Figure 3-1.—Two-wire transmission line.
wires, and the dielectric (air or insulating medium)
between the wires. The following paragraphs will
be useful to you as you study distributed constants
on a transmission line.
Inductance of a Transmission Line
When current flows through a wire, magnetic lines
of force are set up around the wire. As the current
increases and decreases in amplitude, the field around
the wire expands and collapses accordingly. The
energy produced by the magnetic lines of force
collapsing back into the wire tends to keep the current
flowing in the same direction. This represents a certain
amount of inductance, which is expressed in
microhenrys per unit length. Figure 3-2 illustrates
the inductance and magnetic fields of a transmission
line.
Capacitance of a Transmission Line
Capacitance also exists between the transmission
line wires, as illustrated in figure 3-3. 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 3-2.—Distributed inductance.
Figure 3-3.—Distributed capacitance.
Resistance of a Transmission Line
The transmission line shown in figure 3-4 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 3-4.—Distributed resistance.
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 3-5 shows this
leakage path as resistors in parallel connected between
the two lines. This property is called CONDUC-
TANCE (G) and is the opposite of resistance.
Conductance in transmission lines is expressed as the
reciprocal of resistance and is usually given in
micromhos per unit length.
Figure 3-5.—Leakage in a transmission line.
3-2
