Calculate number of coil turns for a given inductance?

Discussion in 'Radio Circuits, Repair & Performance' started by KF5YDR, Jul 11, 2018.

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  1. KF5YDR

    KF5YDR Ham Member QRZ Page

    There are a bunch of online calculators that will give you the inductance for a given coil, but I need to design a coil for a given inductance.
    I’m wanting to build an air-core coil antenna for HF reception only, essentially a continuously-loaded electrically short monopole. I’m using length of two feet, diameter of 1 inch, and estimated capacitance of 12 pF to make things easy (I’d have to dig through Evernote to find the site that gives 6pF/ft as a reasonable estimate for reactance of electrically-short monopoles). An online calculator for loaded monopoles gives me an inductance of 424uH for a 2ft monopole at 3.9MHz. I’m not sure if it’s reasonable to assume for the sake of that calculation that a continuously-loaded antenna will behave like an electrically-short monopole of similar height, so I’m not considering the inductance to be a known variable.

    Is there an online calculator for such a thing, or at least a formula? The variables I have are:
    - resonant frequency
    - length of coil
    - diameter of coil
    - diameter of wire
    - spacing/pitch (insulated wire makes this easy :p)

    I don’t think I need super precision—this is going to be a strictly receive-only antenna, and all it has to do is out-perform a 9ft whip!
     
  2. KD8DEY

    KD8DEY Subscriber QRZ Page

    Professor Coyle....(Also available as a downloadable spreadsheet)
    http://crystalradio.net/professorcoyle/
    Ymmv
     
  3. WB2WIK

    WB2WIK Premium Subscriber QRZ Page

    Interesting, but why would a two foot long monopole outperform a nine foot long one?:)

    You can make it perfectly resonant at any frequency you choose, but it will be very narrowband and may be better to tune out most of the reactance of the 9' whip and do the same thing with a broader curve and better performance.
     
  4. G3YRO

    G3YRO Ham Member QRZ Page

    What exactly are you trying to achieve here?

    If it's a small receiving antenna, then you'd be much better making a small loop. You then have the added advantage that it's balance (so low noise) . . . and directional (so you can null out the noise).

    I recently made one for 160m, and it works fantastically well . . . a square loop on a 6ft cross, with 4 turns on the tuned part, resonated using varicap diodes (as its up in my loft). You could scale that down, and just make it 3ft across for 80m.

    Roger G3YRO
     
  5. SM0AOM

    SM0AOM Ham Member QRZ Page

    What you are trying to do is calculate a "normal mode helix", which is a 1/4-wave self-resonant
    coil with a physical length of a few percent of the wavelength at the design center frequency.

    This is a notoriously difficult electromagnetic problem, and most "ordinary"
    coil design software provide quite incorrect results.

    However, some quite recent research aimed
    at Tesla coils and high-voltage problem has provided more useful approximations. One such calculator can be found at http://hamwaves.com/antennas/inductance.html, and if reasonable values are "plugged into" the software, it can be calculated that
    a space-wound coil of 25 mm diameter , 700 turns of 1 mm diameter wire and a length of 900 mm becomes self-resonant at 3.79 MHz.

    You should however be advised that such an helix would have an intrinsic Q in the order of 1500, which makes it extremely narrow-band
    and also extremely sensitive to its surroundings and of the dielectric properties of any insulating materials.

    73/
    Karl-Arne
    SM0AOM
     

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