Negative to battery: yes or no?

Discussion in 'General Technical Questions and Answers' started by KI7TGX, Mar 13, 2019.

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  1. KD0CAC

    KD0CAC Ham Member QRZ Page

    Talk to any shop working on cars / trucks / equipment .
    Especially after emissions & computers came around .
    Then if near any cost - salt water , or other rust belt , grounds connections loosing good connections .
  2. WA7ARK

    WA7ARK Ham Member QRZ Page

    If your radio has alternator whine, it is because the radio is connected to the vehicle's chassis in more than one place, and the alternator return circuit prefers to flow though nice copper conductors instead of more resistive steel that the car is made of. Connecting the black wire to the battery negative always makes that problem worse; not better! It almost guarantees that you will have alternator whine.

    Most mobile radio installations in vehicles have at least two chassis connections; one is the black wire in the power cord, the other is via the coax shield to where the antenna base is connected to the car body.

    In actual fact, the way to identify if the ground-loop caused by multiple grounds is the source of alternator whine is to lift the black wire and temporarily let the radio get its DC return along the coax shield. That way, the radio has a single-point-ground (SPG), and the alternator whine will disappear.

    I always cut the black wire as short as possible, discard the idiot fuse (it is there to protect the idiots that connect the black lead to the battery), and connect the black wire to solid steel as close to the radio as possible.
    K7JEM likes this.
  3. K3UJ

    K3UJ Ham Member QRZ Page

    What chassis? Everything's unibody now, no frame (unless your in a truck) I don't know how much I'd trust body panel seams for electrical continuity.
    KA9JLM likes this.
  4. WA7ARK

    WA7ARK Ham Member QRZ Page

    One spot weld can easily carry 10s of Amps; think about how much current 10,000 spot welds will carry. Chassis = Body; you can tell how long I have been doing this...
    N0TZU and K7JEM like this.
  5. KA9JLM

    KA9JLM Ham Member QRZ Page

    Never heard of that before. I have never had a problem.

    If that is correct it sounds like the alternator is not grounded properly.

    Set the radio in the seat and use a mag mount antenna and all is good. :rolleyes:
  6. WA7ARK

    WA7ARK Ham Member QRZ Page

    No, it means that by default, the car body is being used to distribute power to every appliance in the car, and that there is alternator ripple current flowing through the car body, which collects all of the current to return it to the battery negative post (totally normal).

    There is a small voltage drop along the body of the car caused by the resistivity of the metal. In most installs, your radio is connected to the car body in two places, one is via the black power lead, the other is the coax shield because the antenna (say an NMO mount) is bonded to the car body under the antenna. Your radio and its wiring is effectively providing a lower resistance path between those two locations on the car body (antenna, and where-ever the black wire goes).

    On some radios, the current that flows between the coax shield and black wire flows along the radio's PCB, and that common-mode current between two points on the PCB can inject the alternator whine either into the speaker audio, or the transmit audio. That current is induced into the radio by the voltage difference that exists between the two places where the radio is connected to the car body.

    You have just complied with the Single-Point-Ground principle by effectively blocking the alternator ripple current that would otherwise come down the coax shield, because the capacitance under the mag mount is too small to pass several kHz alternator ripple, but it passes VHF just fine. The radio is now connected to the car body at a single point, namely where the black wire goes.

    All of the alternator ripple current that flows along the car body ultimately is gathered up in the big ground cable that goes from the engine block to the battery negative post. Say you have a rear-deck mounted antenna, so that connection is near the trunk You have a choice of where to connect the radio's black wire. Where do you suppose is the best place to connect it; battery negative post, engine block, passenger cabin floor, or in the trunk?

    The answer is the trunk; the worst place is the battery negative post. It turns out that the cabin floor is almost as good as the trunk.

    Think of starting the car, and using an AC milliVolt meter with two long leads to measure the voltage between two parts of the car. Measuring from trunk to battery neg might produce 100mVac caused by the normal alternator ripple current flowing along the car body. Turning big loads on/off (headlights/brake lights/turn signals/heater/AC) will change the magnitude of the reading.

    Now leave one probe in the trunk, but progressively move the other probe to the engine block, the passenger compartment floor, and then further back. You will see that as the two probes get closer to each other, the voltage reading will progressively decrease toward zero; the closer the probes get to the same point...

    This is the reason that running the black wire to the battery negative post is the dumbest, worst, most naive, total BS advice that has been promulgated through the ham community.
    Last edited: Mar 15, 2019 at 4:14 PM
  7. WB2LBV

    WB2LBV Ham Member QRZ Page

    In the past I've connected both pos and neg to the battery, but for my most recent install (Yaesu FT8900) I used a chassis ground. The radio is under the seat with a nice short (about 1 ft.) ground cable to a seat bolt. Works great, no noise or voltage drop.
  8. WA7ARK

    WA7ARK Ham Member QRZ Page

    My post #12 and #16 explains why....
  9. K7JEM

    K7JEM Ham Member QRZ Page

    We also need to remember that the vast majority of amateur radios have the negative lead tied directly to the chassis and frame of the radio itself. A properly installed mobile radio will pick up a ground connection directly through the mounting bolts or screws. This is one reason why the negative power connection needs to be made relatively close to the radio itself, it avoids a long ground loop. To verify this, you can remove the antenna and ground connection and see if the radio continues to perform as expected.
    N0TZU and WA7ARK like this.
  10. WA7ARK

    WA7ARK Ham Member QRZ Page

    Of course you are right. However, with newer cars it is sometimes difficult to find a place where the mounting bracket is anchored to metal. In my explanations in post #12 and #16 above, you now have to consider three possible connections to the car body, the antenna, the mounting bracket, and the black wire. In that context, cutting the black wire short, and connecting to the same bolt that attaches the radio's mounting bracket to the metal under the carpet on the transmission hump (for example) makes the most sense. That is how my 2-meter rig is installed in my Chev Silverado P/U. Also how the dual-bander is installed in our Subaru.
    K7JEM likes this.

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