3-12
Capacitance of a Transmission Line
Capacitance also exists between the transmission line wires, as illustrated in figure 3-11. Notice that
the two parallel wires act as plates of a capacitor and that the air between them acts as a dielectric. The
capacitance between the wires is usually expressed in picofarads per unit length. This electric field
between the wires is similar to the field that exists between the two plates of a capacitor.
Figure 3-11.—Distributed capacitance.
Resistance of a Transmission Line
The transmission line shown in figure 3-12 has electrical resistance along its length. This resistance
is usually expressed in ohms per unit length and is shown as existing continuously from one end of the
line to the other.
Figure 3-12.—Distributed resistance.
Q16. What must the physical length of a transmission line be if it will be operated at 15,000,000 Hz?
Use the formula:
Q17. What are two of the three physical factors that determine the values of capacitance and
inductance of a transmission line?
Q18. A transmission line is said to have distributed constants of inductance, capacitance, and
resistance along the line. What units of measurement are used to express these constants?
Leakage Current
Since any dielectric, even air, is not a perfect insulator, a small current known as LEAKAGE
CURRENT flows between the two wires. In effect, the insulator acts as a resistor, permitting current to
pass between the two wires. Figure 3-13 shows this leakage path as resistors in parallel connected
between the two lines. This property is called CONDUCTANCE (G) and is the opposite of resistance.
