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Whip it good

Discussion in 'CB Antennas' started by 3puttbob, Oct 3, 2012.

  1. 3puttbob

    3puttbob Member

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    The 102" whip is an icon in the CB world. I used one 20 years ago with great results (and a few broken fluorescent tubes at the gas station). Anyway, have it back on my Prerunner now but, am experiencing something. Had to add a 10" extension and a 4" spring to bring the match (tested on 2 swr meters) down to 1.2:1 evenly on all 40. I'm attached to the left front bed rail and am somewhat directional of course. But, it seems I'm not getting return Q's from anybody. Occaisionally I'll get someone call back from a few miles away but it's rare.
    Anyone have a theory? I've checked all coax continuity and grounding.


     

  2. W5LZ

    W5LZ Crotchety Old Bastard

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    ... try removing that extension and spring. Or maybe leave the spring if you really need it mechanically.
    - 'Doc
     
  3. 3puttbob

    3puttbob Member

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    Hi Doc
    Did that. Tried to avoid extra hardware. When I did the match climbed to 2.5:1 on the low channels. Don't see that condition too often. Usually too long.
     
  4. psycho

    psycho Running a special on our rooms!

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    I thought the Toyota Tacomas have fiberglass beds.
     
  5. gamegetter

    gamegetter Well-Known Member

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    Sounds like a ground plane or bonding issue.
    As Psycho said if the bed is fiberglass then you have a very much reduced ground plane. If it's metal then bonding may be in order.

    Have you tried running ground strap to frame? Not the ideal solution buy may be a work around.
     
    #5 gamegetter, Oct 3, 2012
    Last edited: Oct 3, 2012
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  6. Wire Weasel

    Wire Weasel Senior Moment

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    Ditto. Likely a poor Ground Plane connection issue. Run a wide braided ground strap from the mount down to the frame. Be sure to clean all connections down to bare metal and periodically check and clean the connections over time.

    On 102's in general if you do the math you find that the Center Frequency for that length comes to 27.525 or thereabouts "Channel 52". 108" would be a better resonant frequency length for straight CB. MFJ and Hustler I think make 108" whips.

    You can try the grounding strap and still have the higher SWR on the CB freqs in which case you will have to extend the whip to achieve more desirable resonance. I have the same issue on my Chevy 1500 and have to use a 6" extender for best resonance and SWR.

    Good luck
     
  7. W5LZ

    W5LZ Crotchety Old Bastard

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    And if you'll keep doing that math you'll find that about 103 - 104 inches works out to 27.185 or channel 19. All of which means that it would be resonant there, NOT that you would see a 1:1 SWR. SWR has nothing to do with resonance, only impedance matching. A resonant 1/4 wave antenna is very, very seldom ever 50 ohms.
    - 'Doc
     
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  8. gamegetter

    gamegetter Well-Known Member

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    And notice I said "may be" a work around because ground loops can also be created in this manner.

    Anything below 1.6 or 1.5 should be fine, your swr does not have to be flat...

    Are you using 4 watts dead key only and no modulation or power mike to test swr?

    on edit in fact anything below 2:1 is ok too.
     
  9. 3puttbob

    3puttbob Member

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    Agreed about the match. If I had a 1.8, evenly I wouldn't worry about it. The bed is all steel but has a bed liner (always wondered about that interfering with the ground plane). I have a swing kit in it but, have it adjusted to a 2 or 3 watt dead key and there's no modulation when I check. The mount however is thin aluminum. I've heard that steel is better so I may change that. I will be adding a ground strap underneath from bed to frame. Even thought of adding a couple copper wires along the top of the bed rail and connecting to the mount. I checked a dead key output around the truck and ranged from 2 to 4.5 because it is a bit directional.
     

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  10. W5LZ

    W5LZ Crotchety Old Bastard

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    As long as the mount is connected to the metal of the bed, the bed-liner will make no difference at all. As long as the bed is connected to the frame, it should work just fine. What that mount is made of, aluminum or steel also doesn't matter if it will mechanically support the antenna. What would make some difference, no idea how much, would be to flip that mount around so that the antenna isn't -in- the bed of the truck but above it.
    -'Doc
     
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  11. ghutch

    ghutch Active Member

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    Bonding straps has really helped my mobile station! the K0BG.com website is great place to start it is a mobile installation site.
    i put my antenna on the cab...the bed did not work for me?? Good Luck
     
  12. gamegetter

    gamegetter Well-Known Member

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    a couple if's here:

    if that mount is directly to bed but bed is not bonded to the frame, such as mine which has rubber bushings,,,then I personally would begin by bonding bed to frame. I would do all 4 corners then test. I think that would give better capacitance to ground instead of the single ground strap from mount to bed...you can decide at that point if you need more...

    take note of doc's post your antenna is resonant at 103-104, i would use the whip and either the ball or the spring to achieve the resonance at the cb band you want, but not both.

    I have run a 102 with just the ball at about that location and it worked out ok with about 1.8 to one swr as best I can remember that was a long time ago. But your situation may be different for some reason...alot of trial and error as everyone is aware.
     
  13. W5LZ

    W5LZ Crotchety Old Bastard

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    And while you're at K0BG's site take a look at the way he explains impedance matching and SWR. It's not the best idea in the world to do one at the expense of the other (talking about resonance and SWR), and you certainly don't have to!
    - 'Doc
     
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  14. gamegetter

    gamegetter Well-Known Member

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    agree with doc, i think resonance is now way off of the cb band as a result of trying to improve match...when capacitance to ground was likely the issue. or we hope any way!:eek:

    good day and good luck
     
  15. The DB

    The DB Sr. Member

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    Being close to the cab, the resonant length of the antenna will be a little longer than the listed amount. That being said the amount that you added seems a bit much. I put forth the possibility that you are going past the antennas resonant point to the next 50 ohm impedance point. A quarter wave whip is not at the 50 ohm impedance point where your SWR would be 1:1, it has a lower impedance and should be closer to 1.5:1.

    If your impedance is below 2:1 even if it is that long it will work and work fairly decently. However, I would consider seeing if someone you know near you has an antenna analyzer and tune the antenna to resonance. The resonant point will work even better. RF bonding will make it work even better if you haven't done it yet.


    The DB
     

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