Also posted to FB group Building Transistor Radios
Watson’s 9 picofarad homemade “gimmick” capacitor
This homemade capacitor can be used for circuits were the capacitance is not critical, such as a wireless microphone for the FM broadcast band. The wire can be obtained from the cable used to connect telephone outlets. Solid (not stranded) plastic insulated hookup wire can also be used.
I used about 300 mm or 1 foot of 24 AWG or 0.5mm solid (single conductor, not stranded) plastic insulated wire.
I cut it into 6 pieces, each 50 mm or 2 inches long. I stripped the insulation 9 mm or 3/8 inch off one end of each of them.
I took 3 of the pieces and twisted the bare wires together and soldered them. I did the same for the other 3 pieces.
I took one wire from each 3 wire piece and twisted them together tightly using my long nose pliers. I made sure the insulation was not damaged. I did the same for each of the remaining wires. I then had 3 twisted pairs, and two bare leads to connect to.
I checked to make sure there were no wires touching on the insulated ends. Then I twisted the 3 twisted pairs into a tight bundle. I measured the capacitance with an LC meter and I got 9 picofarads.
If the capacitance needs to be reduced I can use only 4 wires or 2 twisted pairs. Or I can make them shorter to begin with, or cut them shorter afterwards. I can also untwist the wires to adjust the capacitance and then twist them back together.
As I said, the pF value is not stable because it depends on how much twists, and how much give there is in the plastic after you build it. If you put it in parallel with a variable cap, then you can compensate for changes over time.
