3-8
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 polyethylene, 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
RADIATION and INDUCTION LOSSES are similar in that both are caused by the fields
surrounding 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.
Q13. What are the three types of line losses associated with transmission lines?
Q14. Losses caused by skin effect and the I
2 R (power) loss are classified as what type of loss?
Q15. What types of losses cause the dielectric material between the conductors to be heated?
LENGTH OF A TRANSMISSION LINE
A transmission line is considered to be electrically short when its physical length is short compared
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NOTE: In this module, for ease of reading, the value of the wavelength will be spelled out in some
cases, and in other cases, the numerical value will be used.
