Freightliner M2 OEM 3/8 Stud Mirror Mount Antenna SWR

Discussion in 'CB Radio Forum' started by niloodn, Sep 4, 2022.

  1. niloodn

    niloodn Bobtail Member

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    Sep 4, 2022
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    Hello!

    Looking for some guidance on an CB installation on a Freightliner M2. It is the Freighliner OEM mirror arm that has the 3/8 stud mount and a ground wire on the nut that secures the mount to the mirror arm. I'm currently using a Firestik FS3B 3' Hand Tune Fiberglass Antenna. I also have 10ft of RG8X connected to the mini UHF terminated cable that connects to the mirror. I was checking SWR with an antenna analyzer and could not achieve better SWR than 3.0 -> 4.0 across the 40 channels while adjusting the tuning screw at the top. I realize that this is likely due to insufficient ground plane, but I'm wondering how folks have resolved this issue. I'm also wondering if particular antennas work better in this situation than others. Radio checks perform fine and I'm able to get out a few miles but would like to bring the SWR down at least to 2 or better.

    Any help would or reccommendations would be appreciated.

    Thanks!
     
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  3. niloodn

    niloodn Bobtail Member

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    Sep 4, 2022
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    Just to add some additional information I've tested the resistance from the bolt/ground where the stud mount attaches to the mirror to multiple points on the chasis and have consisent readings of well under 2 ohms. So while the mirror arm mounts onto the door via some composite material bracket the grounding to the antenna would seem to be ok. I see plenty of folks on the road with Antennas(Firestik syle that appear to be for 11 meters) on the Mirrors of the M2 so I'm sure someone has figured this out. Hoping that someone finds this forum. :)
     
  4. Ridgeline

    Ridgeline Road Train Member

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    Dec 18, 2011
    Michigan
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    One question and one comment

    are you checking the swr at the antenna or coax in the truck?

    this is important.

    so here is the thing, you need to have both sides of the feed line (coax) acting as a means to transmit the signal.

    this means you need an RF ground otherwise you get what is called a common mode current problem and a high swr.

    the resistance reading shows only a very little part of the issue, so you need to start bonding the antenna mount to the chassis to create that other half of the feed line (coax) to have the antenna to work better.
     
  5. niloodn

    niloodn Bobtail Member

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    Sep 4, 2022
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    SWR is being checked inside the cab where it would plug into the radio. I've used both a standard SWR meter and a NanoVNA to check at that point.

    Coax is likely RG58 from the Mirror mount into the door, terminated with with Mini UHF connector. I'm guessing its roughly 5 feet long and then I added a 10 foot RG8X extension and routed into the cab.

    Mirror looks like this.
    Freightliner M2 Exterior Mirror Assembly With Electric Operation And Heat

    I believe the 6th PIN on this mount goes right to chasis ground, which is providing the grounding to the mount. Non Antenna versions of this Mirror Assembly only contain 5 pins. As for Common Mode Current, I'm guessing you are correct, however, it does not appear to be enough to alter my transmitted audio characteristics as my signal report indicated good audio. Although it could certainly still be affecting the SWR and your comments regarding bonding make sense, however, I'm not quite sure how I'm going to do this in a tasteful proffesional looking way.

    Thanks for your recommendations!
     
  6. Ridgeline

    Ridgeline Road Train Member

    18,797
    87,194
    Dec 18, 2011
    Michigan
    0
    Pull coax to the mount, one peice.

    The ground you mentioned is an electrical ground for the mirror motors and the heater element if it has one.

    it is not a proper ground.

    you need to properly build an rf ground through bonding and/or use of braided straps.

    is this your truck or a company truck?
     
  7. pixman

    pixman Bobtail Member

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    Oct 22, 2022
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    I am having the same exact issue with mine. Have you found a solution?

    I tried the grounding straps and the SWR has gotten worse .
     
  8. Night Stalker10

    Night Stalker10 Road Train Member

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    Jun 26, 2017
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    Are the mirror mounts pop metal or something similar? The picture makes them look like they are plastic. Unfortunately the manufacturers don't care or don't understand the principal of how a mobile CB antenna works. These trucks might be a good candidate for the dipole antenna using two 4 foot firestick antennas. A dipole antenna won't be the best performance wise, but it would be better than nothing.
     
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