Here is my Mostly Printed Butterfly Capacitor, ready for use in a magnetic loop antenna.
As discussed previously, I rebuilt the prototype with greater spacing between the aluminum plates. Spacing and attachment is provided by nickel plated nuts on 10-32 brass rods. The thickness of the nuts is 1/8”, so that is the spacing between the plates.
My test equipment says that the device provides a range of about 10-60 pF. This is still a bit of a guess, as I know I have a lot of stray capacitance in the test leads. The formulas show that in theory, I should be getting up to 68 pF based on the spacing and the area of the rotor plates. (The area is 3.8 square inches per side.)
I am trying to build a small magnetic loop to cover 10-15 MHz. Calculations show that this range of capacitor should be in the ballpark. If not, I can always make some more plates to extend the range. I purposely cut the rods to provide extra length if needed.
Mostly Printed Butterfly Capacitor Parts
The sides of the capacitor are 3D printed with ABS, using 25% infill. This should be both light and strong enough. The threaded rods are held in place using plastic nuts and washers custom made for the 10-32 brass threaded rod using a configurable model called Nut Job on Thingiverse. Nut Job is great for making plastic bolts, threaded rods, nuts and washers of various sizes.
The rod holding the rotor plates rotates inside some small plastic bushings which fit into center holes in the sides of the capacitor.
Eventually, this capacitor will be mounted on a piece of one inch PVC pipe, which will provide the central support for the magnetic loop antenna. You will notice that there are little 1” tabs at the bottom of the side sections. I will use cable ties to hold the capacitor in place on the PVC pipe.
Once you have learned to use CAD, it is relatively easy to design parts like this for the exact size you want for your capacitor. The cost of this Mostly Printed Butterfly Capacitor is around $30 for materials, as follows.
- Aluminum Sheet $12 – 8 by 24 inches (Home Depot)
- Brass Threaded Rod $12 for three feet (Home Depot)
- Nickel Plated Nuts $6 for 100 pieces (Mode Electronics 54-556-100)
The plastic for 3D printing adds another $2. Other than the 3D printer and CNC, the only special tools needed were some files to smooth the milled plates.