mobile interference

Discussion in 'Mobile Radio Systems' started by VE7MBV, Feb 5, 2011.

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  1. VE7MBV

    VE7MBV Ham Member QRZ Page

    Hello all, I am new to the Amateur Radio world, and I need some help

    I have installed a Kenwood TM-271A in my Jeep, I have two direct wires from the batt. one from postive and one ground.

    When I am NOT TXing or RXing I get interference on almost every channel( more so on the 144-145 fq) I can hear my engie RPM through the interference in the background, but its mostly just a static sound. I also get it with engine OFF ignition on.

    I am running a simple Larson antenna and about 3-4 feet of cable between antenna and radio

    any ideas...will a bandpass filter fix this?
  2. W8JI

    W8JI Ham Member QRZ Page

    It sounds like the computer might be radiating out through wiring in the Jeep, because when the engine is off and the key on the injectors, alternator, fuel pump, and spark are all off. Fuel pumps only run a few seconds without the engine running in a good design fuel system, almost everything else is off right away. Only the computer system is running.

    A bandpass filter at the radio will not help, and almost always the battery connections will not help.

    I would NOT, no matter what anyone says, connect the negative lead of my radio the the battery post. I would connect it to a chassis ground near the battery ground or some other large area of sheet metal, but never directly to a negative post on the battery or a battery negative lead. Nothing good can ever come from connecting to the negative post or battery ground wire.

    Maybe you can sniff around with a handheld and see where the noise is radiating from and perhaps move the antenna. Make sure the antenna has a good ground, where ever it is mounted.

    73 Tom
  3. KA0HCP

    KA0HCP XML Subscriber QRZ Page

    Check out Alan's website Has everything you need to know about mobiles.
  4. K0BG

    K0BG Ham Member QRZ Page

    It is the data busses. If it is a newer Jeep (2008>), there are three. They mix together, and there are birdies clear into the low VHF region. They sound more like a light buzz just over the band's background noise level. Typically, you shouldn't hear them if the squelch is set correctly. If you still are, it might be caused by a ground loop.

    My first question, are you using a mag mount antenna?
  5. WA7PRC

    WA7PRC Ham Member QRZ Page

    I believe Tom meant near the rig. The reason for doing it this way is twofold:
    • Minimize voltage drop in the wiring. At higher current, this becomes more of an issue.
    • Minimize the amount of crud that can degenerate across the wiring (and into the rig). Less R = less noise.
  6. VE7MBV

    VE7MBV Ham Member QRZ Page

    No its not a mag mount, but now that I think about it, It is screwed onto a painted serface( may even have some plastic on it) So I am going to start by running a ground strap for the antenna

    its an 89 cherokee
  7. KE5PPH

    KE5PPH Ham Member QRZ Page

    It's a unibody. Follow the tips on BONDING on k0bg's site.
  8. W8JI

    W8JI Ham Member QRZ Page

    On 2 meters, the ground strap better be less than two inches long.
  9. KH6AQ

    KH6AQ Ham Member QRZ Page

    Rather than using a handheld radio to 'sniff' for noise you can try the 'laying on of hands.' With the volume on the mobile radio turned up loud, the ignition ON, and the engine OFF, touch various items under the hood, wiring harnesses, parts of the exhaust system and so on. You will fine items that are hot with RF. Touching them will increase or decrease the noise. These items can then be treated by bonding or the by the addition of clamp-on ferrite cores.
  10. K0BG

    K0BG Ham Member QRZ Page

    Ferrites are wonderful things, but only if they're applied correctly. Their effectiveness is directly related to the square of the number of turns. Snapping a split bead around a wiring bundle does all most nothing as far as RFI suppression is concerned.

    Mag mounts, clip mounts, and others held on by set screws are not the stuff of champions. If you really want to prevent ground loop problems, which often mimic RFI problems, they mount the antenna correctly, drill a hole, and use an honest-to-john NMO mount (New Motorola), rather than some Pacific Rim knockoff.
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