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Author Topic:   Interference on VHF and Stereo
jason78 posted 08-10-2014 11:44 PM ET (US)   Profile for jason78   Send Email to jason78  
I am noticing interference in my radio and I believe my VHF when running the motor. Assuming it is interference from the Alternator. Any suggestions on how to resolve this? Thanks!
jimh posted 08-12-2014 08:59 AM ET (US)     Profile for jimh  Send Email to jimh     
What kind of motor? Is it an electric motor?
jason78 posted 08-12-2014 11:02 PM ET (US)     Profile for jason78  Send Email to jason78     
It is a Merc 200XL carbd 2 Stroke. I am now thinking it may be a combination of that and my Lowrance HDS sonar.
David Pendleton posted 08-14-2014 03:13 PM ET (US)     Profile for David Pendleton  Send Email to David Pendleton     
Have you considered adding ferrites to your radio and sonar cables? You can buy them online.
jimh posted 08-14-2014 06:23 PM ET (US)     Profile for jimh  Send Email to jimh     
If the onset of the interference is coincident with starting and running the outboard engine, it seems quite reasonable to deduce that the source of the interference is the outboard engine.

Since the outboard engine is a spark ignition engine, it then seems quite reasonable to deduce that the mechanism in the engine that generates the radio frequency interference is the high voltage spark ignition circuit.

In order to operate a spark ignition engine and not have the spark circuit generate radio frequency interference, the spark plugs must be of the type designed to suppress generation of radio frequency interference. Sparks plugs designed to suppress radio frequency interference tend to be in two groups: resistive or inductive.

I recommend you consult the manufacturer of your spark ignition engine for a recommendation for a radio frequency suppressing spark plug. The manufacturer should know precise what brand and model of radio frequency interference suppressing spark plug will be best for that engine.

Let us know when you have installed radio frequency interference suppressing spark plugs. If that does not provide a remedy to the problem of interference, we can recommend additional steps.

jimh posted 08-14-2014 06:32 PM ET (US)     Profile for jimh  Send Email to jimh     
To determine if the interference is being conducted to the other devices by their power lines due to contamination of the power by alternator noise, operate the affected devices on an isolated 12-Volt battery. If the interference is eliminated, then the source was by conduction along the power circuit from the outboard engine alternator.

To suppress alternator noise, often called alternator whine, which, incidentally, is very common on small boat electrical installations, you must insert a low pass filter in the power circuit. The low pass filter must have a cut off frequency that is lower than the alternator whine frequency. Small filters for power lines are available in low-current ratings at very modest prices.

Using ferrite chokes on power lines or other cables will not suppress alternator noise. Ferrite chokes will only be effective on radio frequency noise.

jason78 posted 08-14-2014 09:30 PM ET (US)     Profile for jason78  Send Email to jason78     
I have determined it is associated with the outboard motor. I am pretty sure I am stuck using the spec'd NGK plugs that are in it.

Can anyone recommend a low pass filter that would be sufficient for "marine conditions"? I will give that a try and see what happens.


jimh posted 08-15-2014 09:18 AM ET (US)     Profile for jimh  Send Email to jimh     
Most engines have options for RFI suppressor plugs.

To see if the interference is being carried into the devices on their power lines, run them from an isolated battery. If the spark ignition is the source of the problem, a power line filter may not work well to remove it.

Also, for a depth sounder (or other affected device), be certain to isolate the cable between the head unit and the transducer. Make sure it runs well apart from all other cable, particularly the engine wiring harness. In older engines, the safety lanyard KILL switch circuit is carrying very high voltage, about 300-volts, from the spark ignition primary coils. This generates a noise, and, if a SONAR (or other device's) cable is run alongside the engine harness, the signal in that cable will be contaminated with the electrical noise from the KILL circuit.

jimh posted 08-17-2014 12:56 PM ET (US)     Profile for jimh  Send Email to jimh     
I have not experimented with this particular power line filter, but in examining the available products, it looks like a reasonable choice:

SCHAFFNER Model FN2450G-6-61

The filter has a plastic housing which should be good in a marine environment. It has screw terminal that will be easy to connect with. The filter has a current rating of 6-Amperes. This is enough for a 25-Watt VHF Marine Band radio, considering the transmit duty cycle will probably be about one-percent or lower.

The data sheet fn-2450-safe-and-ergonomic-emcemi-filter-with-very-low-leakage-current. html

shows the filter has very high attenuation of high-frequency components on the power line. The filter is under $30.

If I were trying to reduce interference with an electronic device from power line noise contamination, I think I would try this filter.

Also see other options at

jason78 posted 08-17-2014 09:58 PM ET (US)     Profile for jason78  Send Email to jason78     
Thanks Jim!
jimh posted 08-26-2014 11:44 AM ET (US)     Profile for jimh  Send Email to jimh     
I recommended the 6-Ampere filter because its inductors have the greatest inductance, and it provides the best attenuation at lower frequencies. The similar models with higher rating for current tend to have smaller inductors, and their attenuation at low frequencies is less. This characteristic can be seen in the data sheet.

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