# High-Power RF Capacitors

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Welcome to the fascinating world of «exotic» RF components! «Exotic» means «difficult to locate», because a typical electronics shop will usually not carry these high-voltage items. Below lists of suppliers are non-exhaustive, but will hopefully help you find what you need.

# Ceramic disc capacitors

ceramic disc capacitor
Some ceramic disc capacitor are rated up to the kilovolt range. However, RF current handling is limited due to the narrow lead contacts and the skin effect. Some suppliers:

# Air capacitors

split-stator variable air capacitor
Conventionally-constructed RF air capacitors are not immediately my favourite type of capacitor. Air capacitors collect dust with their open construction. More importantly, air capacitors have a low breakdown voltage. However, one variant of air variable capacitors is unique because of its high current rating. The design of variable split-stator —or «butterfly»— capacitors is such that no RF conduction currents flow through the bearings of the rotor. In low to moderate power applications, air fixed and variable split-stator capacitors can be a viable and budget-friendly alternative to vacuum capacitors. Some suppliers:

# Door-knob capacitors

door-knob capacitor
Door-knob capacitors are a cheap, robust, reliable and easily mountable alternative to expensive fixed-value vacuum capacitors. Suppliers:

# Vacuum capacitors

variable vacuum capacitor
Manufacturers:

Suppliers:

# Vacuum capacitors can die!

Beware of glass vacuum capacitors that have been badly handled or that have been poorly protected during transit. The glass envelope may have cracked, destroying the vacuum. In capacitors with clear glass envelopes and copper electrodes, air ingress is indicated by slow discolouration of the copper. The voltage rating of an air-filled capacitor at atmospheric pressure (760torr or 1.01bar) is about thirteen times lower than that of an otherwise identical vacuum capacitor (at 10 torr).1

# Home-brew RF capacitors

Over the years, many amateur radio magazines have been publishing articles about building transmitting magnetic loop antennas. These antennas appeal by their very compact size, sharp directivity nulls and high signal-to-noise ratio on reception.

The great flaw of these antennas is their very limited radiation impedance. Consequently, the radiating efficiency of these magnetic dipoles is extremely low. —That is, if you are not resorting to super-conducting materials!— Due to the resulting high Q-factor of these antennas, voltages and currents are extremely high. This lead to a number of very interesting ideas about home-brewing high-power RF capacitors being published in articles about magnetic loop antennas.

One idea I successfully employed on VHF, consisted out of a copper fitting, lined with a 1.5mm-thick PTFE (Teflon) sheet, and inserted by a snugly fit brass bolt.

# References

1. Knight D. Vacuum capacitors. In: From Transmitter to Antenna. Available at: http://www.g3ynh.info/comps/vac_caps.html.

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