Hooray! My first butterfly capacitor is assembled.

It’s just a prototype. But, it’s made and it works. The aluminum capacitor plates were milled on my Mostly Printed CNC and the rough edges have been largely filed away. The frame structure is 3D printed with ABS filament. The hardware is largely 10-32 brass rod and 10-32 nickel plated nuts. The bushings for rotating the first butterfly capacitor, some of the nuts and a few spacers were also 3D printed.

The idea with a variable capacitor is that the “rotor” plates rotate within the “stator” (or stationary) plates. Maximum capacitance is achieved when the rotor is fully enmeshed between the stators. Minimum capacitance is achieved when the rotor and stators do not overlap. Adding more sections of plates increases total capacitance.

In this prototype, the plates are spaced 1/8 inch apart, and the spacing is set by the nuts between the plates. The alignment of the rotor plates is offset by a 1/16 inch spacer, so they fit between the stationary plates. The closer the spacing the more capacitance you can achieve. But also, the closer the spacing, the less voltage that can be handled without arcing. The spacing in this prototype is too small to be useful in a magnetic loop antenna, but okay for a test.

Brass and nickel plated hardware was chosen because these are relatively good conductors, as is aluminum. Stainless steel is a poor conductor, so was not used.

Testing My First Butterfly Capacitor

To the left of the capacitor in the picture is my RigExpert Antenna Analyzer. This handy device is very useful for testing capacitance and inductance, as well as antennas and transmission lines. After subtracting the stray capacitance, my first butterfly capacitor was measured at around 10 to 60 Pico Farads (pF). This is excellent and close enough to what the formulas predicted.

The capacitor rotated smoothly. The threaded rod for the rotor plates turns inside a small bushing connected to each of the plastic structures holding it all together.

My next step is to take the prototype apart and rebuild it with 1/4” spacing between the plates. This will reduce the capacitance but increase the breakdown voltage to the level needed for a magnetic loop. I will also begin to design some gears and couplings for a stepper motor to turn the device remotely.