4-9
line, then the same frequency and the same level of power should be transmitted to the other end of a
transmission line. Because all transmission lines contain some degree of resistance, some loss of power
will occur during the test.
Exposure to the elements over a period of time causes transmission-line deterioration. To determine
the accuracy of this test, you should use the power meter to measure the output of the signal generator at
the end of the test cable to be attached to the transmission line. Any power loss associated with the test
cables should be recorded and subtracted from the measurement taken with the transmission line
connected.
You should note that transmission lines, like all other electronic components, are designed to operate
over a specific range of frequencies. It is not uncommon for a transmission line to operate improperly at
one frequency, yet operate properly over the remainder of its frequency spectrum. You should check
transmission-line losses over their entire frequency range. Insertion loss measurements are normally taken
when a system is first installed or the transmission line is replaced. Periodic measurements should be
performed to enable you to determine if system performance is being degraded by transmission-line
deterioration.
Q-4.
Is it possible for a transmission line to operate improperly at certain frequencies and properly
at others?
TRANSMISSION-LINE FORMULAS
Transmission lines are engineered and manufactured to meet certain specifications. The most
important of these specifications relates to frequency, power-handling capabilities, and characteristic
impedance. The dielectric constant (K) of the insulating material is probably the manufacturer’s most
important consideration and is the primary factor that affects the size of the coaxial cable. The formulas in
the following sections discuss some aspects of coaxial transmission-line engineering.
A cross section of a coaxial line is shown in figure 4-6. The characteristic impedance of a coaxial
line can be determined by the following formula:
