Mobile Grounding

Discussion in 'Mobile Radio Systems' started by K7KWN, Apr 6, 2012.

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  1. K7KWN

    K7KWN Ham Member QRZ Page

    I just installed an older (2003) dual band mobile. The instruction book says to ground and power directly from the battery. Just to be safe, that is what I did, but am wondering if that is really necessary as opposed to grounding to the frame someplace.
  2. KC9UDX

    KC9UDX Platinum Subscriber Life Member Platinum Subscriber QRZ Page

    Yes. Resistive losses along the body and/or frame can be significant. It increases with age, but, can also be high from the factory. Running high currents through the body/frame accelerate the increase.
    Last edited: Apr 6, 2012
  3. K9STH

    K9STH Ham Member QRZ Page


    Actually, it is the opposite! Running the negative wire directly to the battery effectively almost doubles the IR voltage drop to the radio. Commercial two-way radio manufacturers have been grounding the equipment directly to the body/frame for many decades!

    The main reason that most, if not all, Japanese radios come from the factory with both the positive and negative leads the same length and recommend connecting the negative lead directly to the battery is left over from the 1970s when those manufacturers were making "CB" radios. In that time frame, there were still a lot of "tractor" units in "18-wheelers" that were positive ground. So that those "CB" units could be used in vehicles with either positive or negative ground, the chassis was isolated from ground and, therefore, if the red wire was connected to the positive terminal of the battery and the black lead was connected to the negative terminal, it didn't matter which battery terminal was connected to the frame of the vehicle.

    When those manufacturers started making amateur radio equipment for export, they continued making the power leads the same length even though the amateur radio equipment no longer had the chassis isolated.

    From what I understand, the European Union is now requiring that radios installed in vehicles must have the negative lead connected to the body/frame of the vehicle as close to the radio as possible.

    If, for some reason, the connection between the negative terminal of the battery and the frame/body of the vehicle comes loose, if the negative lead from the radio is connected directly to the battery, then the entire current supplied by the battery will be carried through this relatively small wire to the radio to the frame of the vehicle either by the bracket holding the radio connected to the chassis of the radio to the vehicle and/or the braid of the coaxial cable to the antenna. This can definitely result in overheating of the negative wire and/or the shield of the coaxial cable. That is definitely not a good thing and, in some cases, can cause a fire! If the negative lead is connected to the frame/body of the vehicle, if the primary negative lead comes loose from the battery then there is no path through the radio to complete the circuit.

    Glen, K9STH
  4. KC9UDX

    KC9UDX Platinum Subscriber Life Member Platinum Subscriber QRZ Page

    You apparently do not live in the 'rust belt'..! I agree with everything you say except that in every vehicle I've owned, a direct connection to the battery is always better than a connection to a point on the body or frame any large distance from the battery.
    So, in the long run, it probably makes sense to run the cable to the battery, and also to connect to the body or frame.

    When I wire vehicles, I make sure to connect the negative lead from the battery to the engine block near the starter, to the frame, and to the body. Then I run one from the engine to the frame and another from the body to the frame (if equipped). It sounds excessive, but it makes the engine much easier to start when it's ten degrees below zero. In any event, if your battery cables are loose, you have other problems besides starter current running through the radio chassis.
  5. K0BG

    K0BG Platinum Subscriber Platinum Subscriber QRZ Page

    What you need to do here, is follow the manufacturer's recommendations. That is directly to the battery, or jump point, and not use the body of the vehicle.

    One has to remember, the body is this case is acting as a conductor. Glen is assuming the body has not resistance at all. It does, and is it more than copper losses. Worse, using the body as a ground return, can cause a ground loops to occur between the various CPUs, and their respective sensors. That is something you don't want to do.
  6. K9STH

    K9STH Ham Member QRZ Page


    We, and others, have gone round and round about this before! What companies like Motorola recommend for installation practices are, at least to me, what need to be followed since their radios are in use in hundreds of thousands of vehicles. If Motorola recommends a different installation for certain vehicles then that definitely needs to be taken into consideration. But, in general, Motorola has used the frame/body of the vehicles for over 7 decades.

    As for the resistance: Yes, steel does have a higher resistance than copper when the conductors have the same cross sectional area. But, where the frame/body of the vehicle are concerned, the cross sectional area is hundreds, even thousands, of times greater than that of a single copper wire generally used with amateur radio units and, therefore, the actual resistance is considerably lower.


    I am originally from northwestern Indiana and that is, as far as I know, part of the "rust belt". When the connection to the frame/body of the vehicle is properly weatherproofed, that connection will last for decades. Again, follow what Motorola (definitely a "rust belt" company being in Schamburg, Illinois) recommends and you won't have any problems.

    Glen, K9STH
  7. K0BG

    K0BG Platinum Subscriber Platinum Subscriber QRZ Page

    Glen, I am not naive, and I am aware of what Motorola used to do. I did, in fact, work for a Motorola MR. I am also aware of Tom Rauch's, W8JI, comments on same. But, we're not talking about 50s, 60s, 70s, or even most of the 80s vehicles. We're talking about modern, computerized vehicles. What used to be the status quo, isn't nowadays.

    Every passenger vehicle in the world, will be required to have idle shut off by 2014 (most 2012 models have it now). This requires that the vehicles have battery monitoring systems, and other electronics to make sure the battery is up to snuff. They're typically Hall devices. Ford and others have chosen to mount the device as part of the negative battery connector. There is both a negative, and positive jump point on vehicles so equipped. Wiring through the body in the vehicles, circumvents this function.
  8. KA9JLM

    KA9JLM Ham Member QRZ Page

    If you run to the battery and fuse both the + and - Polarities you will be OK. That will be best.

    Then ground at the nearest point to the radio.

    If your battery is connected to the frame like it should be, then you are good to go.

    No big deal.

    Connect direct to the battery if it is possible.

  9. KA9JLM

    KA9JLM Ham Member QRZ Page

    Alan you may be able to read minds.

    But you can not over write them.

    I hear you... You are correct.

  10. KC9UDX

    KC9UDX Platinum Subscriber Life Member Platinum Subscriber QRZ Page

    I probably sound like I'm trying to be argumentative, but I promise you I'm not: If I connect the 10W 2M rig in the car I currently use as a daily driver the way Motorola recommends, it will not get enough power to run. It's not the connection from the battery to the frame that's the issue. It's the fact that the body is pretty well rotted. Thanks to this road salt stuff...
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