the signal source.  If you have a RF noise problems and can’t figure out how remove, try passing your transducer, GPS, and other data cables through magnets. You can find ferrite rings at any electronics supply store. Place them as close to the control head as possible, and often times, this will significantly reduce your noise problems.

Compounding Complications

Sometimes a combination of conditions can prove too much for your sonar to handle. Keep in mind that your sonar is processing EXTREMELY small signals, and constantly switching between sonar signal processing algorithms to figure out and display what is under the transducer.  You might have everything set up correctly, and you still might see erratic behavior on occasion.

Some challenging conditions for your sonar might be:

Very dirty water – suspended particulates, plankton clouds, very muddy water, floating leaves,…all will mess with your sonar signal. Keep in mind that plankton will move in the water column between daylight and darkness—all else being equal, you may see a performance difference between day and night.

Very disturbed water – such as in a swiftly flowing river.

Clouds of baitfish – sometimes bait clouds can be so thick that the sonar paints the digital depth on top of the cloud, not on the bottom below. You can count-down a sinking lure to verify the correct depth.

Combine these “normal challenges” with some that we introduce on the boat, and you could have serious problems. What you need to know from the facts of finicky behavior is that there can be a fine line between a clean display and a messy display. Troubleshooting often involves a lot of trial and error to finally obtain a clean display. Sometimes a single change makes all the difference. Sometimes, you have to re-wire the boat!

There have actually been “identical” setups coming out of OEM plants: same boat, same units, same installations, and some boats have trouble and some don’t. Some are easy to fix, and some leave you scratching your head.

If you have noise problems, follow the principles here and just do “something” to change the conditions on your boat. Then put it on the water and check it out. If after a few iterations you’re throwing wrenches across the garage, visit your local installer. Believe us, you aren’t the only one with a noise problem!
« Last Edit: April 09, 2013, 06:53:33 AM by RGecy »
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