Mobile Radio Interference
Here’s a problem that has been escalating as the automotive industry brings out the newer models. Computers that aren’t shielded, electric fans that replace the old belt driven ones, electric fuel pumps, and higher output alternators all of which have less than adequate RFI filtering.
The RF emissions from the devices can cut your receive range substantially. And why listen to any more noise than you have to.
Before any action can be taken the cause needs to be narrowed down. Disconnect the antenna with the vehicle running. If all the noise goes away it’s RF and coming in through the antenna.
If you hear a whine that increases in pitch when the engine is revved up, it’s coming through the power leads. Run the power connection to the battery and if that doesn’t work use a power line filter (LM-P-10) in the positive lead at the radio.
Radiated interference that is received through the antenna can be reduced or eliminated in many ways. I’ve found that the use of a Wilson 1000 antenna reduced interference on half and cured the problem on the other half of the tests made at my store. A popping sound that increased with the speed of the engine is ignition noise.
Insure that your vehicle has suppresser plug and wires. Wires may need replacement, one bad wire can create an intermittent problem. Install a LM-20RF on the battery side of the ignition coil.
This is the most common new problem, Electric fuel pump whistle. It’s a steady hashy whistle as soon as the ignition is turned on. Fords are the worst offenders. The fuel pump is in the tank. Locate the positive and negative feed lines and install a LM-20RF on each line.
Mount the filters to a good chassis ground cleaned to bare metal. Test and make sure you have the feed lines and not the fuel level indicator leads.
A less common problem is the radiator fan interference. It causes an audible static when the fan is running. Disconnect the fan to verify the problem, if it’s a culprit install a LM-20RF in line with the power lead.
These LM-20RF filters will work with other motors such as the heater blower motor and the windshield wiper motor. ©CBWI
More information and these products are available from:
MTI Land Mobile Division PART# MSRP
Marine Technology, Inc. LM-P10 $25.95
2667 E. 28th Street, #505 LM-20F $10.95
Signal Hill, CA 90806 LM-25ORF $27.95
(800) 772-0796, (310) 595-6521 LM-TVX2 $37.95
Also Available From Bob’s CB & Wireless