im a little confused on mobile grounding

Discussion in 'Mobile Radio Systems' started by GROUNDWIRE, Nov 18, 2018.

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  1. GROUNDWIRE

    GROUNDWIRE QRZ Member

    they say run your dc ground directly to the battery and the rf ground to the body and frame to avoid ground loops, here is my question though, the body doors and hood are bonded to the frame for rf ground, but the battery negative post is grounded to the frame as well. sooo isnt that a ground loop right there? also dc grounding to body goes to frame which goes to battery negative. am i missing something here?
     
  2. K0BG

    K0BG Platinum Subscriber Platinum Subscriber QRZ Page

    This is a good question, and one often ask. The short answer is... What you don't do, is use the body for the power ground return for the transceiver and accessories.

    It always appears that everything is connected to the chassis, as there is continuity, albeit a few ohms. But closer examination will reveal that there are grounding points (for lack of a better word) throughout the body. These points are for common feeds, like the head and running lights. Interior accessories like the AM/FM radio, HVAC, etc. have their own grounding points. The key is all of this, are those few ohms I mentioned, which are of special importance on aluminum-bodied vehicles. The definition of a ground loop, is a differential between two points. Place enough current between any two points, and galvanic action will occur.

    Here is a question, but in reality a thought provoking comment... Guess what would happen, if you hard-grounded your aluminum radiator to the chassis?
     
  3. KB7FSC

    KB7FSC XML Subscriber QRZ Page

    Good question. So what can be the repercussions of dc grounding directly at the battery but not rf grounding the radio chassis to the body? Ground loops? I’ve always left the chassis of the radio alone regarding grounding and now I’m second guessing my habits.

    Wane - KB7FSC
     
  4. K0BG

    K0BG Platinum Subscriber Platinum Subscriber QRZ Page

    The issue at hand, in this case, is the battery monitoring system. They typically have a Hall device (load current sensing) on the battery's negative lead and/or build into the negative connector (brand/model specific). Bypassing the LCD can cause all sorts of unwanted consequences.
     
  5. N7EKU

    N7EKU Ham Member QRZ Page

    Hi,

    The idea to run DC ground directly to the battery is to avoid the voltage drop that you would get by e.g. running from a lighter socket. Also, many power lines coming from the fuse panel are not designed to handle high current. The idea to run RF ground to the body is because you want a very good RF ground as directly under the vertical antenna element as possible -- not routed from the battery through all sorts of iffy connections. In the end though, ground loops -- at the radio -- are eliminated this way because the DC ground ends up being connected to your RF ground through the radio chassis as it connected to both the DC ground and RF ground through the coax shield.

    That's the way I see it anyway.

    73,


    Mark.
     
  6. KB7FSC

    KB7FSC XML Subscriber QRZ Page

    I guess that is the way I've thought about it also, so I haven't directly grounded the chassis of my radio to the body of the vehicle with a separate ground strap. Rather, as you describe, I've relied on the coax shield to act as the rf ground and the dc ground goes directly back to the battery ground directly in front of any current sensor. The root of my question was if NOT providing a rf ground from the chassis of the radio to the body of the vehicle was considered poor practice? As I read your description and Alan's comment, it appears that NOT directly putting a ground strap between the radio body and chassis is OK. Let me know if I'm interpreting this incorrectly.

    Thanks much for the input!

    Wane - KB7FSC
     
  7. GROUNDWIRE

    GROUNDWIRE QRZ Member

    all my rf / chassis grounds are done thru the coax. radio gear to coax to a puck mount in the roof to antenna. power for gear is thru 1/0 excelene welding cable to positive terminal of a 4 battery bank., charged by two 350a alternators. dc ground is thru same 1/0 excelene welding cable to frame ground. reflect is 3w at 1200w foreword. no ground loop issues that i can detect. noise floor is zero except for my stupid fuel pump, ( but its going out) voltage drop on transmit absolutely none. thats why im wondering do i really need to make a dedicated 15ft ground lead run to the battery?
     
  8. K0BG

    K0BG Platinum Subscriber Platinum Subscriber QRZ Page

    Hum, I can't come up with the same voltage drop you did. Twelve hundred out, is about 2,300 in, or about 165 amps peak. So the peak voltage drop is .49 for a 15 foot run, and this does't include fuses or connections.
     
  9. AA7QQ

    AA7QQ Ham Member QRZ Page

    Do you mean that you are using the frame & chassis for the negative conductor? If so, you should see a noticeable difference with a full run back to the battery/chassis connection point (Depending on year/make/model) or direct to the battery.

    THe least it would do is to lessen the strain on the ground strap system of the vehicle, as you are drawing all of that current through them. If the right one(s) get overtaxed & flake out, you could lose important systems of the vehicle.

    Ed
     
  10. M0GVZ

    M0GVZ Ham Member QRZ Page

    The first reason is because of what Alan K0BG wrote about modern charge systems.

    The second reason I can answer this because I've seen it happen. In the event of there being a break in the vehicles main negative wire from the battery to the vehicle body then the path to the negative terminal for ALL current being drawn by the vehicle by ALL electrical systems is through anything connected directly to that battery terminal which also has contact with the vehicle body. If you've hooked up your radio to the negative terminal that means your radio because there is a DC connection from the antenna mount via the coax to the radio and then to the battery via the radio power leads.

    Where I saw this happen was on a tractor being used on a farm. Over time the bolt securing the vehicle negative cable to the chassis had worked loose and the cable became disconnected at the chassis end. The resulting current flowing through the VHF radio completely blew it to bits inside the case.
     

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