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Author Topic:   Interference to SONAR from Motor
fisher801 posted 09-06-2006 10:26 PM ET (US)   Profile for fisher801   Send Email to fisher801  
My Humminbird Matrix 37 GPS and SONAR fails to work (as a depth sounder) whenever I have my outboard running, but when it is not, my unit's sonar somehow calibrates and I get an actual depth reading along with a correlating image. The unit will read and display depth when I do have other electrical accessories on, although I still see interference (in the way of fuzz) and my sensitivity calibration levels are set to "low". I would suspect the problem could be excessive EMI whenever my outboard is running, and if it is, what is the proper method to regulate it?
Thanks again.
jimh posted 09-06-2006 11:02 PM ET (US)     Profile for jimh  Send Email to jimh     
Please clarify is the problem occurs when the boat is underway or even when the boat is not in motion.

The ignition system of older outboard motors can generate a lot of radio frequency interference. Typically these older motors did not use RF suppressor type spark plugs. This type of interference shows up as lines in the raster display. The spacing of the line varies with the engine speed.

fisher801 posted 09-06-2006 11:08 PM ET (US)     Profile for fisher801  Send Email to fisher801     
Yes, the problem occurs only when I am underway and/or the engine is in idle. The motor is a 2005 75-hp Evinrude E-tec.
crabby posted 09-07-2006 08:26 AM ET (US)     Profile for crabby  Send Email to crabby     
Is the unit wired directly to the battery or does power come from a feed that is common to other equipment (or even the battery connection on the ignition switch)? Most sonars I have owned or installed recommend a direct run to the battery, although it is not always necessary it is likely a good practice to follow.
jimh posted 09-10-2006 10:17 AM ET (US)     Profile for jimh  Send Email to jimh     
There are two paths for the interference from the motor to reach your SONAR.

Because the SONAR transducer is located close to the motor at the transom, it is very likely that radio frequency interference (RFI) from the ignition circuit of the motor is being propagated to the SONAR transducer. The only effective way to reduce this interference is to suppress the generation of radio frequency interference from the motor. Most modern outboards use techniques to suppress RFI in order that their own electronics will not be affected. Older outboard motors which lack sophisticated electronic controls often generate considerable RFI. For example, my c.1987 Yamaha motor produced a huge amount of RFI. My c.1992 Evinrude motor, which much more electronics of its own, produces very little RFI.

The RFI can enter the SONAR signal path either via the transducer itself or via the cable connecting the transducer to the control head. It is often seen that these connecting cables lack proper shielding. You may want to experiment with the routing of the connecting cable to see if you can affect the ingress of RFI by changing the location.

If you discover that the source of your interference is from RFI propagated from the motor to the transducer or wiring, you will have to apply RFI suppression techniques to your motor. Please begin a second discussion in order to pursue advice on how to accomplish that.

A second path for the interference from the motor to reach your SONAR is via the power distribution wiring. If there is any appreciable impedance in the power distribution wiring, it may be possible for RFI to be induced into the power leads. However, in most OEM installations on Boston Whaler boats the secondary power distribution of the battery circuit is done with 8-AWG cables. These cable represent a very low impedance to the battery. It is generally not seen that direct connection of the SONAR to the battery is necessary.

To determine if your problem is occurring as a result of power distribution wiring problems, bring a second battery aboard your vessel and temporarily connect the SONAR device to this battery. This will completely isolate your SONAR's power from the vessel power. If the interference is via the power distribution, it should stop when you use this isolated battery.

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