3-7
seldom used because the dielectric losses for all known dielectric materials are too great to transfer the
electric and magnetic fields efficiently.
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.
We will not consider the waveguide operation in this module, since waveguide theory is discussed in
NEETS, Module 11, Microwave Principles.
Q4. List the five types of transmission lines in use today.
Q5. Name two of the three described uses of a two-wire open line.
Q6. What are the two primary disadvantages of a two-wire open line?
Q7. What type of transmission line is often used to connect a television set to its antenna?
Q8. What is the primary advantage of the shielded pair?
Q9. What are the two types of coaxial lines in use today?
Q10. What is the chief advantage of the air coaxial line?
Q11. List the three disadvantages of the air coaxial line.
Q12. List the two common types of waveguides in use today.
LOSSES IN TRANSMISSION LINES
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 of three types—COPPER, DIELECTRIC, and
RADIATION or INDUCTION LOSSES.
NOTE: Transmission lines are sometimes referred to as rf lines. In this text the terms are used
interchangeably.
Copper Losses
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 resistance higher than solid
tubing, this power loss is higher.
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.
