___________________________________________________ Principles of Transmission Lines
3-22. A waveguide may be classified according to its cross section
(rectangular, elliptical, or circular), or according to the material used in its
construction (metallic or dielectric). Dielectric waveguides are seldom used
because the dielectric losses for all known dielectric materials are too great to
transfer the electric and magnetic fields efficiently.
3-23. The installation of a complete waveguide transmission system is
somewhat more difficult than the installation of other types of transmission
lines. The radius of bends in the waveguide must measure greater than two
wavelengths at the operating frequency of the equipment to avoid excessive
attenuation. The cross section must remain uniform around the bend. These
requirements hamper installation in confined spaces. If the waveguide is
dented, or if solder is permitted to run inside the joints, the attenuation of
the line is greatly increased. Dents and obstructions in the waveguide also
reduce its breakdown voltage, thus limiting the waveguide's power-handling
capability because of possible arc-over. Great care must be exercised during
installation; one or two carelessly made joints can seriously inhibit the
advantage of using the waveguide.
LOSSES IN TRANSMISSION LINES
3-24. The discussion of transmission lines so far has not directly addressed
line losses; actually some line losses occur in all lines. Line losses may be any
Note. Transmission lines are sometimes referred to as RF lines. In this text
the terms are used interchangeably.
Copper Losses
3-25. One type of copper loss is I2R loss. In RF lines, the resistance of the
conductors is never equal to zero. Whenever current flows through one of
these conductors, some energy is dissipated in the form of heat. This heat loss
is a power loss. With copper braid, which has a higher resistance than solid
tubing, this power loss is higher.
3-26. 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.
3-27. Because resistance is inversely proportional to the cross-sectional area,
the resistance will increase as the frequency is increased. Also, because
power loss increases as resistance increases, power losses increase with an
increase in frequency because of skin effect. Skin effect is a tendency for
3-7
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