Split-stator capacitors
The split-stator capacitor is one in which two variable capacitors are mounted on the same shaft. The split-stator capacitor normally uses a pair of identical capacitors, each the same value, turned by the same shaft. The rotor is common to both capacitors. Thus, the capacitor will tune either two tuned circuits at the same time or both halves of a balanced-tuned circuit (i.e., one in which the inductor is centertapped and grounded).
Differential capacitors
Although some differential capacitors are often mistaken for split-stator capacitors, they are actually quite different. The split-stator capacitor is tuned in tandem, i.e., both capacitor sections have the same value at any given shaft setting. The differential capacitor, on the other hand, is arranged so that one capacitor section increases in capacitance and the other section decreases in exactly the same proportion. Differential capacitors are used in impedance bridges, RF resistance bridges, and other such instruments. If you buy or build a high-quality RF impedance bridge for antenna measurements, for example, it is likely that it will have a differential capacitor
as the main adjustment control. The two capacitors are used in two arms of a Wheatstone bridge circuit. Be careful of planning to build such a bridge, however. I recently bought the differential capacitor for such an instrument, and it cost nearly $50!
“Transmitting” variable capacitors
The one requirement of transmitting variable capacitors (and certain antenna tuning capacitors) is the ability to withstand high voltages. The high-power ham radio or AM broadcast transmitter will have a dc potential of 1500 to 7500 V on the RF amplifier anode, depending on the type of tube used. If amplitude-modulated,the potential can double. Also, if certain antenna defects arise, then the RF voltages in the circuit can rise quite high. As a result, the variable capacitor used in the final amplifier anode circuit must be able to withstand these potentials.
Two forms of transmitting variables are typically used in RF power amplifiers and antenna tuners. Figure 7 shows a transmitting air variable capacitor. The shaft of this particular capacitor is nylon, so it can be mounted either with the frame grounded or with the frame floating at high voltage. The other form of transmitting variable is the vacuum variable. This type of capacitor is a variation of the piston capacitor, but it has a vacuum dielectric (K factor = 1.0000). The model shown in Fig. 8 is a 18- to 1000-pF model that is driven from a 12-Vdc electric motor. Other vacuum variables are manually driven.
Figure 7 a transmitting air variable capacitor
Figure 8 Vacuum variable capacitor
Solid-state capacitors
One of the problems with variable capacitors is that they are large, bulky things (look at all the photos) that must be mechanically operated. Modern electronic circuits, including most radios today, are electrically tuned using a varicap diode for the (Ct) of a PN junction diode is a function of the reverse bias voltage applied across the diode. The varicap (a.k.a. “varactor”) is therefore a variable capacitor in which the capacitor is a function of an applied voltage. Maximum capacitances run from 15 to 500 pF, depending on the type. capacitor function. These “capacitors” operate because the junction capacitance
Varactors come in several different standard diode packages, including the twoterminal “similar to 182” package shown in Fig. 9. Some variants bevel the edge of the package to denote which is the cathode. In other cases, the package style will be like other forms of diode. Varactors are used in almost every form of diode package, up to and including the package used for 50- to 100-A stud-mounted rectifier diodes.
Fig. 9 Typical varactor cases.
