When soldering the segments of a collinear antenna, care must be taken not to shorten a segment's inner conductor to its braided screen at segment joints. This spacer keeps the inner connector isolated from the braided screen at joints. Further, it gives the joint some rigidity avoiding fatigue.

You may have to customize the dimensions of your coax cable into your specific .stl file

• by means of the Customizer (link above, below picture) or
• by editing the dimensional parameters at the top of the OpenSCAD source file (see below).

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As always I include the OpenSCAD source file for you to adapt and/or improve:

• Download and install OpenSCAD from https://www.openscad.org
• Launch OpenSCAD and open the enclosed OpenSCAD source file (/File/Open).
• Amend the parameters at the top of the file or even amend the code.
• Save your changes (/File/Save).
• Render the design (/Design/Render).
• Export the rendered design e.g. as a .stl file (/File/Export/Export as STL).
• Slice and print.

See my other designs at
https://www.thingiverse.com/thinger13/designs