Homebrew Butterfly Capacitor

The butterfly capacitor is the best option for the magnetic loop construction.

Butterfly capacitors are hard to find, expensive but they can be home made.

I start this project with some hand made drawings that was photographed and transferred to the PAINT BRUSH.

The PAINT BRUSH drawing can be seen on the left top of the above figure, this technique gives me the freedom to test designs of the fixed and the moving plates and select the appropriated hardware.

There are two basic parameters to design a variable air capacitor:
1) Plate’s area
2) Distance between plates.

High voltage capacitors needs more spaced plates to avoid arching, at QRP levels the plates can be closer from each other resulting on more compact designs.

There is a very usefull program written by KI6GD called Capacitor Calculator. It works very well to me in designing all kind of variable capacitors.

KI6GD Gleen Sperry

A4 page label sheet printed ready to be applied to the aluminum foil

The final drawing was a 4.5 centimeters by side square copied 12 times filling
3 colons and 4 rows.
This figure was printed on a full page size label to be applied directly over the
aluminum 0, 5 centimeters foil.

Cutting with a metal scissor

Puncturing the points to be drilled

Aluminum foil with drilled holes

The above photos shows the 12 squares drilled

The next step is carefully cut the crossed lines with a DREMEL machine

Pell off the guide drawings on the printed label sheet over the pieces and start to work on the plates to make them polished and as plain as possible.

Polishing the butterfly capacitor plates

Screw mounted capacitor plates

Front and back acrylic parts

On my model I have used some thin washers between the inner face of acrylic front and back mounting pieces and the extreme nuts on the screw used to avoid mechanical instability.

This procedure is needed to an appropriate free aisle movement keeping it firmly like the professional units.

Finished butterfly capacitor

Minimum capacitance

Maximum capacitance

PY1 AHD ALEX (Alexandre Grimberg).
e-mail: py1ahd@ig.com.br