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Curing (or searching for) noise...

Discussion in 'Ham Radio - CB - Trail Communications' started by pickupman, June 12, 2005.

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  1. pickupman

    pickupman Active Member

    September 1, 2004
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    City, State:
    Cedar Rapids, IA
    Year, Model & Trim Level:
    2002 XLT
    I've noticed a few people asking about different noises and how to cure them...

    not bieing the vehicle expert, I can't vouch for the cause (and my truck seems, at the moment, to be pretty quiet...), but i have a few ideas for locating the source of the noise... and some general notes on RFI/EMI problems...

    There's 2 basic kinds of noise; radiated and conducted

    Radiated noise is jsut that, noise that comes off of a wire or device and gets in to your two way equipment through the antenna. There's a few ways to see if this is your problem:

    the most difficult method is to move the antenna. As an example, you may be able to move it from near the fuel tank (like on the bed of a ranger) to the roof of the cab, and the noise may subside....

    one of the easier methods is to rmove the antenna from the mount, or remove the coax from the radio. If the noise stops at this point, chances are your noise is radiated.

    Cures for radiated noise depend on what is radiating the noise... it oculd be as simple as putting a capacitor across the leads of the fuel pump (if that's what is radiating) (and at this point, I'm hoping someone else will chime in with a response of other fixes for the fuel pump, i know there's a sticky on the subject), or could be much more difficult if it's something else.

    The radio shack noise filters placed on the input power wires of your equipment will NOT help if your noise is radiated.


    Conducted noise is noise that is generated by something in the system (let's say alternator whine, since that's somewhat common) that is carried into your radio by the power lines.

    If you have conducted noise, removing the antenna should have no effect, moving the antenna should have no effect, etc.

    As a side note, conducted noise is VERY common when using the cigarette lighter or fuse panel as power sources as opposed to runnign dedicated power lines from the battery.

    one method to test for conducted noise is to connect your radio to the antenna as normal, but to power it from a battery (one that is completely independant of the vehicle's power system), then run your truck as normal (in this case, caution must be observed on how your battery is connected, make SURE your polarity is correct.... it is possible to expose your radio to 24V if the battery used for the power supply is hooked up backwards... it's not likely, but worth being cautios of). another note is that you do NOT need to have a high current power source as a second battery for this check, you only need to test with your radio receiving, usually, so even a bunch of AA cells in a holder will work for most radios....

    the Radio Shack noise filters placed on the input of your radio may help in this case, but are not necessarily a cure-all for fixing radiated noise.

    just some random notes from an EMI engineer....
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  3. mizzotch

    mizzotch New Member

    August 2, 2006
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    City, State:
    Honolulu, HI
    Year, Model & Trim Level:
    1992 XLT
    A definitive site for HF mobile

    Here is a very informative site written by a guy who really knows his stuff. Specific solutions are described and the pro/cons of different methods are explained. Seriously good reading if you intend to operate HF mobile. He covers it all: What antennas work and don't work, grounding/bonding, finding and curing noise, etc.

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