Since I am planning to resume this blog, I thought I’d share a fairly recent project. For a magnetic loop antenna build, I needed a variable capacitor that can handle relatively high voltages. These can be somewhat hard to come across today, and you have to work with what you can find on eBay voltage and capacitance wise. That’s why I came up with a variable capacitor that can be mostly 3D-printed, with the addition of some hardware store parts. The best thing about it is that the design is free for everyone to use on OnShape and it can be easily customized by adjusting the respective parameters.

The capacitor plates are cut from 0.5 mm aluminium sheet, which should be readily available at most hardware stores. They are stacked on top of each other with washers to set the distance. I had some problems with the plates bending and shorting out, so I covered them in some tape. M4 threaded rods form the fixed pins as well as the rotating shaft in the middle. A 16:1 transmission on the top allows for very fine adjustments, which is essential for achieving a good SWR on shortwave. The transmission belts consist of 1.75 mm TPU filament that is glued into rings with superglue. Getting the length correct is really important. Cutting them too long means that the belts will slip, cutting them too short will cause them to break prematurely. All rotating shafts are supported on both ends by small ball bearings to ensure smooth operation. The plastic parts are printed in blue PLA. With the capacitor shown here I can operate between 40 m and 20 m, although it is a bit lossy and gets warm when I push 100 W of RF power through it. Otherwise I think this is quite neat as it allows you to completely customize the capacitor.

I would love to see your results if you try to replicate this. Also, I am planning to start posting again about my projects and I have some microwave PCBs on order, so stay tuned for that!