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Coax ground to backing plate or roof?

Discussion in 'CB Antennas' started by groundwire, Mar 15, 2020.

  1. groundwire

    groundwire Active Member

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    How do you guys ground your coax in the mobile using a puck mount and backing plate? Do you ground the coax to the backing plate or between the backing plate and roof (direct roof connection)
    Im grounded to the plate but looking at it, i was thinking the ground going thru the plate then capacitivly grounding to the roof cant be as good as a direct connection to the roof. So i took out my vom and like i thought, there is resistance between the two. I got a 1.2 ohm reading. What are your thoughts and why? oh yea, YES i took all the paint and primer off the whole square on the roof so it is raw metal to metal contact


     
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  2. WECSOG

    WECSOG Well-Known Member

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    That 1.2 ohms will indeed equal loss. Anything you can do to improve the connection to the roof will help.
     
  3. groundwire

    groundwire Active Member

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    Yup, and im thinking thats where my swr issue is comming from
     
  4. 2NC995

    2NC995 DAN

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    The 1.2 ohms doesn’t mean much. Could be your leads, or just the connection. Can you get a 0.0 touching the leads together?

    It would be an open at DC if connected only via capacitance.
     
    WECSOG likes this.
  5. groundwire

    groundwire Active Member

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    My test leads have a .3 ohm resistance so my 1.2 ohm reading is minus the the lead resistance
     
  6. 2NC995

    2NC995 DAN

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    What is the backing plate made out of?
     
  7. groundwire

    groundwire Active Member

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    Im 95% shure its aluminum. Its from breedlove mounts
     
  8. Tallman

    Tallman W9WDX Amateur Radio Member, KW4YJ EXTRA class

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    I would go to the backing plate.
     
    binrat, 2NC995 and tecnicoloco like this.
  9. M0GVZ

    M0GVZ Sr. Member

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    With the puck mounts you use one of the mounting bolts.

    The reading you get with a multimeter doesn't mean anything as it's only measuring a DC resistance and AC/RF works completely differently. Also too late now but you shouldn't have removed the paint and primer as you've removed the zinc rustproofing.
     
    2NC995 likes this.
  10. M0GVZ

    M0GVZ Sr. Member

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    At 0Hz DC but he's not transmitting at 0Hz.
     
    2NC995 likes this.
  11. groundwire

    groundwire Active Member

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    The coax shield is connected to one of the bolts.
     
    543_Dallas likes this.
  12. 543_Dallas

    543_Dallas Sr. Member

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    Connect the shield to the backing plate with the puck bolts. I split the braid and connect to 2 bolts with soldered on ring terminals. Keep the center conductor that sticks out as short as possible.

    Sand the bottom of the cab to bare metal where the backing plate makes contact. Use your favorite compound between the roof and backing plate to prevent corrosion. I've had no issues with the Noalox on the puck I did almost 10 years ago.
     
  13. groundwire

    groundwire Active Member

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    Haha, thats a coincidence, thats exactly how i have mine done. Except the braid is omly on one bolt.
     
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  14. WECSOG

    WECSOG Well-Known Member

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    If there's resistance at DC, that resistance is still there with AC. Granted, capacitance between the plate and the roof will act in parallel (bypass), but that is not even close to a perfect capacitor. It's a good idea to get that resistance as low as possible.
     
  15. Handy Andy

    Handy Andy Do Your Research First, Then Decide...

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    Wow...Went to the site to find what you're looking at...

    Aluminum - hmm...lots of them are Anodized, so the issue of good ground and the type of mount used, both go hand in hand.

    Are you using this mount?
    upload_2020-3-16_19-47-59.png
    When I see "Backing Plate" since the above is a 3-hole punch...

    I take it you also are installing this...
    upload_2020-3-16_19-53-27.png
    Is that correct?

    Because I found this - which was helpful but not sure what you're up against...
    [​IMG]


    But this doesn't look exactly like the one I'm thinking you have...​
     

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