Advice on CB antenna for Pickup

Discussion in 'CB Radio Forum' started by American-Trucker, Mar 20, 2010.

  1. volvo244t

    volvo244t Road Train Member

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    Jun 24, 2010
    Bettendorf, IA
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    Go to Pilot, get a Terminator II and toss it on a tri-mag mount on the roof at center of available mass, i.e. in the middle of the roof, not the rear. I have a 102 on the roof of my station wagon this way, SWR is 1:1. The T-II will not have quite as good of SWR as that, but it will be close, you could expect 1:1.2.

    [​IMG]

    Not quite center of available mass in my case, I was doing as good as I could with available coax length. Made it slightly directional toward the front, but we are talking very slight.
     
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  3. james28138

    james28138 Bobtail Member

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    May 30, 2014
    Yadkinville, NC
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    View attachment 66678 This is my setup. 96 inch Francis fiberglass whip mounted to tailgate side of tool box. Works really well for me. I also run 102 inch stainless whip sometimes but seem to get more noise and swr is higher when i run it. Swr with the francis is 1.2:1. But I amt thinking about moving antenna to middle of top of tool box so it will have a complete ground plane. Can I get some thoughts on this idea.
     

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  4. mike5511

    mike5511 Road Train Member

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    May 15, 2011
    NW Arkansas
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    My thoughts are you are lucky it works that well on a tool box. They usually don't.
     
  5. james28138

    james28138 Bobtail Member

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    May 30, 2014
    Yadkinville, NC
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    mike5511, Dont seem to have any problems. I have the antenna grounded to the frame of the truck. Radio and amp is grounded to battery. I do have some noise when the truck is running and dont k now what to do about that but it is not too bad. Also have a ground wire from amp to radio.
     
  6. handlebar

    handlebar Heavy Load Member

    James,
    It's possible that your Fibreglas® antenna seems to hold a better SWR (negligible as the difference is) than the steel because it's more likely to remain more nearly straight with wind drag when you're moving. As the steel bends in the wind, the changes between its distance from the whip to the surrounding metal (or, indeed, its loss of effective counterpoise perpendicular to its intended axis (straight up) is being lost.

    Also, unless you've got either a 4-6" extension at its base, or a pot-bellied spring there, then the steel whip is more nearly tuned in the 10 meter band than the middle of the CB range.

    At my shop, my backyard is liberally festooned with huge wire array antennas, but my sole CB antenna is a 102" stainless Rat Shack whip mounted to the aluminum fascia running about halfway between the two ends of the fairly flat roof over that part of the house. It's resonant at about 28.110, which is good for me for CW work. At 29.6, it approaches 2.0:1. Down in the CB spectrum, it's around 1.9:1 at Ch 40, 2.01 at Ch 20, and back up to around 2.2:1 at Ch 1. Since I never run around 90 watts on 10 meters and 4 watts on CB (I'm located up along a long, flattish ridge), the aluminum fascia running around the three sides of the room provides an adequate counterpoise.

    The one time I did try a 6" extension as an experiment, my SWR plummeted in the CB range down to about 1.1:1 at Ch 20, but even at the bottom end of 10 meters I was already above around 2.5:1. And since 10 meters is nearly 2 MHz wide, and I spend more time there, I pulled the extension out. The 2.0:1 bandwidth for the silly thing is acceptable from the bottom of CB to the top of 10 meters, and with a T-style tuner I've been able to get acceptable DX out of 15 and 20 meters as well, including Qatar and U.A.E.

    But it sounds like you're doing well. Assuming your toolbox is bonded electrically to the bed, you're apparently providing sufficient counterpoise for a good match. Mind you, a good SWR has nothing to do with radiation efficiency (you could as easily have a dummy load on the end of your coax) but if you're getting out OK, I'd go looking for your noise sources with the amp *out* of the line, fix them, and *then* reconnect the kicker -- if you really decide you need it.

    HTH,
    73
     
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  7. james28138

    james28138 Bobtail Member

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    May 30, 2014
    Yadkinville, NC
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    Thanks handlebar for your reply. Although you seem to know what your talking about, way beyond what I know. I do have a "pot belly" on the antenna. I was considering putting another one on. Several years ago I ran a 102 stainless with two springs and it dropped swr to almost nothing.I have a bedliner in the truck so the back side of antenna have no ground plane so to speak, so I made two radials for coat hangers, lol and mounted them to the bottom of the antenna mount. Not sure if this was a good idea or not. Dont know if it helps. But at least signal has something to bounce off of. I considered mounting it to the top of box to give it a better ground plane. Whats your thoughts on this? The tool box is screwed to the bed but I do also have the antenna ground to the frame of the truck.
     
  8. handlebar

    handlebar Heavy Load Member

    James,
    I believe the "radials" you made from the base are actually just a way of bonding the antenna mount to the pickup box, which is a lot more than many ops do. For them to be a "ground plane" by themselves, they'd each have to be about 9 feet long. But, as they say around here, "Ya dun good."

    Cheaper than grabbing another $15-20 spring, just try taping a foot or so of bared Romex house wiring (or even wire coat hanger) pretty tightly to the top inch or two of your whip. Get in the cab, close the door, and read the SWR, which should be pretty high. Get out, snip off an inch of the taped-on extension, back inside & close the door, and check SWR; should have dropped. Keep doing it an inch (or a half-inch) at a time, until it starts to creep up again. (It may not change much as you approach the existing whip length, since the SWR is already about perfect.) If you've still got 2 inches of extension left at the SWR's lowest point, and since your "unencumbered" SWR is so good already, I'd just pull off the extension, chalk the time up to a learning experience, and then operate to your heart's content with whichever antenna best serves your sense of aesthetics and how enjoyable it is to let your steel whip swing in the direction of other cars stopped at a traffic light ;) Remember, before every SWR check, get inside and close the door(s). At frequencies as low as 27 MHz, they help define the RF counterpoise provided by the vehicle.

    I hit stuff all the time, and most of the time I've got base loaded commercial VHF & UHF whips littering the roof of my van. The only time I put on the 102 steel and stainless potbelly spring are when I'm making a road trip out of town and want to try to hear every little bit of signal (I use the same whip for 10 meters CW, so lots of signals are pretty weak there).

    I'm used to the odd looks. I just look like an older, fatter version of the undercover peace officer I was for years in The Great Northwest, but now it's increasingly unlikely cuz, well, what narc drives around in a car with 6 antennas, anyway? :)

    73
     
    Last edited: Jun 6, 2014
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