Another type of copper loss is due to SKIN
EFFECT. When dc flows through a conductor, the
movement of electrons through the conductor’s cross
section is uniform, The situation is somewhat different
when ac is applied. The expanding and collapsing
fields about each electron encircle other electrons.
This phenomenon, called SELF INDUCTION, retards
the movement of the encircled electrons. The flux
density at the center is so great that electron movement
at this point is reduced. As frequency is increased,
the opposition to the flow of current in the center of
the wire increases. Current in the center of the wire
becomes smaller and most of the electron flow is on
the wire surface. When the frequency applied is 100
megahertz or higher, the electron movement in the
center is so small that the center of the wire could
be removed without any noticeable effect on current.
You should be able to see that the effective cross-
sectional area decreases as the frequency increases.
Since resistance is inversely proportional to the
cross-sectional area, the resistance will increase as the
frequency is increased.
Also, since power loss
increases as resistance increases, power losses increase
with an increase in frequency because of skin effect.
Copper losses can be minimized and conductivity
increased in an rf line by plating the line with silver.
Since silver is a better conductor than copper, most
of the current will flow through the silver layer. The
tubing then serves primarily as a mechanical support.
Dielectric Losses
DIELECTRIC LOSSES result from the heating
effect on the dielectric material between the conductors.
Power from the source is used in heating the dielectric.
The heat produced is dissipated into the surrounding
medium.
When there is no potential difference
between two conductors, the atoms in the dielectric
material between them are normal and the orbits of
the electrons are circular. When there is a potential
difference between two conductors, the orbits of the
electrons change. The excessive negative charge on
one conductor repels electrons on the dielectric toward
the positive conductor and thus distorts the orbits of
the electrons. A change in the path of electrons
requires more energy, introducing a power loss.
The atomic structure of rubber is more difficult
to distort than the structure of some other dielectric
materials. The atoms of materials, such as polyethyl-
ene, distort easily. Therefore, polyethylene is often
used as a dielectric because less power is consumed
when its electron orbits are distorted.
Radiation and Induction Losses
RADIAION and INDUCTION LOSSES are
similar in that both are caused by the fields surround-
ing the conductors. Induction losses occur when the
electromagnetic field about a conductor cuts through
any nearby metallic object and a current is induced
in that object. As a result, power is dissipated in the
object and is lost.
Radiation losses occur because some magnetic lines
of force about a conductor do not return to the
conductor when the cycle alternates. These lines of
force are projected into space as radiation, and this
results in power losses. That is, power is supplied
by the source, but is not available to the load.
VOLTAGE CHANGE
In an electric circuit, energy is stored in electric
and magnetic fields. These fields must be brought
to the load to transmit that energy. At the load, energy
contained in the fields is converted to the desired form
of energy.
Transmission of Energy
When the load is connected directly to the source
of energy, or when the transmission line is short,
problems concerning current and voltage can be solved
by applying Ohm’s law. When the transmission line
becomes long enough so the time difference between
a change occurring at the generator and a change
appearing at the load becomes appreciable, analysis
of the transmission line becomes important.
Dc Applied to a Transmission Line
In figure 3-7, a battery is connected through a
relatively long two-wire transmission line to a load
at the far end of the line. At the instant the switch
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