10. The coaxial line.
a. The coaxial line is used widely as a transmission line at
frequencies up to 3000 megahertz.
It consists of a hollow copper tube
through which another copper tube is run coaxially (having the same axis)
with the outer tube. See Part A of Figure 5. Current flows along the outer
surface of the center conductor and along the inner surface of the outer
conductor. The outer conductor may be grounded since current flows only on
its inner surface.
b. The inner copper tube is insulated from the outer conductor by
spacers made of pyrex, polystyrene, or some similar insulating material.
The conducting surfaces are often silverplated to reduce losses due to
Also, the coaxial line is usually pressurized to keep out
moisture.
Moisture inside the cable causes the RF energy to arc over,
resulting in a power loss.
c. The coaxial transmission line may be made flexible as shown in Part
B of Figure 5.
The space between the wire inner conductor and outer
conductor is filled with a flexible plastic insulating material called
polyethylene.
The outer conductor is made of copper braid.
The flexible
coaxial line has somewhat higher losses than the rigid line because of the
polyethylene.
Figure 5.
Types of Coaxial Transmission Line.
d. Coaxial lines are superior to two-wire parallel lines because of the
perfect shielding provided by the outer conductor.
Shielding not only
prevents radiation but also prevents noise pickup from external sources.
This is one reason why coaxial lines are widely used for television lead-ins
in noisy locations.
e. The chief disadvantage of coaxial lines at the higher frequencies is
the loss of power due to skin effect. Skin effect is the result of current
flowing on the outside of conductors.
The power loss is due to the
increased resistance
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