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Multiple negative ground radios on a single power supply

Discussion in 'General Technical Questions and Answers' started by NQ8J, Sep 10, 2020.

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  1. KF5LJW

    KF5LJW Ham Member QRZ Page

    I respect your opinion, you understand what is going on, but I disagree you should allow DC current to flow. What is kind of strange you then gave some RF Issues that can be caused from it. RFI sounds like an effect to me.

    But here is my point. All grounds should not be carrying any normal operating currents because that defeats their primary purpose of providing 0-volt touch potential. Current flowing on a conductor will develop a voltage drop along its length. This is why electrical codes forbid normal operating currents flowing on ground circuits. Secondly DC current is destructive to your ground electrode system if you bond the negative polarity. That DC current flowing will greatly accelerate corrosion on your ground electrode system. This is the exact reason why telephone companies, cellular, data, you name it bond the Positive polarity to protect the ground electrode system and greatly slows down corrosion.


    Respectively I find that funny. I use a lot of ironic humor, and this is great material. Example I often say: Hams love having messed up grounding and bonding because it gives them something to fix and spend more money and time on. :) Ironic huh?

    Why give yourself the problem to start with? It is a simple 10- minute zero cost fix.
  2. K7JEM

    K7JEM Ham Member QRZ Page

    This simply is not true. The vast majority of LMR mobile radios, or rack mounted base radios have their chassis tied directly to the negative power lead. In most cases, there is absolutely no difference in applying the negative voltage either through the chassis, or though the attached power lead.
    N0TZU likes this.
  3. KF5LJW

    KF5LJW Ham Member QRZ Page

    My bad should have said some commercial LMR do not bond chassis to battery negative. Same is true with some amateur radios. However today radios used in racks and equipment enclosures for commercial operations (LMR), use 48 volt battery. Some 24 volt older stuff out there . Neither positive or negative is bonded to the equipment chassis
    There is one heck of a big difference you keep and a lot of folks keep missing. It puts you in a ground loop a real nasty ground loop. I call that a huge difference.
  4. K7JEM

    K7JEM Ham Member QRZ Page

    There is no difference in applying the battery negative either to the chassis, or to the negative terminal, which is simply a wire attached to the chassis. There can be no difference, since the wires go to the same places.

    A good example is a Kenwood TKR-850 rack mount repeater, which runs off 12v, not off 24 or 48 volts. In that radio, it has a negative terminal on the back of the radio. That terminal goes to a wire that immediately bonds to the negative ground plane on the PA deck, which is screwed with multiple screws right into the chassis and frame of that radio.

    This is not an isolated design issue. Almost all radios are built this way.
    N0TZU likes this.
  5. KF5LJW

    KF5LJW Ham Member QRZ Page

    Sorry but you are dead wrong. With that bonding jumper inside you DC supply means you have absolutely no ground of any kind. You are hopelessly lost in a ground loop. If you would simply look at the circuit can clearly see that if you can read a circuit diagram. You do not have a Ground, you have a Bonding Jumper between two earth electrodes and all your radio toys are nothing more than a long piece of wire some 20 to 60 feet or longer in some cases.

    I have drawn the circuit many times. Perhaps if you drew it in your mind can see it. Your bonding jumper starts outside where your coax shield is bonded to the shack ground rod. It enters on your coax shield which is not a ground, From the Shield to the chassis of your radio, From the radio out on the negative 12 volt conductor to the transformer inside your DC Power Supply. Inside the DC PS jumps on the bonding jumper to the DC PS chassis. From the DC PS chassis to the green power cord equipment ground conductor plugged into a wall socket. From the wall socket back across the house to the AC Breaker panel AECG ground bar. Then finally arriving at the end it leaves the ACEG ground bar and terminates to the AC Ground Rod on the other side of the house. That is no ground of any kind. It is a serial daisy-chained bonding jumper you call a radio system stuck inside a ground loop. Can you see it now?

    Remove that bonding jumper and all that goes away. Now you can have a Ground if you run a ground wire in instead of using your your coax shield. It makes a huge difference.
    Last edited: Sep 10, 2020
  6. K7JEM

    K7JEM Ham Member QRZ Page

    You can't remove a bonding jumper if the whole radio is bonded from the circuit boards to the chassis in multiple places. I think you are missing what I am saying. Every single board inside the radio is screwed directly to the chassis/frame of the radio. There is no way to eliminate that.
    W9WQA and N0TZU like this.
  7. KB7WG

    KB7WG Ham Member QRZ Page

    There could be other things going on here. We don't know the feedline layout, outdoor grounding/lightening setup and the antenna orientations. Does the same current appear at every band? We don't know his measuring equipment or the possible influence on them. Some equipment is excitable and biased in an rf field.

    Un-bonding the PS secondary, would not prevent the dc sharing. But it should defeat a rf path.
  8. NQ8J

    NQ8J Ham Member QRZ Page

    Just to be clear: the bonding jumper was removed from this power supply within 5 minutes of pulling it out of the cardboard box it was shipped in.

    What I wanted to draw attention to is that, even with the bonding jumper in the power supply removed, as soon as you connect a second transceiver, and your coaxes are bonded where they enter or at a transceiver selector switch, you just created another loop.

    The power leads are what came with the radios, maybe 5-6ft long. If they where short and very large you could reduce the amount of current through the unintended path but not eliminate it.

    For example if you have two 6' #6 wires (0.39mΩ per foot) feeding both radios, you are 20' away from the service ground where both coaxes have their shields tied together, and are using RG-213 (Belden 8267 outer shield 1.2mΩ per foot), you have two paths for current on the negative side to flow into the radio:

    1) Radio 1 negative wire, 2.34mΩ
    2) Radio 2 negative wire, 2.34mΩ + 40'round trip of RG-213, 48mΩ for a total of 50.34mΩ.

    Our parallel circuit is 2.236mΩ and with radio 1 drawing 20A we have a voltage drop of 44.6mV between the negative terminal of the power supply and our radio.

    18.9A flowing on our intended path
    0.9A flowing on our unintended path.

    Yes these don't totally add up, only for the reason that I did not want to work with 8 digits past each decimal place for every number, but if you do this exercise you'll find they add perfectly.

    Now nobody is going to use #6 for their power feed to a 100W radio. Also if RG-58 (5.5mΩ per foot) was used we would have less current on our unintended path.

    Let's take another scenario: Let's say we have two radios, both fed with #6, but instead of being bonded through their coax 20' away, they are connected to a coax switch connected to a single antenna, 3' away. Our round trip trough the coax is now 6' (7.2mΩ).

    1) Radio 1 negative wire, 2.34mΩ
    2) Radio 2 negative wire, 2.34mΩ + 40' of RG-213, 7.2mΩ for a total of 9.54mΩ.

    Our parallel circuit is 1.879mΩ and with radio 1 drawing 20A we have a voltage drop of 37.6mV between the negative terminal of the power supply and our radio.

    16A flowing on our intended path
    4A flowing on our unintended path.

    Remember, this is with #6 wire power feed. Our unintended path current would be higher with #12 or 2x #14 typically used.

    One more:

    What if we have all of our desk gear connected to a "shack ground", a 1" copper pipe running along the back of our desk, and each piece of gear is connected through a #12 jumper, 1' long. Power is supplied through the more common #12 wire (1.6mΩ per foot). Lets pretend radio 1 and 2 are next to each other and their jumpers are 1' away from each other.

    I'm not going to bother including the 18uΩ per foot resistance of the pipe. Also going to ignore current through coax for the moment.

    Now you have an even lower resistance path for unintended current.

    1) Radio 1 negative wire, 9.6mΩ
    2) Radio 2 negative wire, 9.6mΩ + 2' round trip of #12 grounding jumper, 3.2mΩ for a total of 12.8mΩ.

    Our parallel negative circuit is 5.486mΩ and with radio 1 drawing 20A we have a voltage drop of 109.7mV between the negative terminal of the power supply and our radio.

    11.4A flowing on our intended path
    8.6A flowing on our unintended path through radio #2

    Even with the negative-ground bond in the power supply removed, you can't eliminate the unintended current with multiple negative ground radios connected to the same supply in an NEC compliant installation.
  9. W9WQA

    W9WQA Ham Member QRZ Page

    you can toss the neg fuse,many do that, not neede4d with chassis ground.

    waay waaay tooo much worrrying goin on here...
  10. W9WQA

    W9WQA Ham Member QRZ Page

    i never read w o w wall of words

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