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Mobile High swr !!!. Need a 27mhz ground independent antenna

Discussion in 'CB Antennas' started by johnxb351, Nov 24, 2020.

  1. Crawdad

    Crawdad Well-Known Member

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    And it DOES NOT require the 6 inch long heavy duty spring to get the whip to the "right" length. It will tune just fine as is unless you are going way below 27MHz. Chances are you will not have to tune it at all to reach a 1.5:1 or lower VSWR if installed properly and mount is well grounded.



    Again, as Slowmover posted, K0BG's website will help solve most of your issues.

    73
     
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  2. Slowmover

    Slowmover Elmer

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    C07BADCA-A91A-43C8-87B8-49A6CAB8319D.jpeg
     
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  3. Crawdad

    Crawdad Well-Known Member

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    What was it that Mr. Barnum said ?
     
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  4. Slowmover

    Slowmover Elmer

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    We covered it in Whack-pactive thread.
     
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  5. johnxb351

    johnxb351 Member

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    I read some where make a coil out of 22 gauge wire wraped 21 time around 1/4 '' pvc pipe and put it in serious at feed point and other end to a ground to help tune the antenna. but this does not mean much to me!!!. can some one put up a diagram in very simple terms as am new to all this .
     
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  6. johnxb351

    johnxb351 Member

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    IS that a mobile 27mhz ground plan. who makes it and how big is it. does it work
     
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  7. Slowmover

    Slowmover Elmer

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    No. A gimmick. Comic relief.
     
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  8. sp5it

    sp5it Master of puppets

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    Useless for CB antenna. Work nice for 2m :)
     
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  9. M0GVZ

    M0GVZ Sr. Member

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    Solution is actually very simple and you already have it. Use the coax connected to the antenna as a RF ground.

    When an installation has a piss poor RF ground the antenna system is trying to use the coax as one to try to compensate - it's the whole reason the old wives tale of "you have to cut your coax to 9,18,27 or whatever feet to get a low SWR" came into being. Your coax forming part of the RF ground is generally a VERY BAD THING. You get common mode RFI, lots of noise and interference.

    HOWEVER if we know that the coax is being used as a RF ground we can make it work to our advantage and we can use a solution to deal with the downsides.

    So how to achieve this. A fooking good RF choke. So what you want to do is to measure back from the antenna mount an electrical quarter wave length of coax which we're going to deliberately allow to be used as a RF ground. So it'll be 234/freq (in MHz) x velocity factor of the coax so 234/27.5 x 0.66 (RG58) so 5.6ft. That's the bit we're going to deliberately use as a RF ground.

    We now need to put in a RF choke at that point so we're not getting common mode RFI and all the problems that come with it on the rest of the coax. We want to make a temporary one at first as we may need to do a bit of tweaking with the length of the coax to get the SWR lowest. So we'll do it by making a coil of 5 turns of the coax at that point with a 4.25" diameter centre. If the SWR is still high once you've tried tuning the antenna you can unwind the coil and start winding it a few inches nearer or further away from the antenna, re-adjust and retest and move again if necessary. You may find you need to lengthen that bit of coax by a couple of feet before winding the choke if your coax has a velocity factor of 0.85 which is the other velocity factor sometimes found in RG58. So don't get disheartened if you lengthen the bit from the mount to where you start winding the choke a few times and it's not working well - its most likely the VF of the coax is 0.85, not the most common 0.66.

    Now once you've found the best length to have between the RF choke and the antenna mount you can either make a better one by wrapping 8 turns of the coax around a FT240-61 ferrite at that point or cut the coax there, fit a PL259 on the end and use a high quality commercially made RF choke. Or if that coil you made doesn't bother you just tape it up somwhere.
     
    #24 M0GVZ, Dec 8, 2020
    Last edited: Dec 8, 2020
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  10. johnxb351

    johnxb351 Member

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    MOGVZ Thank you for you time and help with that info. I will try that on my next day off
     
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  11. Road Squawker

    Road Squawker Sr. Member

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    [​IMG]

    here ya go
     
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  12. johnxb351

    johnxb351 Member

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    OK thanks for the photo. Will that work on 27mhz
     
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  13. Marconi

    Marconi Usually if I can hear em' I can talk to em'.

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    I was looking for this image I posted in the PDF file below, thanks for the link RS. Here is the text that went with this image. I don't see any frequency noted, but this site is a Ham operator. http://www.k0bg.com/match.html

    upload_2020-12-8_14-20-0.png
     
    #28 Marconi, Dec 8, 2020
    Last edited: Dec 9, 2020
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  14. Marconi

    Marconi Usually if I can hear em' I can talk to em'.

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    351, do you know how tall the coil you described above is? If it says it is for 11 meters, it sure looks different from what RS posted. I assumed it might be 2" inches tall and it does not work with a modest simulated mobile GP.

    Generally, my software does not have enough segments to model a coil antenna and a full mobile body with both requiting lots of wires and over 500 segments, so I tried a simulated mobile as a ground plane, but the coil dimensions don't seem to work. So, I stopped until I get some better dimensions and for the coil and for sure a good frequency tor this idea.
     
    #29 Marconi, Dec 8, 2020
    Last edited: Dec 9, 2020
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  15. johnxb351

    johnxb351 Member

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    HI MARCONI all i can tell you it was for 102 inch antenna on 11 meter band. I will try mogvz surgestion 0f using the coax it self as a rf choke 5.6 ft from antenna and 5 turns around 4.25 inch pvc pipe. this will have to wait till after my work . so all surgestions are wecome in the mean time. thanks. john
     
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