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Coaxial cable capacitors are easily made from RG-8 or RG-213 which have approximately 30pF capacitance per foot. Divide the capacitance needed by 30 to get the number of feet required. Allow extra length for tuning and connections.
The coax is gradually shortened until the desired capacitance is obtained. Leave extra center insulation extending beyond the braid at both ends. Trim back a little of the outer jacket at the high voltage (center conductor) end and then cut off the braid flush with the jacket. Each inch removed equals 2.5pF.
Direct connections using the coax center conductor and braid can be made, but soldering smaller diameter wire leads to them makes things a little easier. The center conductor at the braid end is hot and must be clear of other objects.
KR1ST has designed a unique small magnetic loop using coax cable capacitors for 17 and 20 meters.
Coax length calculator
Precision capacitors from about 1 pF upwards may be made from short lengths of coaxial cable, with the inner conductor being one capacitor plate and the outer shield being the other.
Using this capacitor calculator by Mike Yancy, the coax length can be calculated given the desired capacitance in picoFarads (pF).
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