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Modeling coil-loaded dipoles in CocoaNEC 2.0

Discussion in 'Antennas, Feedlines, Towers & Rotors' started by KH6AN, Dec 13, 2018.

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  1. KH6AN

    KH6AN Ham Member QRZ Page

    Aloha All,

    I am a new-minted General licensee (WH6FZA), living in a condo on Maui and hoping to operate QRP HF in the near future. Due to space constraints and restrictions, I need a compact antenna. Due to budgetary constraints, I need an inexpensive antenna. A friend suggested a Buddipole, but I'm considering the homebrew precursor instead. I'd like to model the antenna to get a ballpark idea of what to expect in terms of impedance and radiation, but I'm a Mac user running CocoaNEC 2.0 and can't figure out what figures to enter where in order to model the loading coils.

    I am relatively new to all things HF (was a Technician for years, and not much of one at that), so I welcome any help in reducing my ignorance.

    Mahalo,
     
  2. N0TZU

    N0TZU Platinum Subscriber Platinum Subscriber QRZ Page

    Congratulations on your General class license!

    I hope you can get CocoaNEC to work for you. I briefly tried to use it a few years ago and ran some test cases against EZNEC (Windows). Unfortunately it had some erroneous results. That plus being very bare bones caused me to give up on it, and just resign myself to doing antenna modeling in Windows under a virtual machine on the Mac.
     
  3. W1VT

    W1VT Ham Member QRZ Page

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  4. KH6AN

    KH6AN Ham Member QRZ Page

    Mahalo,

    Can you advise me on what figures to enter where? On the Loading tab in the Element Inspector, I can choose from:
    • None
    • Series RLC (with potential entries for R, L, and C and load locations of Center segment, All segments, or a range of segments)
    • Parallel RLC (with the same entries as Series RLC)
    • Impedance (with potential entries for Real, Imaginary, and the same load locations as above)
    • Conductivity (entries for Conductivity and the same load locations
    • Other (NEC-2 Hollerith Card Image)
    Again, I'm grateful for any help.
     
  5. KC8VWM

    KC8VWM Ham Member QRZ Page

    Remember these coil calculators are not an exact science. They are only intended to get you inside the right ballpark of things.

    You would typically construct the coil according to the calculations but in most cases need to take actual measurements to see where they resonate on the desired band.

    I typically recommend first time builders copy an existing design, rather than calculating their own newly constructed coil design.

    The reason for this is because this will help to provide you with a hands on approach to gaining a better understanding of the coil construction principles and theory of operation associated with their design.
     
    Last edited: Dec 13, 2018
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  6. W1VT

    W1VT Ham Member QRZ Page

    Series RLC would make a lot of sense for a loading coil, while Parallel RLC would make a lot of sense for a trap. But, I've never used that program or a modern Mac. The refurbished i7-2600 Quad core I bought earlier this year to replace my nine year old PC only set me back $245. It already had a serial port, so I only needed a high quality sound card.
     
    Last edited: Dec 13, 2018
  7. KH6AN

    KH6AN Ham Member QRZ Page

    I'm copying the existing design...just thought modeling it would provide useful information.

    Mahalo,
     
  8. KC8VWM

    KC8VWM Ham Member QRZ Page

    This is sort of a reverse engineering project.

    So what kind of existing electrical measurement data and physical size parameters from this existing coil and antenna element length information do you already have to feed the calculator?
     
    Last edited: Dec 13, 2018
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  9. KH6AN

    KH6AN Ham Member QRZ Page

    The directions indicate the wire type (#20 Radio Shack speaker wire; I had to make a substitution), form (1/2" ID CPVC), number of turns, and total length.
     
  10. K7TRF

    K7TRF Premium Subscriber QRZ Page

    This is what I'd use for modeling loading coils.

    R is ideally determined from coil Q as it's the resistance at operating frequency you're interested in. Some calculators like the Hamwaves coil calculator give decent estimates for this
    L is the desired coil inductance
    C should be determined by coil self resonance, either from measurement or based on estimates from something like the Hamwaves calculator or simply set to zero for a decent first order estimate of loading coil performance (assuming the coil is operated at a frequency well below its Self Resonant Frequency)

    Note that the Hamwaves coil calculator assumes air coils not something wrapped on a dielectric like PVC pipe but the air core models will still get you close though both Q and SRF will vary once you wind the coil around a dielectric.

    https://hamwaves.com/inductance/en/index.html#input
     
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