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Thread: Question: Ground Plane for Mobile VHF Antenna

  1. #1

    Default Question: Ground Plane for Mobile VHF Antenna

    I was wondering if I mounted my NMO mount VHF mobile antenna to my spare tire mounts plastic 3rd brake light (Jeep Wrangler JK Unlimited), and used a jumper wire between the base of the mounting bracket, and the exposed metal of the tailgate would that provide a sufficient ground plane?



    I realize that the plastic I have chosen isn't the strongest option, but without a tire carrier it's my only decent option I can think of. I don't want to use the tail light mount on the corner because, well, it's on the corner and really low. At least here, it will be up higher and clear of the tailgate.

    Thanks in advance to anyone who answers.

    Gerard (a.k.a. G!)
    KE0MBG
    Arvada, CO

  2. #2

    Join Date
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    Remember that 19 inches is 1/4 wavelength on 2m. That piece of aluminum could act as the lower half of a 2m vertical dipole. Your jumper wire will become part of the antenna. If it is 19" long it won't have much effect except to distort the radiation pattern. Ground plane configurations that work at HF may not work at VHF. I would try it first without the jumper wire.

  3. #3

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    Ground planes that produce omnidirectional decent radiation patterns have the antenna in the centre of a large conductive plane - a damp bit of string can have a good VSWR on a meter, but is a rubbish antenna. Your radio will probably be happy with your system, but how well it works will be down to luck!

  4. #4
    Super Moderator 5B4AJB's Avatar
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    Can't you mount the bracket straight onto the tailgate or the side of the box?

    If your antenna hits a branch it'll snap that plastic thingumabob.

    Off road antannas need spring mounts...

  5. #5

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    Quote Originally Posted by W5DXP View Post
    Remember that 19 inches is 1/4 wavelength on 2m. That piece of aluminum could act as the lower half of a 2m vertical dipole. Your jumper wire will become part of the antenna. If it is 19" long it won't have much effect except to distort the radiation pattern. Ground plane configurations that work at HF may not work at VHF. I would try it first without the jumper wire.
    I have so much to learn still....thank you.

    Quote Originally Posted by paulears View Post
    Ground planes that produce omnidirectional decent radiation patterns have the antenna in the centre of a large conductive plane - a damp bit of string can have a good VSWR on a meter, but is a rubbish antenna. Your radio will probably be happy with your system, but how well it works will be down to luck!
    I had considered trying to figure out how to attach a pizza pan to the inside of my fiberglass roof and use my mag mount but I quickly realized that was not ideal because it would keep getting knocked over.

    Quote Originally Posted by 5B4AJB View Post
    Can't you mount the bracket straight onto the tailgate or the side of the box?

    If your antenna hits a branch it'll snap that plastic thingumabob.

    Off road antannas need spring mounts...
    Yeah, that was a big consideration of mine, and then I thought about the ground plane issue also.


    Thanks for the answers everyone. I guess I am gonna just buy the bracket to mount it on either the drivers or passenger fender. Lord only knows why I went with the NMO mount instead of a UHF tho.

  6. #6

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    OK here is some more brainstorming to go along with my original ground plane question. Let's assume I am not going to use a ground lead to any chassis ground as originally stated.

    Let's also assume that I will isolate the antenna from the bracket in the above image so it is not a functional part of the antenna but only the structure it is attached to.

    I am devising a spring mount for my ham antenna and need some guidance. Why is it so hard to find spring mounts for ham, or am I over complicating this too much?

    On the left in the attached image is a design that would use the existing antennas PL-259 mount, and the right side with the black spring is if I were able to omit the bottom portion of the antenna.

    Does that bottom portion with the PL-259 mount on my antenna (Tri Band, 2m/1.25m/70cm) have any function, and can it be deleted in my design?
    Antenna Mount.png

  7. #7

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    It could be a loading coil I know my MFJ antenna is similar and has a load coil...

  8. #8

    Join Date
    Nov 2017
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    With my wife
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    Here, try this site. Best if you set aside some time to check it out, it's quite extensive.
    https://www.k0bg.com/

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