Odd (to me) grounding issue

Discussion in 'Mobile Radio Systems' started by KM4IY, Mar 15, 2017.

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  1. KM4IY

    KM4IY Premium Subscriber QRZ Page

    In the 30 some years i've been mobiling, i've never seen this. yesterday i wanted to tune an 80 meter whip with the analyzer, so i disconnected the antenna coax from the tuner. the radio was on at the time... as soon as i dropped the coax, the radio went off... like it was losing ground. checking the connection to the battery ground, it appears solid, but i'll make sure today.

    this is in a honda element, btw, maybe there's something different in hondas battery grounding? but the mobile 2/440 rig i was using didn't notice it...

    i'm puzzled.... thanks...
     
  2. AI3V

    AI3V Ham Member QRZ Page

    That's right.

    You lost your negative.

    Rege
     
  3. KG7SWP

    KG7SWP Ham Member QRZ Page

    If the rig has a fuse in the negative, check it. If good, cycle it in & out of the fuse holder a few times with silicone di-electric grease. Some build up oxidation that breaks the contact. Do the + fuse while you are at it.

    Ed
     
  4. K0BG

    K0BG Platinum Subscriber Platinum Subscriber QRZ Page

    Ed's comment is dead on. This occurs most of the time, because the fuse isn't really carrying any load to speak of. The reason is, the negative lead is also connected to the chassis of the transceiver, and if it is grounded, a portion of the current flows through the chassis. In this case, through the coax shield. What bothers me is, why didn't you notice? I don't know the current draw of the transceiver in question, but if it is say 10 amps, that would be enough draw to lower the input voltage under load. This would cause the display to dim when transmitting.
     
  5. AI3V

    AI3V Ham Member QRZ Page

    Actually in a correct installation the negative lead carries exactly as much current as the positive lead.

    Randomly connecting a transceiver is poor practice, if you use the chassis of the car - or any other conductor carrying current- and connect to it without regard to existing current you open your install up to all sorts of ground loops and rfi.

    Don't connect wires to random potentials.

    Rege
     
  6. KM4IY

    KM4IY Premium Subscriber QRZ Page

    as it turns out, Ed IS corrrect. NO, i didn't see the display dim... the only grounds in use in my installations are at the battery and the antenna. this hasn't appeared to have a "negative" effect on the performance of the radio. nearly every time i've made a call on 40, i've made a contact, several on 80 and at least received recognition for calling on 20 this morning, they heard me but i was in the hash. maybe THIS will make it even better.
     
  7. AI3V

    AI3V Ham Member QRZ Page

    May I suggest a test?

    Temporarily connect the rig to a dummy load, have the rig DC wires connected, and the radio and dummy setting on the seat so the only connections to the car are the DC wires.

    Make sure you have rated voltage at the rig while you tx at full rated power.

    Compare the voltage you measure now with the voltage you had when you first installed the rig.

    Rege
     
  8. KG7SWP

    KG7SWP Ham Member QRZ Page

    I had the same thing happen to my IC7K at the shack. Googled it & of course found @Kobg answering this issue more than once, so I have tried to distribute his knowledke to make this dumb blonde seem smart... Oh, I have also been in electrical/electronics as a profession since 10th grade. Residential/commeccial electrician, Automotive electrical/electronics,Marine electrical/electronics/diesel/gasoline mech, now automation (We flew Lady GaGa for the 1/2 time. Many times, the stupidest of things can stop a whole show. And, it usually is the stupidest of things.

    I will say that I thought I knew it all about CB/ham installs in vehicles. That may be because I installed in such a bulletproof way, due to fixing all them cheap butt splices, 3M taps & wire nuts over the years. But, when I found Alan's page, I doubted some of his statements. I was wrong. They have all rang true in the last couple years of getting back into ham radio. Read his page. Read it again. Keep going till you have it verbatim. When doing anything, even though you know it word for word, re-read it again.

    Ed
     
  9. N8EKT

    N8EKT Ham Member QRZ Page

    I have had service calls because of similar installs done by a local radio shop

    By the time they call ME, they have usually melted the coax
     
  10. WA7PRC

    WA7PRC Premium Subscriber QRZ Page

    Obviously, larger coax is indicated. :p
    Or, get rid of the olde fuse in the negative lead. :)

    Tom W8JI 'splains it: http://www.w8ji.com/mobile_ground.htm
     

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