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Drafting a spiral?

Discussion in 'Electronic Design' started by Charles Lind, Feb 21, 2009.

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  1. Charles Lind

    Charles Lind Guest

    I need to draw a 32 turn spiral pattern, for a "pancake" coil antenna,
    that will be suitable for etching PCB's. IOW clean edges and uniform
    widths for the tracks and spacing. Diameter is approx 150mm. Can
    anyone please suggest how to do this accurately, preferably by

    Charles Lind
  2. Tim Williams

    Tim Williams Guest

    Parametrically, you want r(theta) = pitch * theta / 2pi, with theta =
    (0, N * 2pi), N turns, for theta in radians. The centerline of the
    trace follows this path, so you need to offset it (either side,
    perpendicular to the path) by half the width to get the actual
    profile. Then add caps on the endpoints to make a contiguous

    If all you have are circles and splines, you might get away with
    increasing radius semicircles joined end-to-end. It will look wrong,
    but the overall effect will still be right.

  3. Joerg

    Joerg Guest

    If this is for hobby or non-profit use: Cadsoft Eagle has a free version
    that allows small circuit board sizes (but I guess you could zoom that
    up with some other SW for the antenna if needed). Install that. Then
    download "" from here:

    I have not used this particular download file myself but it should
    unpack a *.ulp file (User Language Program) which creates spiral PCB
    inductors. This ULP file needs to be copied into the same directory
    where all the ULPs are that come standard with the software. File should
    be safe and tested quite well, Mr.Zaffran is one of the engineers who
    works at Cadsoft.

    Try the schematic editor as well, maybe you like it.

  4. You design a form on the computer that you can make in manufacturing.
    The form is a cardboard (paperboard) helix that you print out, and roll
    up onto a tube, so you can paste it onto a cardboard tube. The print job
    prints the line of your antenna onto the paper. The finished tube has
    lines on it now that you use a form for the positioning of the antenna
    elements. Once everything is assembled, you remove the tube from within
    the antenna, and all that is left is the antenna itself.

    So the CAD design is for the printed helix before it gets rolled up onto
    the tube form that is the diameter of your helix.

    Done right, it will have your layout lines in all the right places so
    that the antenna builders can build the antenna. Automated process can
    be derived from similar fixtures and forms methods, but this antenna
    style sounds difficult to automate too much.
  5. Ralph Barone

    Ralph Barone Guest

    old_x = 0
    old_y = 0

    for turns = 1 to 32
    for theta = 0 to 359
    r = (360 * turns + theta) / (150/32)
    new_x = r * cos(theta)
    new_y = r * sin(theta)
    drawline (old_x, old_y, new_x, new_y)
    old_x = new_x
    old_y = new_y
    end for
    end for

    or something like that...
  6. Guest

    It's a cheap but excellent PCB layout program that has spirals and
    polygons built in. Set turns, track width, start radius,distance
  7. MooseFET

    MooseFET Guest

    Others have suggested good ways to do the figuring of the spiral.
    Making it into Gerber plots is another step.

    The GEDA PCB program stores its PCB layout in an easy to understand
    format. You can make your program produce the spiral in a file that
    you can then copy and paste into the ASCII file that is the layout.
    Then you can make Gerber plots. This way, you can do other parts too.
  8. linnix

    linnix Guest

    I think you want:
    (turns + theta/360) * (150/32)

    For gerbers:

    MOVE(0,0); // D02
    for turns = 1 to 32
    for theta = 0 to 359
    r = (turns + theta/360) * (150/32)
    new_x = r * cos(theta)
    new_y = r * sin(theta)
    DRAW(new_x, new_y);
    end for
    end for
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