Identifying and Solving Sonar Interference Problems
First off, understand there are five different types of interference problems you may face. They will all look similar on your screen and distract from your ability to get a clear picture of the bottom below, but the remedy for each is quite different.
- Acoustical Interference is caused by faulty transducer installation. You will see this noise on the screen only when the boat is traveling across the water, at or beyond the plane speed of the boat. It's caused by an uneven, or turbulent, water flow across the face of the transducer. This applies to transom and in-hull mounted transducer applications.
- Sonar Cross-Talk Interference is created by another sonar of similar transmit frequency nearby. It will show itself as lines rotating around the dial of a flasher or diagonal lines across the screen of a graph. When two depth finder's transducer cones intersect, each unit will get confused as to which signals are which.
- Ignition Interference comes from engines that introduce ignition noise into the power circuitry. These are power spikes that travel through the power wiring and into your sonar, causing noise to be displayed on the screen or heard in your radio.
- Conducted Interference is usually caused by electric trolling motors that incorporate Pulse Width Modulation (PWM) in their speed control. This noise will be evident only when the trolling motor is activated and can vary in intensity through the range of motor speeds. The noise is conducted through the power line and enters your depth finder through the power connection.
- Electromagnetic Interference (EMI) is created by the powerful electric trolling motors on the market today. EMI is radiated from the trolling motor's lower unit and power wiring into the air. It is then absorbed into the transducer cable that's attached to the trolling motor. This is a relatively new problem. As trolling motors get more and more powerful, the noise gets worse and worse, overwhelming the cable shielding and noise reduction circuitry that used to protect sonar from this type of interference.
Being able to pinpoint the type of interference you have will be the first step in getting a clearer sonar signal in all waters and conditions. Here are some simple and easy to follow steps to target and then address the problems you may be having.
If your transducer is mounted on the transom or glued in-hull for the purpose of reading depth while the boat is traveling across the water, you may be subject to acoustical interference being displayed on your depth finder. This type of interference makes reading the bottom almost impossible, once you reach a certain boat speed. The screen often is a total clutter of bad signals with a total loss of bottom signal. This noise can be reduced a great deal by adjusting the water flow rate over the transducer surface. This can be done by re-positioning the transducer.
Transom Mounted Transducers
If you do not have enough of downward slop to your transducer, turbulent water will develop under the face of it. Increasing this angle by lowering the back of the transducer will help. Additionally, lowering the whole transducer can help you get down into a more smooth water flow area. However, going down too low can cause the transducer to shoot up water in a "rooster tail". If you have adjusted the angle and not gotten much improvement, then the transducer has been set in a position of what we call “dirty water”, where bubbles are coming off the hull due to a rivet, bad weld or bend in the hull. You'll need to remove the transducer and reposition it in another location on the transom where the water flows more smoothly. Many newer hulls have very steep keel angles, so mounting a transom style transducer so you're shooting at a downward angle can be a challenge. Generally, you want to mount as close to the keel line as possible, but you'll have to balance performance and location. Be sure to follow your transducers mounting instructions carefully.
In-Hull Mounted Transducer
With this mounting application you have limited adjustment options. The key is to make sure you choose a good location and get a good installation in the first place. However if your puck is already stuck to your hull, than it's time for some rework to get things right again. Before you get out the hammer and chisel, see if you can improve the reading by changing the weigh distribution or modifying the trim setting of your boat. Getting the bow to run a little lower may help things considerably. If not, then you'll need to remove the transducer and reattach it in a better location. Usually, dead center, about a foot from the transom, is best. But if there's a strake or a bad spot in the fiberglass there, you'll need to work around that. Testing the location, by setting the transducer in about a half inch of water while you run across the lake, is good way to find the best spot. Doing this is easier said than done and may require ingenuity or a willing helper in some cases.
If your unit has interference with nothing other than another sonar running in your boat, then you have sonar cross-talk interference. For most depth finders, the only real solution is to move the transducers further away from each other. This can help keep the transducer cones from intersecting, but because cones get wider as the depth increases, the problem can not usually be totally solved by position only. Changing one of the sounders to another model that runs on a different frequency than the first will solve the problem. Some modern depth finders, like the Vexilar FL-12 and FL-20, have interference rejection technology that will permit you to simply knockout the interference by pressing a button repeatedly until you see the cross-talk signal disappear on both units. These depth finders can have transducers mounted right next to another unit that runs at the same frequency.
Ignition noise can be a sign of problems with your engine. However, if it seems to be performing well, or you would just rather run her 'til she blows, a Ferrite Core can stop the noise from getting into your depth finder. These are small parts that you pass the depth finder's power wire through, often wrapping it through several times. For Vexilar units, contact Vexilar to get one.
Interference from Electric Trolling Motors
Whenever you use an electric trolling motor on the same boat as a depth finder, you are likely to see noise from the motor on the display of your sonar. Today's high-tech motors perform better than ever, but can be very unforgiving to other electronic devices in the area.
If you disconnect your transducer from your depth finder and you continue to see noise when the trolling motor is running, then you are dealing with conducted interference coming in through the power connection. This can happen when the boat's electrical system does not have one common ground that connects to the water. Using a multi-meter, available at places like Radio Shack, check for proper grounding in your boat's electrical system. Set the meter to measure resistance or continuity. Then connect one lead to the negative battery post that connects to the trolling motor and the other lead to the negative post on your starting battery. The reading should show zero and the meter may beep. Also, check between the negative starting battery post and an unpainted portion of the outboard motor, like the prop shaft or trim tab. You should also measure a complete circuit here as well. If the ground connection is missing between the trolling motor battery and the starting battery, add a small gauge wire, about 18 AWG, with a 1 amp fuse. This will complete the ground connection here. If the ground to the outboard motor is missing you should take your boat into a service shop to see why the ground is missing. All outboards should have this ground. If yours is missing something is wrong. If all grounds are in place, but noise is still a problem, a Ferrite Core can solve the problem.
While you’re electric trolling motor is running, disconnect your transducer from the back of your unit. If the interference goes away, then you have EMI created by the rapidly switching voltage and powerful DC motor in the trolling motor's lower unit. Proper grounding and Ferrite Cores can be a good remedy for this type of interference. First, be sure your system is grounded properly as described above. Incorrect grounds are a common source of problems. Vexilar has even incorporated a very unique grounded puck transducer in our sonar systems that will insure the electrical system is grounded to the water at the point of the interference. This helps keep EMI under control, but may not eliminate it completely.
By knowing how to identify interference sources, you will be able to eliminate or greatly reduce the troublesome noise that can render your electronics worthless. It may take a little time and effort in some cases, but it will let you get the most out of your sonar in all conditions.