This is a protective case for the 40m coil available from Aliexpress, Ebay and others. It also adds a switch to use it on the 20 and 30m bands.

The 40m coil brings one of the 5.6m telescopic antennas into resonance on 40m. The drawback of the coil is that you have to mount and unmount it to switch bands. And, 30m is only usable when shortening the antenna to less than 3 meters which will reduce efficiency.

So I created a protective case with a switch that bypasses the coil. Toggling between 40m and 20m is then done by just flipping the switch. By tapping the coil exactly in the middle (see photos), and using an on-off-on switch, it is also possible to use it for 30m without changing the length of the telescope. Higher bands can be used by switching to 20m and collapsing the telescope antenna partially as usual.

Available versions
You can build either a 40/20m version (without 30m) or one for 40/30/20m. For both versions you need:

• two 10mm ring terminals,
• two M3x8 self-tapping screws
• about 20cm of 1mm² flexible wire

The other parts differ for the two versions:

For a 40/20m version (without 30m):
If you are sure you don't need 30m, you can use the full enclosure version of the case. So you need to print

• coil-case-lower-40m.stl
• coil-case-upper-40m.stl
• Either coil-case-panel-without-labels.stl or coil-case-panel-40m.stl/coil-case-labels-40m.stl for a panel with embedded labels.

In addition, an on-off switch with a 6mm shaft is required to bypass the coil.

For a 40/30m/20m version:
The 30m version has a 90° opening that allows to tap the coil. Tapping is preferably done before the coil is placed in the case! You need to print

• coil-case-lower-30m.stl
• coil-case-upper-30m.stl
• Either coil-case-panel-without-labels.stl or coil-case-panel-30m.stl/coil-case-labels-30m.stl for a panel with embedded labels.

In addition, an on-off-on switch with a 6mm shaft is required. One of the "on" positions will bypass the complete coil, the other will bypass just half of it for 30m resonance. The "off" position in the middle enables the full coil for 40m. Please note that this switch position doesn't have a label because there is no room for it.

Assembly
Solder the 30m tap if you want to build the 30m version. Prepare both ring terminals with about 7cm of wire. Place both caps onto the coil. Mount the switch into the panel, then fasten the panel using the screws. This should keep upper and lower parts together. If you don't trust it, add a drop of superglue. Finally solder the ends of the wires to the switch.

Other configurations
The model comes with the OpenSCAD source that allows to change many parameters to your liking. You can change

• the size of the switch mounting hole,
• the opening angle,
• wall size,
• label text

and many other parameters by just editing the source or using the customizer.

Caveats
I have created my versions on a BambuLab P2S using PLA. You might prefer to use something more sturdy depending on your needs. If you are using higher power with the coil, warming may be a problem since the case prevents free air flow, so you should keep an eye on it. I have used the 30m version with 100W CW without problems, but your mileage may vary.

Final version update 2026-01-02
I have created another version using a much larger on-off-on switch from Aliexpress. While doing so, I made a few minor adjustments to the source as follows:

1. The inner radius seemed to be a bit tight for a new version of the coil, so I increased the default by 0.2mm.
2. The position of the labels do now adjust automatically depending on the switch mounting hole diameter.
3. Fractions can now be used in the customizer.

Using these changes, the version with the larger switch (see photos - it's the yellow one) can be generated by just using the OpenSCAD customizer. I have added a JSON file with the settings but not added STL files, since the larger version doesn't seem to be worth the trouble. The larger switch has slightly better haptics, but it's more difficult to assemble and needs longer wires, so I would advise against using it.

With these changes, the case can be considered final.