If a section of transmission line is less than 1/4 wavelength long, and is left open atthe far end (rather than shorted out), it will act as a capacitor. The capacitance will in-crease with length.The most common transmission-line capacitor uses two telescoping sections oftubing. This is called a coaxial capacitor and works because there is a certain effectivesurface area between the inner and the outer tubing sections. A sleeve of plastic di-electric is placed between the sections of tubing, as shown in Fig. 11-10. This allows thecapacitance to be adjusted by sliding the inner section in or out of the outer section.Tolerance20911-9A trimmer capacitor.Coaxial capacitors are used in radio-frequency applications, particularly in antennasystems. Their values are generally from a few picofarads up to about 100 pF.ToleranceCapacitors are rated according to how nearly their values can be expected to match therated capacitance. The most common tolerance is 10 percent; some capacitors are ratedat 5 percent or even at 1 percent. In all cases, the tolerance ratings are plus-or-minus.11-10A coaxial variable capacitor.