Trying to copy a parabolic curve from a PDF

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Trying to copy a parabolic curve from a PDF

Postby seans_potato_biz » Wed Dec 05, 2018 11:53 pm

I want to use Inkscape to design parts of an antenna. The parts will be cut from acrylic on a laser cutter and use to hold together some sheet metal and wire which will form a reflector and the antenna itself. The problem I have is creating the parabola from the PDF supplied here: ... pqYmM/view

I create a bezier curve which I've been adjusting to try to match the inside curve but I'm wondering whether someone knows a better way than manually tweaking it like this? Maybe importing the PDF in a different way gets some more useful lines?

The antenna is shown and described in this YouTube video: ... CcQakg3Y3I
Win 7/10, Inkscape 0.92.2

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Re: Trying to copy a parabolic curve from a PDF

Postby Lazur » Thu Dec 06, 2018 7:03 am


To plot a parabola I'd rather suggest using the function plotter extension.
The function is y=x*x, should be quite simple setting the rest.

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Re: Trying to copy a parabolic curve from a PDF

Postby ragstian » Thu Dec 06, 2018 7:44 am


As Lazur said, use Extensions -Render - Function plotter.
Make the whole of the canvas visible by pressing the number five key on the main keyboard (not the num 5 on the numpad)
Draw a rectangle filling most of the canvas. Let the rectangle stay selected.
( If it's not selected Press F1 and then left click on the rectangle border)

Use these values for an initial setup:


This will render the function as this.


It's now ready for further editing in Inkscape.

To plot the focus point you have to work out the equation: The result is: X=0.0, Y=0.25, in other words in the middle of the square.

Please observe that the intersection of the three straight lines in your figure is not the focal point of the parabola.

To plot the same figure in python use this example from the matplotlib examples:

Good Luck!
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