14-05-2012, 03:59 PM
TRANSMISSION LINES AND WAVEGUIDES
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TRANSMISSION LINE THEORY
The electrical characteristics of a two-wire
transmission line depend primarily on the construction
of the line. The two-wire line acts like a long
capacitor. The change of its capacitive reactance is
noticeable as the frequency applied to it is changed.
Since the long conductors have a magnetic field about
them when electrical energy is being passed through
them, they also exhibit the properties of inductance.
The values of inductance and capacitance presented
depend on the various physical factors that we
discussed earlier.
LUMPED CONSTANTS
A transmission line has the properties of inductance,
capacitance, and resistance just as the more
conventional circuits have. Usually, however, the
constants in conventional circuits are lumped into a
single device or component. For example, a coil of
wire has the property of inductance. When a certain
amount of inductance is needed in a circuit, a coil of
the proper dimensions is inserted. The inductance
of the circuit is lumped into the one component. Two
metal plates separated by a small space, can be used
to supply the required capacitance for a circuit. In
such a case, most of the capacitance of the circuit is
lumped into this one component.
DISTRIBUTED CONSTANTS
Transmission line constants, called distributed
constants, are spread along the entire length of the
transmission line and cannot be distinguished separately.
The amount of inductance, capacitance, and
resistance depends on the length of the line, the size
of the conducting wires, the spacing between the
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.