RF Capacitors- using standard plumbing parts
15mm, 22mm and 28mm copper pipe, in any combination make useful concentric high voltage capacitors - especially in loop antennas, where they can be integrated into the loop element itself. The dielectric can either be air (using a few thin spacers to keep the tubes truly concentric), or one of a variety of readily available plastic tubes (a useful way of increasing the capacitance per metre). Alternatively, strimmer cord, or such-like, can be spirally wound onto the smaller of the tubes, to act as a centralizer (especially useful for 22/28mm pipe combinations).
Insulating tubing between conductors not only allows more capacitance per unit length, but is also useful for preventing corona type breakdown where there are high RF voltages present. If the actual length of dielectric tube within the conductors can be varied, it can also be used to provide a degree of tuning. All potential dielectrics should be tested for loss in a microwave oven (+ cup of water).
In the UK, copper tubes used in the plumbing industry are sized by quoting by their outside diameter.
Wall thickness will be between 0.5 and 1mm.
Capacitance values for air spaced tubes:
15 within 22mm = 210pF/metre
22 within 28mm = 365pF/metre
15 within 28mm = 114pF/metre
These values do not reflect the effect of any inherent series inductance.
Dielectrics other than air:
a) 20mm flexible plastic conduit:
Useful for spacing 15 and 22mm conductors (Check internal diameter - some makes are too small to admit a 15mm tube) Capacitance is approximately 344pF/metre.
b) 20mm solid plastic conduit:
Useful for spacing 15 and 22mm conductors (but note, not a tight fit to the 15mm pipe). Capacitance for 'heavy duty' grade conduit is approximately 412pF/metre
c) 20mm semi-flexible high pressure water pipe:
Useful for spacing 15 and 22mm conductors, this type is usually a tight fit for both inner and outer conductor. Capacitance is approximately 431pF/metre.
d) 20mm solid high pressure water pipe:
Useful for spacing 15 and 22mm conductors - is also usually a tight fit for both inner and outer conductors. Capacitance is typically 531pF/metre
Site under construction.....