Ground Test

Discussion in 'General Technical Questions and Answers' started by KF5LJW, Mar 21, 2020.

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  1. KF5LJW

    KF5LJW Ham Member QRZ Page

    Ok this is going to generate a lot of replies, a very good thing. Or at least I think it will. You are stuck inside the house, you got nothing better to do.

    I want each of you to do a simple experiment by performing a DC Ground Loop test. It is really simple most of you can do. Should be educational and fun.

    Some conditions must be met. Most stations meet the conditions.
    1. A 12-Volt CB, HF, VHF, or UHF Transceiver. Example a 100 watt HF Kenwood Transceiver, or even a 50 watt VHF/UHF.
    2. External 12 volt power supply like an Astron RS-35
    3. External antenna you think is properly bonded to ground.

    Most of you meet these conditions. So here is the test procedure.

    1 Turn off radio.
    2. Disconnect the Radio's black negative wire from your power supply. Leave Positive, antenna, and all other connections connected normally to radio.
    3. Turn Radio ON.

    Pretty simple test just about anyone can do. If your radio turns on, you got yourself an extremely nasty ground loop. Last place you want your radio to be is in a ground loop of any kind, and DC is the worse of them all because it means you are in both a DC and AC Ground Loop. It means you got all kind of common mode noise running through all your ground wires and your radio is now a circuit conductor full of signals you do not want.

    I will not tell you what is causing it right now, but I will give you a hint with another test, step four.

    4. Disconnect antenna from back of your transceiver. Did you radio turn off?

    There is another way to do the same test if you have a Clamp-On Amp Meter or even an ole fashion shunt amp meter. Measure the DC current in the positive and negative battery to your radio. They should be equal, but for most hams will not be equal.

    Some of you will get what you expected, disconnect the power, and the radio will not turn on. However the majority of you are in for a SHOCK when your radio turns on.

    Warning do not try to TX with the negative disconnected. You voltage will sag to low if you try.

    Report back what happens.
    Last edited: Mar 21, 2020
    K0UO likes this.
  2. N3PM

    N3PM Ham Member QRZ Page

    Should be easy to do by pulling the fuse in the negative lead. Most rig cords have 2 fuses. I first saw this a while back when the negative lead fuse died in a mobile installation. Back when everyone ran both + and - back to the battery. Found it when the radio shut off when the coax was disconnected.
    Mike N3PM
    K0UO likes this.
  3. W9WQA

    W9WQA Ham Member QRZ Page

    step 4 , when i unhook the antenna the radio stays on but no signals.?
    my radio is bolted to the frame of the car.
    even without the black wire it works?
    did i do something wrong?
    looks like i have 3 grounds, the chassis,the black wire,and the ant coax.
    what should i do?
    is there a short in my radio?
  4. W9WQA

    W9WQA Ham Member QRZ Page

    oh, one more thing. if a radio was built with positive ground does that mean i cant ground it? i have a portable like that...
  5. KA0HCP

    KA0HCP XML Subscriber QRZ Page

    No, there is nothing wrong with your radio. It is doing exactly what you have wired it to do. It is using the body ground for power. The suggested test was intended for house stations using an external power supply, not mobile stations.

    KF5LJW, is being coy. What he is doing is flogging a repeated theme of his about external power supplies (possibly) having house safety ground tied to the Negative DC output.

    The simpler and less dramatic way to test your power supply is to use a VOM and check for continuity between the AC plug ground pin and the DC Negative output terminal.

    'LJW as previously recommended altering the power supply to eliminate the tie point if it exists. He has also stated that Astron power supplies are not consistent as to whether they have this connection; some do, some don't.

    FWIW, I don't disagree with him on this point. Correcting this issue will provide better electrical/surge/lightning safety and may reduce household RFI and Common Mode current.

    I haven't checked my PS, but with this thread encouragement will do so this afternoon! b.
    Last edited: Mar 21, 2020
    N0TZU likes this.
  6. W9WQA

    W9WQA Ham Member QRZ Page

    if you cut the wire you will find it makes no difference whatsoever...

    and,it went right over your head!

    "No, there is nothing wrong with your radio. It is doing exactly what you have wired it to do. It is using the body ground for power. The suggested test was intended for house stations using an external power supply, not mobile station"
  7. KF5LJW

    KF5LJW Ham Member QRZ Page

    You are not doing anything wrong. That is how it was designed to work in a car. Your test is invalid, you did not meet the conditions. The test is for your radio to be inside your home aka Shack using an external DC power supply integrated with your house wiring using an outside antenna properly bonded to meet code.

    Having said that what you are doing is making the point. Most Ham radios are designed to work in a vehicle with negative 12 volt battery systems and not in your house. The difference between a radio made to work in your car, vs one made for the house is mobile equipment uses ground as both a circuit conductor and a Ground. No other industry does that because it would be dangerous and noisy. Equipment made for a facility DO NOT BOND Chassis and Battery Negative together, it is kept isolated. You get a third wire called Equipment Ground.

    Your radio works in your car because it is designed that way and you do not need a negative wire. The vehicle chassis is the negative conductor and ground.
    KA0HCP likes this.
  8. K8JD

    K8JD Ham Member QRZ Page

    One of my antennas has balabnced line feeders coming in to the room, I unhooked them from the MFJ 974 balanced tuner and measured the voltage and risistance to ground from both wires in the feedline, Read zero volts and infinite ohms. I think this is good !
    I have a FT990 that plugs right into the AC wall socket, would be a lot of work to get it open and measure the power supply black lead to the circuit boards.
    I have one GOOD loop, but it's a 40 M fullwave loop antenna up in the trees.
    Went to the Vehicle and found red and black wires and a plug , Coax and a mobile antenna, but there is no radio in place. Lost interest in getting yelled at by XYL when I wanted to use the rig in the vehicle.
    With the Quarentine on, I have a lot of free time on my hands.
    Play and learn !
    Last edited: Mar 21, 2020
  9. KF5LJW

    KF5LJW Ham Member QRZ Page

    No argument here, but the test method shows why it is a real problem, and wakes you up to the fact test positive your radio is in a DC and AC ground loop.

    There are many here who deny that can happen or do not think is a real problem. All one has to do is see the circuit and realize you are in both a DC and AC Ground Loop. Last place you want any sensitive electronic equipment to be in. It can be extremely dangerous, and wreak havoc with RFI and common mode noise problems. All because they fail to see the big picture and do not understand one ground from another ground.
    Last edited: Mar 21, 2020
    KA0HCP likes this.
  10. W9WQA

    W9WQA Ham Member QRZ Page

    so i added wire outside while copying weak signals and viewing the 7300 display.
    as figgered, absolutlty no difference whatsoever.
    its just 2 wires in parallel. lower resistance...note pix with white "ground " wire, pix with black pulled.
    this supply has open inside

    Attached Files:

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