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Cant get a good SWR

Discussion in 'Technical Chat' started by SR-71A, Jul 25, 2018.

  1. Jul 25, 2018 at 5:04 PM
    #1
    SR-71A

    SR-71A [OP] Define "Well-Known Member"

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    So, the short-ish version is I finally got my ARB bullbar mounted up on the front of my truck a few weeks ago. Now I'm getting around to mounting my new Firestick CB antenna up there on one of the dedicated antenna tabs.

    For the past year I've been running a mag mount Cobra (36-ish inch) up on the roof in the center. Wire run down the back, in the rear passenger door, and under the trim to my Cobra 75WXST which lives under the front passenger seat. This setup gave me SWR readings around 1.3, and just about 5W of Tx power according to the meter. Pretty good as far as Im concerned.

    Now, with my new 3 ft Firestick, heavy spring, mounting stud & misc hardware, and new 18' coax all set up on the ARB and I get shit for Tx power and SWR. SWR readings are: about 2.4 on Ch1 and 2.6 on Ch40.

    First thing I tried was making up a dedicated ground wire from the base of the mounting stud to a known good ground near the radiator. I figured the antenna might not be grounded through the thick powder coat. Didn't really help much (brought the SWR readings down a tiny bit to the numbers above)..

    I've tried different combos: adjusting the tip of the Firestick from all the way in to all the way out, spring, no spring, even took the tip off completely to make the antenna as short as possible. Nothing is making a huge difference. SWR readings are still hovering right around the numbers above.

    So, anyone got any pointers - what am I missing here? Anyone else running their CB antenna up front like this?
     
  2. Jul 25, 2018 at 5:07 PM
    #2
    SR-71A

    SR-71A [OP] Define "Well-Known Member"

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    Also, I ohmed out the new coax and it seems good. Near zero through the center conductor and through the shield. No reading between the shield and center pin. So I tend to think my cable is good..
     
  3. Jul 25, 2018 at 5:08 PM
    #3
    Comatose

    Comatose You snuff it, we stuff it.

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    Take pictures of your mounting job. And post them. There's a good chance you are getting reflected power from the mount.
     
  4. Jul 25, 2018 at 5:09 PM
    #4
    EatSleepTacos

    EatSleepTacos MOBTOWN OFFROAD AMBASSADOR

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    Did you grind the PC off on the bottom of the hole?
     
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  5. Jul 25, 2018 at 5:09 PM
    #5
    EatSleepTacos

    EatSleepTacos MOBTOWN OFFROAD AMBASSADOR

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    [​IMG]
     
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  6. Jul 25, 2018 at 5:17 PM
    #6
    kbp810

    kbp810 Well-Known Member

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    Instead of the wire, you might could try a grounding strap (wider the better). Sometimes that can help. Remember that the antenna wants a ground plane, not an electrical ground. Though, not a bad idea to scrape off some powder coat where the strap meets the mount.

    Also, being on the bumper might be a little too low, especially for a 3 foot. A longer antenna, or one that is specifically top loaded, could possibly help as well.
     
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  7. Jul 25, 2018 at 5:17 PM
    #7
    SR-71A

    SR-71A [OP] Define "Well-Known Member"

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    Here ya go. top and bottom. And no @EatSleepTacos living in Maine (aka salt belt) I dont ever want to remove factory coatings if I can help it :boom:

    20180725_200949.jpg

    20180725_201017.jpg
     
  8. Jul 25, 2018 at 5:18 PM
    #8
    EatSleepTacos

    EatSleepTacos MOBTOWN OFFROAD AMBASSADOR

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    Honestly I think what you need to do is remove the PC on the bottom. On my old set up I didn't, and had similar results to you. After I did, my SWR was much better.
     
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  9. Jul 25, 2018 at 5:21 PM
    #9
    SR-71A

    SR-71A [OP] Define "Well-Known Member"

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    All Firesticks are top loaded aren't they? Kinda why I went that route over the roof mount in the 1st place.

    Guess I dont fully understand antenna ground vs ground plane.

    Shouldn't I get the same result tho, running my ground wire? (The smaller one you can see in the 2nd picture)
     
  10. Jul 25, 2018 at 5:21 PM
    #10
    113tac

    113tac Well-Known Member

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    I agree with @EatSleepTacos and @kbp810 try the powder coat and maybe ground strap. My SWR was terrible with a 4ft firestik, after ground strap it was below 2 without tuning if I remember correctly.
     
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  11. Jul 25, 2018 at 5:22 PM
    #11
    EatSleepTacos

    EatSleepTacos MOBTOWN OFFROAD AMBASSADOR

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    Trust me, I don't get it either. Here's what I did on my old set up

    Mounted - SWR was meh
    Ground off bottom PC - SWR was better
    Added 8 gauge grounding cable to frame - SWR was the tits
     
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  12. Jul 25, 2018 at 5:26 PM
    #12
    SR-71A

    SR-71A [OP] Define "Well-Known Member"

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    Right on. Well maybe Ill have to do that and just fluid film that area after the final install. At least it'll be on the underside :anonymous:
     
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  13. Jul 27, 2018 at 6:28 AM
    #13
    SR-71A

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    After doing some research I think I understand the difference between antenna ground plane and an electrical ground. Elect. ground = a good electrical bond between the antenna mount and the frame (low resistance). Ground plane = large flat metal surface to bounce the signal back up the antenna and radiate it out into the atmosphere w/ 360 coverage.

    Which explains why my old mag mount in the center of the roof was so good. Whereas a tiny little tab on the bar = shit ground plane. Also for those who might read this in the future, if the body of your CB is well grounded (like mine) you probably don't need an electrical ground on the antenna. The shield of the coax should tie that all together.

    So, has anyone tried a No ground plane antenna?
     
  14. Jul 28, 2018 at 1:02 PM
    #14
    License2Ill

    License2Ill Shitiful

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    Yeah. I mounted 2m to my ARB seeing the SA guys doing it. And you get it, it's not ideal. But swr doesn't have to be 1.5 to 1.

    I think those guys are running 1.8 or 6 meter.
     
  15. Jul 29, 2018 at 5:31 AM
    #15
    SR-71A

    SR-71A [OP] Define "Well-Known Member"

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    Is that a yeah you run a no gorund plane antenna, or yeah you agree with my rant from above? Haha I was hoping there's someone on here that could confirm this helps or not. Those antenna setups are actually kinda expensive :(
     
  16. Jul 29, 2018 at 5:40 AM
    #16
    JdevTac

    JdevTac Bawnjourno

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    Make sure your ground wire is as short as possible. If that thing is like 4ft long to the frame then it can almost act like it’s own antenna and screw readings up. What you’d ideally want is an actual ground strap, like the woven kind, with a short a run as possible. Ideally less than 2ft.

    In all honesty though I would’ve just kept the roof mount. Roof mount is as perfect as you can get on a Tacoma. An antenna on the arb mainly only gets a ground plane from the hood which is unidirectional in relation to the antenna, where on the roof you get full 360. Now if you just need the CB for trail use then none of that stuff matters.
     
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  17. Jul 29, 2018 at 9:46 AM
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    SR-71A

    SR-71A [OP] Define "Well-Known Member"

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    Yeah, I rarely transmit unless I'm on a trail somewhere. But thats also the main reason I'm getting rid of the roof mount. 1st low tree branch I come across and there goes the antenna, so thats no good
     
  18. Jul 29, 2018 at 6:46 PM
    #18
    License2Ill

    License2Ill Shitiful

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    Your rant. Haha.
     
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  19. Sep 24, 2018 at 6:27 PM
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    SGT. Kelly

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    The no ground plane set up is mostly used for boats, and other non metallic vehicles. I have a Wilson Trucker 2000 antenna on a fire stick hood mount on my 2013 TRD Sport. The antenna base should be grounded to the chassis, and the antenna ground or woven outer conductor grounded to the base. The center conductor should not be grounded to either of those. In other words when I check my antenna with an ohm meter the antenna base should show continuity with the chassis but not the antenna itself.
    If you have a SWR of 4 or higher on your antenna you likely have grounded the antenna to the base or chassis.
    When installed mine I had an SWR of 2.5 and adjusted it sits at o.1 on channel 1 and o.7 on channel 40 and the rest sit somewhere in between. The key to a mount not on the roof is to get the center coil (if your antenna has one) or the majority of the antenna above the roof line. This can be a challenge, and most people would say put a mag antenna on your roof. They come pre-coiled in the base and the magnet causes the entire surrounding surface to become an Omni directional antenna since nothing is blocking it. I got a replacement 10” stud for my antenna at a truck stop and added to the bottom of my antenna which put the center coil on mine above the roof line, and have no directional problems with mine. I can transmit and receive with no issues no matter which way I’m facing. I have a heavy duty base stud and large spring loaded connector for my antenna to sit on so the antenna will not break off at interstate speeds. It flaps like a 3 legged duck on a pond but never hits the cab or blows into surrounding traffic.
    Again make sure the antenna center conductor is not touching or grounded to the base or chassis. You can’t adjust the SWR on a grounded center conductor. It will change, fluctuate, go up, go down, etc. and will always be high.
    Always go somewhere open and free of utilities, poles, houses, and adjust your antenna in the vehicle with doors and windows closed.
     
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