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Author Topic:   AM Radio Interference
mrdrago posted 08-02-2005 12:36 AM ET (US)   Profile for mrdrago   Send Email to mrdrago  
I am having trouble with my AM radio picking up alternator or engine noise when my Mercury 90HP 1987 engine is running. My radio antenna is a Shakespear type and I am using a Motorola adaptor that allows using the antenna for VHF and FM. It works great except the AM does not work only when the engine is turned off. Does any have suggestions for how to correct this problem?



bsmotril posted 08-02-2005 09:24 AM ET (US)     Profile for bsmotril  Send Email to bsmotril     
Put a noise filter on the input power leads to the radio. Any good Marine supply place will have them. BillS
Robob2003 posted 08-02-2005 11:50 AM ET (US)     Profile for Robob2003  Send Email to Robob2003     
I have been told that most standard outboard plugs are not resistor type so will cause radio interference, but resistor plugs are available. I think they cost a bit more.

Bob on Tampa Bay

mrdrago posted 08-03-2005 01:23 AM ET (US)     Profile for mrdrago  Send Email to mrdrago     
Could you provide a bit more specifics about what I need. Radio Shack sells a filter that is used on car radios would that work? Must it be purchased from a marine store? To make sure I understand, this is a filter for the power source not the antenna correct?

Could you provide some more details on the spark plugs? What are they and were can I get them?



jimh posted 08-03-2005 09:19 AM ET (US)     Profile for jimh  Send Email to jimh     
Alternator noise and ignition noise are two separate problems. You must use different techniques to reduce the interference.

Alternator noise is a audible whine whose pitch varies with engine speed, roughly from about 1,000-HZ and higher. This type of noise is conveyed by the power supply connections. It can be reduced by use of a filter in the power leads between the radio and the battery.

Ignition noise is a staccato popping noise. It sounds like a rough modulation of the signal. The rate of the popping also varies with engine speed. This type of interference is generally conveyed by radio propagation. In some cases it may also be conveyed by the power line. It cannot be filtered out using power line filter techniques. It cannot be filtered out using antenna filters. The only way to suppress this interference is to eliminate it at its source.

To determine the source of the interference, disconnect the antenna from the radio and open the squelch. If the noise stop, it was probably being conveyed by the antenna.

The source of most ignition noise is from the spark plug sparking itself. If the spark plugs do not contain ignition noise suppressors, the noise will radiate from the spark plug wires, too. The first step in reducing this noise is to use spark plugs which contain radio frequency suppressors.

Spark plugs intended to suppress radio frequency interference (RFI) are generally made in two styles, resistive and inductive. Resistive plugs contain a small series resistance in the plug body to suppress RFI. The resistance also affects the spark slightly, tending to reduce it. Inductive plugs have a small coil or inductor in the plug body which is more effective at suppressing RFI. Inductive plugs do not affect the spark as much.

The spark plug gap affects the generation of RFI. As the gap increases, the amount of RFI increases. Check your spark plug gap. On some OMC engines the specified spark plug gap has been reduced to 0.030-inch from the original 0.040-inch dimension in part because of a desire to reduce RFI from the ignition system.

In general, older motors were designed with not a moment's concern for RFI and most small boats did not have a great deal of radio or electronic equipment. Modern motors have a great deal of electronics under their cowling and thus are designed to suppress RFI from their ignition systems.

If you motor is an old motor with a very hot spark, you may not be able to suppress the RFI to the point of making AM radio reception interference free. The AM modulation technique is especially prone to such interference. This vulnerability is inherent and cannot be avoided.

bsmotril posted 08-03-2005 09:28 AM ET (US)     Profile for bsmotril  Send Email to bsmotril     
Great information Jim about noise source and elimination! I hope you move this to the reference section where it it easy to find 2 years from now when I forget about this thread. BillS
jimh posted 08-03-2005 02:14 PM ET (US)     Profile for jimh  Send Email to jimh     
I don't believe there is any product sold at Radio Shack that I would recommend for use on a boat.

Spark plugs with radio frequency interference (RFI) suppresors must be matched with the ignition system used on an outboard motor. Some spark generating devices do not work well with certain types of RFI suppresor spark plugs. Check the owner's manual or the factory maintenance manual for your outboard motor to see which type of plug is recommended.

The inductive type suppressor plugs usually cost more, and they are usually recommended for use with more modern ignitions systems but, again, check with the manufacturer of your motor to see what they have found most effective.

Sparks plugs with RFI suppressors can be obtained from most normal sources of spark plugs. If the engine has any under-cowling electronics of its own, it probably already has (or had)RFI plugs.

Reception of AM signals without any interference requires a very high signal to noise ratio. Typically the desired signal must be 1000-times stronger than the interference to properly mask the interference effects. In FM signals, this difference is drastically reduced, and often noise-free reception is possible with signal to interference ratios as small as 2:1. This is one of the reasons the FM transmission techniques were developed. They have been in use now for 60 years, and have practically driven AM techniques from use in broadcasting and communications.

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