Tuning a Francis antenna?

Discussion in 'CB Radio Forum' started by bucksnort, Dec 5, 2014.

  1. bucksnort

    bucksnort Light Load Member

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    I have a Volvo 670 running a mirror mount on the right side mirror bracket. Bought a Francis 4.5' antenna and getting a 2.5 SWR. Changed out the Francis with a Firestick and getting 1.5. SWR is pretty much even between 1 and 40 on both antennas. Do I need to cut down the Francis? Everything is grounded well.
     
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  3. generallee

    generallee Medium Load Member

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    So much for pretuned huh
     
  4. bucksnort

    bucksnort Light Load Member

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    That's why I picked it up. I understand cutting if one band is higher than the other or putting a spring in the mix to lengthen, but this talk about them being pre tuned is BS
     
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  5. mike5511

    mike5511 Road Train Member

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    They usually work without tuning, but you can cut some off and fine tune them a little better, or add a spring. With a 2.5:1, I'd say you have a RF ground problem. If the Firestick has a sure enough 1.5:1 SWR, then it is a no brainer on which one to use. It might be you got a bad antenna in the Francis as well.........
     
  6. Neverready

    Neverready Medium Load Member

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    if your swr is higher on 40 than 1 general rule is to trim a bit. If your getting 1.5 across 40 channels with the firestik I would go with the firestik.
     
  7. bucksnort

    bucksnort Light Load Member

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    Francis has continuity from base to the tip wire. There's good ground all the way to the cab frame
     
  8. handlebar

    handlebar Heavy Load Member

    Your SWR is fine. Keep in mind that a dipole antenna that's resonant will have an impedance of about 70 ohms. A quarter wave ground plane style over an excellent counterpoise (in this case, your truck cab and stuff bonded to it, as long as you're mounted immediately above or off the edge of the roof) has an impedance of around 32 ohms.

    Divide either one to get a positive number, i.e., 70/50 or 50/32 (because SWRs are *always* expressed as a positive ratio), and you'll find just about 1.5:1.

    Rather than get too hung up on the actual numerical value of your SWR, simply tune for the minimum reflected voltage (cuz voltage is what a typical SWR bridge shows), read the value, and note it somewhere in your vehicle maintenance log or a piece of tape on the back panel of the radio itself.

    That way, if the SWR changes suddenly or radically, you'll know "sumpin's up", to use the technical jargon. :)

    For years, amateur and HF commercial land mobile stations measured the RF current in a transmission line, and made sure that the "final input power" was within prescribed limits. Hence all the loading and tank circuit adjustments on boat anchor gear, or even early-ish tubed CB gear. The CB spec used to be 5 watts input to the final stage (usually a vacuum tube) as measured by a voltmeter on the plate supply and the amount of current being drawn. Ohm's Law would allow one to compute the final *input* power at the final tube

    These days (geez, I *am* old!) nearly everything is solid state, and it's easy/cheap to get modest but serviceable wattmeters to be close enough to spec, and even the FCC changed their spec to output power. Since they no longer require us commercial radio license holders to be responsible for power and purity of most transmitters (they've shifted the burden to the licensee or user), the law sorta encourages experimentation, which I applaud. My applause diminishes somewhat every time someone who uses their CB for work brings me a radio and it's missing coil slugs, modulation circuit limiters, proper coil shapes, and even TVI traps that have been tuned to allow maximum signal to escape the radio, regardless of the likelihood that that extra energy strangled out of the poor wee beastie is in an entirely different frequency range than what's on the radio's channel dial.

    So having a 1:1, or 1.1:1 SWR, often means the coax or some other element in the output stage or antenna system in introducing some "reactive" losses, allowing the system to seem better than, well, what one should be able to calculate as "perfect SWR".

    Sorry; I digress -- I'm old. But my point is to do what you can to your antenna system to enhance its counterpoise, and then meter the SWR with appropriate tweakage to the antenna's radiating length to try to get the whole shebang (more technical talk there) into resonance at the frequency(ies) needed.

    But I digress -- I'm old.

    73
     
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  9. TazAZ

    TazAZ Bobtail Member

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    I didn’t know that you can trim a Francis antenna
     
  10. Xcis

    Xcis Medium Load Member

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    .
    .Agreed, absolutely go with the Firestick antenna.
    .
    .When the Firestick is 1.5 and the Francis is 2.5, it is clearly an antenna tuning problem not an antenna ground plane problem. If both a antennas were at 3.0 or higher that would indicate an antenna ground plane problem.
    .Francis antennas are broad band not pre-tuned. Common misconception. Yes you can tune a Francis.
     
  11. Meteorgray

    Meteorgray Heavy Load Member

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    Pre-tuned antennas sounds like an advertising trick. How would the manufacturer know what kind of counterpoise any particular antenna will be fit on.

    They must be justifying such a term by claiming their antennas are broad-banded enough to make them "pre-tuned" for various counterpoises compared to others. This thread seems to find the fault line of that gimmick.
     
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