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Equipment chassis grounding in mobile environment

Discussion in 'Mobile Radio Systems' started by K5JPR, Jan 7, 2020.

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  1. K5JPR

    K5JPR Ham Member QRZ Page

    I'm putting the finishing touches on my brand new hf portable ham shack that consists of a Yaesu FT-891, Ameritron ALS 500MR, LDG 1000PROii and MFJ-828, along with other supporting components. The mockup, once fully debugged, will be transferred into my semi truck and feed into a center fed, center supported aluminum tube dipole mounted atop a military surplus QEAM mast. So far, things are working well and I'm getting good signal reports while using this mockup in the gazebo behind the house, powered by my Dodge dually parked next to it, and all the cases grounded to a single point, then extended to a ground rod in the dirt.

    Now, once I transfer to its final installation point, the chassis ground rod goes away. In preparation for the final install, I tested that connection back to the battery ground post, and the result appears to be a tremendous ground loop into my headphones. I hear my transmitted voice in a very muffled and distorted way that increases when the output wattage increases. Perhaps RF? Not sure yet. Full disclosure: I'm still testing for cause and solutions on the weekends when I'm home but the idea just came to me to use a 0.5uf high voltage capacitor (like a motor start capacitor or doorknob capacitor) between the ground point and the battery terminal or vehicle frame however, my knowledge of electronics and DC vs RF grounding is less then adequate to know if I'm barking up the right tree or not. Ultimately, I need to ground these radio chassis to something...perhaps a rubber static strap to the pavement below? I dunno.

  2. M0GVZ

    M0GVZ Ham Member QRZ Page

    Absolutely nothing ever needs connecting to that little screw on the back on any radio. It's literally put there to keep old timers happy and give those without adequate RF grounding something to bolt a bit of wire to to make them feel better.

    The battery is NOT a RF ground, it can't be used for one, it'll never be one. You don't connect anything to do with your radio to the negative terminal on the battery, instead you should connect the negative cables to where the main battery negative connects to the vehicle frame/body.

    All the answers for any questions you can ever have about mobile installations can be found at Alan K0BG wrote the mobile section of the ARRL book.
    AK5B and N0TZU like this.
  3. K5JPR

    K5JPR Ham Member QRZ Page

    Yes, I've read nearly all of the stuff that k0bg has posted. Excellent stuff. I've also read the exact opposite of your recommendation in regards to the little post on the chassis so, I flipped a coin and it came up Grounding. When I get back home this weekend and set the gear back up in the gazebo, I'll start the process of elimination by removing the chassis grounds as you advise.

    With regards to the 12vdc ground, yes, when I install in the semi, I planned to connect the ground lead to the same point on the engine block that the battery ground is connected to in order to eliminate the risk of using my equipment as a ground path for the rest of the truck. For the purpose of testing in the back yard, I just start the dually, then connect the 2ga leads to the battery after the engine is running for convenience, not the final connection plan of action.

    I'm also planning to replace all of my audio interconnections that I custom made using shielded pair cables with mix31 chokes with rg-174 and mix31 chokes. Of course, I'll be doing only one change at a time and testing, just to make sure that what I do either improves, worsens or does nothing.

    Now, all of my low draw 12vdc connections are also custom made using the same shielded pairs however, on those, I've connected the shield bleed wire to a ground block at the distribution end, and placed a mix43 split at the equipment end. All of those shield grounds will still connect to the vehicle frame, correct?
    AK5B and K0UO like this.
  4. AI5DH

    AI5DH Ham Member QRZ Page

    That is where you went wrong. Where is the Negative Black Wire from the radio terminated too? Please do not say directly to the Battery Negative Term Post.. If that is where it is at explains all your problems, and yes that is an EXTREMELY NASTY GROUND LOOP.

    There are only two places you want to terminate the Negative Black Wire from the Radio. The absolutely best place is right next to the radio to the vehicle chassis hard point used for ground. I do mean as short as possible. 1 foot or less.

    However before you commit, test and make sure it provides a good low voltage connection before cutting cable. . Some vehicle chassis are not bonded from front to back. If power is low from excessive voltage drop, run black cable to engine compartment and Bond to where the vehicle battery is bonded to firewall. In fact you do not even have to penetrate the firewall. Just find the Hard Point under a kick panel, loosen up a bolt, and terminate ring terminal with locking hardware.
    AK5B likes this.
  5. AI5DH

    AI5DH Ham Member QRZ Page

    Forgot this diagram.

    AK5B likes this.
  6. K5JPR

    K5JPR Ham Member QRZ Page

    "...that is an EXTREMELY NASTY GROUND LOOP."

    Just for clarity and to make sure I fully understand, given that all of my equipment is currently mounted to a piece of plywood sitting on a wooden picnic table, with the entire antenna assembly erected 15 feet away from, and neither is touching the power supply (i.e., my truck), that connecting both the positive and negative power leads to the battery creates a ground loop? I realize that also connecting the equipment chassis ground lugs to the negative battery terminal does create a ground loop, and that experiment failed spectacularly. My real head scratcher is how I prevent the risk of rf bite in a 400+ watt portable rig that's mounted in a semi truck without driving a ground rod into the pavement at the truck stop each night. Some say there is no risk of rf bite. Others say there is, leaving me to realize that the correct answer is both.
  7. W5UAA

    W5UAA Ham Member QRZ Page

    With so much plastic in vehicles today, I'm surprised no one has mentioned that if your radio is not touching the vehicle's chassis, this is of no concern. I have an IC-V8000 mounted under the dash of my Tacoma. The mounting bracket is bolted to plastic. So I ran the power cord straight to the battery. It's been working for years that way.
  8. K5JPR

    K5JPR Ham Member QRZ Page

    Yup. The entire cab of the semi it fiberglass...all except the back of the cab. I'll be building 3 tier custom shelving over the passenger seat to mount all the gear and the electrical distribution made from ABS sheets, framed out with aluminum angle. The only part of the rig that will come into contact with any metal will be the QEAM attached to the headache rack. I still plan on taking the 2ga ground cable to the engine block at the same post that the battery ground is attached, unless I find that it's easier to get into another threaded hole near by.
  9. AI5DH

    AI5DH Ham Member QRZ Page

    It does not form a ground loop in that configuration.

    OK good question and is easily answered by basic electrical fundamentals. In order for current to flow requires 2-Nodes, 1 node for current to enter, and the 2nd node to exit. Think of it this way. Say we have a 100-volt battery laying on the ground not connected to anything. What happens is we ere to touch either the Positive or Negative Terminal?

    Nothing happens because we do not have a complete circuit. Now what happens if we were to bond the Positive Battery Term Post to a Ground Rod. What happens if we were to touch the Negative Term Post standing on dirt with conductive shoes? We get nailed with 100-volts and taught a nasty lesson. We now have a complete circuit referenced to dirt ground. You become the load resistance.

    Another example is anyone's home wiring. If you touch the Line aka Hot conductor you get nailed with 120 VAC because the system is a Grounded System. All a Grounded System really means is one of the Circuit Conductors is REFERENCED TO GROUND or something in place a ground like a ham sandwich, or chassis of a car or truck. Industrial applications do not use Grounded Systems because they are too dangerous and prone to unnecessary outages. FWIW it is called a Open Delta System. If there is a Ground Fault on any of the 3-phases, nothing happens except an alarm goes off alerting plant operations there is a Ground Fault and they need to schedule a plant shutdown to make repairs. Imagine what would happen if a Glass Extruder or refinery lost power. You have a very expensive outage and will takes a long time to recover from cleaning hardened glass stuck inside the Extruder.

    So to answer your question directly is with the Truck being bonded to earth, makes the conditions to cause a RF Electrical Shock because you Grounded it and referenced the antenna to dirt. Take the Earth Ground off and that goes away.

    AK5B likes this.
  10. KA0HCP

    KA0HCP XML Subscriber QRZ Page

    -I've experimented with grounding the radio in two cars over 20 years and found that it made no difference. I see no advantage to doing so, and logically it does create the possibility for introducing problems due to having multiple ground points (loops). Conclusion: Don't do it.

    -Lowering the hostility: Alan K0BG no longer recommends blanket connection of power to the battery. In new cars that have current sensors on the negative battery lead, he recommends using either the closest chassis point to the battery or using one of the battery ground nodes throughout the car.

    In my present car which has current monitoring, I was having battery drain problems and have switched my power point to the nearest chassis point. No problems since then.
    KC1OCA, KB0MNM and AK5B like this.

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