Inside a vacuum capacitor, two metal (copper usually) electrodes are separated by a vacuum. The outer part of the capacitor is made from either glass or ceramic. They usually have a low capacitance in the range of 10 to 1000 pF and high voltages, typically 5000 volts and higher. Vacuum capacitors find wide application in industrial applications of radio frequency energy. They are used in equipment such as high powered broadcast transmitters, Amateur radio RF Amplifiers and large antenna tuners. Vacuum capacitors can also be used as the impedance variation part in automatic matching. Variations of vacuum capacitors include fixed and variable. The main difference between fixed and variable is that variable vacuum capacitors are able to be adjusted.
Vacuum variable capacitors need to be able to handle high voltages. A typical high power ham radio or AM broadcast transmitter will have a dc potential of 1500 to 7500 V on the RF amplifier anode, dependent on the kind of tube. In the case of AM, this potential can double. Certain defects within the system may also increase RF voltages, so a variable capacitor used in the final amplifier anode circuit must be capable of withstanding these potentials.
Vacuum is the most perfect of dielectrics with a zero loss tangent. What this means is that very high powers can be transferred without significant loss and heating; the units are self-healing after moderate overloads. Compared to other variable capacitors, vacuum variables, because of the vacuum itself, are more precise and stable. The sealed chamber means the dielectric constant remains the same over a wider range of operating conditions. Through the use of vacuum variable capacitors, designers have been able to produce smaller transmitters that offer better, more reliable performance, and are easier to tune and keep in tune.
For high power transmitters and other high voltage capacitor applications, vacuum capacitors are used for power amplifier tank circuits, output of pi networks, neutralizing circuits, grid and plate blocking circuits, antenna coupling and “rejecter” tap circuits, pulse shaping in the output circuit of magnetrons, feed-through capacitors for harmonic attenuation, dielectric heating equipment tank circuits, low-inductance, high-current bypass applications, and non-magnetic capacitors for resonance imaging. Vacuum capacitors are employed in broadcast transmitters for long wave, medium wave (MW or AM), short wave, and VHF (FM and TV). They also serve in broadcast antenna systems, particularly in AM directional antenna phasing systems, diplexers, triplexers and line tuning units.
Source by Patrick Stox