Parametric OpenSCAD model for making coil forms for coax-traps for wire antennas. The official home for these files is https://github.com/SmittyHalibut/coax-trap-form (Sorry Thingiverse, you're just a point-in-time snapshot when I think to make it.) Look to github for the newest versions of all this.
Electrical design from the 2015 ARRL Antenna Handbook page 10-15, section 10.2.2, "Five Band W3DZZ Trap Antenna" The idea is a trap resonant at 7.2MHz, which natively builds a 40/80m antenna. But the series capacitance in the trap allows the total antenna length to resonate at 20, 15, and 10m as well, at higher harmonics. The idea being a 5 band antenna with only a single pair of traps.
The book shows two designs, depending on which diagram you look at:
- Traps: 60pF, 8.2uH, 22' outer wire length
- Traps: 100pF, 4.9uH, 21' outer wire length
Both have 32' inner length, and are fed with 75ohm.
My goal is to make these traps using the coax trap design discussed later in the chapter.
Using: https://www.qsl.net/ve6yp/coaxtrap.zip coax trap calculator, and C/ft numbers for coax I had on hand (RG8/X and RG58), I wasn't able to make a coax trap with the exact values above; all common coax has too much C/ft to get down to 100pF. The best I could do was with my RG8/X, down to 137pF and 3.575uH, 6.25 turn on a 3.1in form.
The included STL is that form, with holes in the right places for coax and attachments. Printed vertically like a tower, THIS WILL NOT BE STRUCTURAL! My intention is to feed the antenna wire through a nylon rope so the rope is tensioned, but the wire is not. The trap structure will just be loosely hanging from the wire ends that stick out from the rope an inch or two.
I also made a 20m trap at 14.15MHz, and included the STL for it. The frequencies are indicated in the file names. Just in case I can't get the above 40m trap antenna design to tune up on higher bands, I want an option for at least getting on 20m during field day. :-)
But the OpenSCAD file is entirely parametric.
Install OpenSCAD, open the .scad file, and adjust the parameters as listed below to make your own form. Then Render the form (will take several minutes), and save your STL. Then you can slice it as normal.
You can adjust the variables:
// Calculator: https://www.qsl.net/ve6yp/coaxtrap.zip
form_diameter_in = 3.1;
num_turns = 6.25;
// RG8/X, LMR240 = 0.242
coax_diameter_in = 0.242;
// No 10-24
bolt_diameter_in = 0.200;
form_thickness_in = .200;
If you build something with this model, I'd appreciate it if you sent me a picture and/or description of what you built. I'm @SmittyHalibut on Twitter.
73 de KR6ZY, @SmittyHalibut -Mark