FreeCAD basics for RF geometry

Geometry creation in FreeCAD

For geometry manipulations CENOS Radio Frequency uses a very strong geometry editor – FreeCAD. This editor is used in the Geometry Editor workflow, so it is important for us to understand how to work with FreeCAD.

Watch a short video on how to use FreeCAD for CENOS Radio Frequency app:

Geometry import

Sometimes when building an RF simulation, you will already have pre-made geometry, which you can import into FreeCAD for additional geometry manipulations.

STEP/IGES

You can easily import your CAD file in IGES or STEP file formats.

To import such files, open Geometry Editor and simply drag & drop your geometry into FreeCAD!

KiCAD PCB files

To import KiCAD PCB files directly into CENOS : RF, select the Geometry Editor approach and there you will need to click on the folder icon to load your geometry.

Once you have selected your file you will return to the main CENOS interface where you will now click on the play icon. Then the software will ask you to save the case and, after this, FreeCAD will automatically open with your geometry!

You have the option to use your model completely in 3D or flatten the thin conductive parts, such as tracks, to have them in 2D. We recommend selecting the Faces mode, as the results will not be significantly affected but the mesh will be easier to create and the calculation time will be reduced!

Note: We recommend that for the substrate you use a geometric figure without curvatures since this would cause troubles reading the geometry and it will not be imported correctly into FreecCAD. If the curvatures in the substrate are important in your model, you can create it within FreeCAD once the model has already been imported.

Unit scaling

CAD files which are imported in FreeCAD must be in the correct units – milimeters. If your geometry is scaled incorrectly, you need to rescale it, otherwise meshing problems will occur.

To scale your geometry:

• Switch to Draft workbench.

• In Tree View select your geometry and click Clone (this function will create a scalable copy of your geometry).

• Once the clone object is created, scale your geometry in Properties. Make sure that the scale factor is the same in every direction to avoid distorting your geometry!

When you close the sketch, you will notice that the sketch will be on the surface you selected!

3D volumes

For a lot of times it will be necessary to create 3D geometries, not only 2D sketches. Fortunately 3D modeling in FreeCAD is very straightforward!

Turning a Sketch into a 3D Shape: Pad Tool

Once your sketch is ready, you can transform it into a solid using the Pad tool. You can choose how far it extends and in which direction.

You need to first create a sketch you need.

Then switch to Part Design workbench, select the sketch desired and click Pad a selected sketch.

In the Pad properties window define the pad length. Once done, click OK.

Pocket tool

The Pocket tool is used to remove material from an existing solid body by cutting it along a 2D sketch. This operation is perfect for creating holes, grooves, slots, and other cutouts in your design.

Start with a solid body. You need an existing 3D solid to use the Pocket tool. Typically, this is created using the Pad tool.

Then create a sketch on a face:

• Select a flat face of your solid.

• Click “Create Sketch” and draw the 2D shape you want to remove (e.g., a circle for a hole, a rectangle for a slot).

Apply the Pocket:

• With the sketch selected, click the Pocket icon in the Part Design toolbar.

This opens the Pocket parameters dialog where you can choose from several extrusion modes and set the depth.

Click OK to finalize the cut.

Basic Shapes

For the simplest volume objects like cubes, spheres or cylinders, FreeCAD offers an easy way of creating such geometries. As you open FreeCAD, you will be automatically put in Part workbench. There you can simply select any geometry you want and build it.

As you select an object, it will be built with a pre-defined size. To change the size of your geometry, select your object and change its properties.

Component library

You can use CENOS Component Library templates to build or add antenna or SMA connector geometries.

Antenna models

Select the model you want to build and click Create.

You can change the template parameters by clicking on the spreadsheet titled with the name of the model you selected from the component library.

Coaxial edge connector​

Select Coaxial edge connector and click Create.

The connector will be automatically placed in your workbench, so you can position it by right-clicking on it in the tree view and selecting Transform.

To change the size of the connector, select it in the tree view and edit the Property tab to suit your application.

Wire thickness

To create a wire antenna from scratch, you need to create a sketch of your antenna, and then define the thickness of your wire:

1. CREATE A SKETCH
   
   1. For 2D sketches you can set sketch position in its properties (In sketch properties Base → Placement → Position)
      
   2. To build 3D sketches, use Draft Workbench → Drafting → Line. There you will need to build lines one by one and adjust the Start and End points to match your design.
   3. For helixes or spirals use Create Primitives function in Part workbench.
      

    

2. Create a wire with volume from the edges, select your shape, click Wire thickness tool in Part Workbench and define the wire thickness.

 

Note: Sometimes when building wires, the visualization will appear like this:

This is a visualization problem, which does not affect the geometry itself. You can continue and use this geometry in your setup!

Useful Tips, Tricks and Shortcuts for working with FreeCAD

Common tasks

Useful shortcuts​

Function	Shortcut
Toggle object visibility	Space
Set camera rotation center	Middle-click
Pan camera	Left-click
Rectangle select objects	Shift+B
Rectangle select elements	Shift+E
Import File	Drag&Drop