Base loaded multiband vertical - how?

Discussion in 'Antennas, Feedlines, Towers & Rotors' started by 9A3AKF, Feb 21, 2019.

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  1. 9A3AKF

    9A3AKF Ham Member QRZ Page

    Hi folks,

    finally, I got a 3d printer and a whole new world opened up in DIY projects and ham radio.
    I intend to build a buddistick clone. I already ordered a 9.5ft telescopic whip and want to build a loading coil which has several separate windings for multiple bands. I want to use it on my 6m fiberglass pole, maybe 3ft high. Or when portable, just clamping it to a fence or table. Speed in setup is the essence!

    For several days now I am struggling to find a reliable written guide on how to calculate the windings in regard of the whip length to make it somewhat resonant.

    Can someone point me to useful literature or share his personal experience in building such antennas?
    Honestly, paying 130 bucks for a buddistick is too much :confused:. Also I kinda enjoy building that stuff more than actually using it :rolleyes:

    I got the inspiration from this: https://www.thingiverse.com/thing:3197539

    Thanks in advance for any help :)

    73!
    Aleks
     
  2. G6YYN

    G6YYN Ham Member QRZ Page

    The way I did it was just to get a coil former and wind it with as many turns as I could with flying leads at both ends.
    Once the coil was assembled below the whip I adjusted the coil taps with an antenna analyser for the bands that the coil covered.

    Ken
     
  3. 9A3AKF

    9A3AKF Ham Member QRZ Page

    Hmm clever approach Ken!
    So when there are not enough turns, the resonant frequency is gonna be higher I assume, right?
     
  4. N7EKU

    N7EKU Ham Member QRZ Page

    KE0NSK and 9A3AKF like this.
  5. K7TRF

    K7TRF Premium Subscriber QRZ Page

    I usually model antennas like what you're proposing to estimate necessary loading inductance. But you can also use the calculator on this website remembering that you want a resonant 1/4 wave vertical which is just half of the loaded dipoles described here: http://www.66pacific.com/calculators/coil-shortened-dipole-antenna-calculator.aspx

    I haven't personally used this one, but here's another version of the loaded vertical (or dipole) calculator: https://m0ukd.com/calculators/loaded-quarter-wave-antenna-inductance-calculator/

    In terms of figuring out how to build an inductor that meets the needs determined above, I find this air coil calculator to be very handy: http://hamwaves.com/inductance/en/index.html#input

    Lot's of ways to wind inductors, but this method is very good and not hard: http://www.ad5x.com/images/Articles/CoilRevB.pdf You don't necessarily need to use the plumbing hardware to build it just like Phil describes but the grommet strip method for winding air coils works really nicely. I've used variations on this theme to build several bug catcher coils as well as trap coils and other air inductors.
     
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  6. N8CMQ

    N8CMQ XML Subscriber QRZ Page

    May I suggest 4NEC2?

    I used to do the same, build and experiment, however, to understand why what I built worked,or in
    most cases, didn't work, I needed to learn theory, that is where the Antenna Handbook comes in handy.
    There are free online copies I use as well as my paper version.

    Now, because of time and the desire not to spend money without knowing if the project will work, I
    desire to simulate antennas, which is why I recommend 4NEC2. If you Google 4NEC2, you will find
    the free download area for it.

    It will seem daunting at first to use the program, but with a little practice, and good luck, you will see
    if the antenna you want to build is worth it. And the program will help with coil or capacitor values, as
    well as other related bits, like conductor size and such.

    Good luck with your project, and I hope it works!
     
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  7. 9A3AKF

    9A3AKF Ham Member QRZ Page

    Wow thanks a lot to all of you :) A lot of useful information, I am gonna soak that in today. Thanks again!

    All the best,

    73
    Aleks :)
     
    N7EKU likes this.
  8. AA5MT

    AA5MT Ham Member QRZ Page

  9. KC8VWM

    KC8VWM Moderator Volunteer Moderator QRZ Page

    Loading coil designs are based on the Wheeler formula.

    L (uH) = r^2 * n^2 / (9 * r + 10 * l)

    Where:

    r = coil radius in inches
    l = coil length in inches
    n = number of turns

    It's also necessary to determine the amount of inductance ( L or uH) you will need for a desired antenna radiator length used for a particular operating frequency before you can perform the Wheeler calculation. To simplify matters, you can find the correct inductance value (uH) you will need to perform the Wheeler formula calculation using this online calculator.

    https://m0ukd.com/calculators/loaded-quarter-wave-antenna-inductance-calculator/

    You can also double check your coil winding dimension math using this online calculator too: http://www.66pacific.com/calculators/coil-inductance-calculator.aspx

    As a sidenote, nothing is super critical when constructing the antenna loading coil at HF frequencies. The calculations are simply intended to get you in right ballpark of things with room for error, so relax and feel free to slightly deviate from the calculated dimensions if necessary. Getting close is good enough. :)

    Once the coil has been constructed and it's attached to the antenna radiator, you find the precise tap location experimentally by "raking" the coil along it's entire length with the feedline connection wire at various spots along the coil to find the tapping point needed for the operating frequency you will be using. When the antenna is connected to your radio receiver, listen for an increase in signal noise levels while you are raking along the coil. This will get you very close to the coil tapping point you are looking for. An antenna analyzer may be useful to check things like SWR at the tap locations you suspect are close and then minor tuning adjustments can be made as necessary.

    When you find the sweet spot which exhibits the best readings on the antenna analyzer or SWR meter etc, mark it with a paint marker for future reference, or simply solder the feed point wire permanently to this tap point on the loading coil.
     
    Last edited: Feb 23, 2019
    AG5DB, 9A3AKF, N8CMQ and 1 other person like this.
  10. 9A3AKF

    9A3AKF Ham Member QRZ Page

    I want to share the progress of building the loading coil. Again, thanks to all of you for pointing me into the right direction. The worst problem was adapting the 3/8 inch thread on the whip to a metric one. I cut the thread on the whip to 8mm and used a corresponding nut that I welded on a big screw that is screwed in the coil for stability. I bought the wire for winding but in the end I went short for about 1m... I should have calculated the wire length before buying it I guess :mad:

    The coil fits perfectly on my 4th element of my fiberglass pole, in about 2m height above ground. I intend to use it on the fiberglass pole with one resonant radial. Also I made those cutouts in the coil for tapping it on resonant positions for each band and copy those buddistick taps for fast band hopping. Still a lot to do...


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