65.
Summary.
a. Telephone lines may be constructed in the form of cables, open wire, or field wire. Cables or field wire may
be supported on poles similar to open wire lines, or may be laid on the ground. Cables may also be laid underground.
b. Transmission lines are considered electrically short if their length is shorter than the wavelength of the
transmitted signal; they are considered electrically long if their length is approximately equal to or longer than the
wavelength of the transmitted signal.
c. The electrical properties of a pair depend on its parameters, R, L, G, and C, all expressed in values per unit
length (commonly 1 loop mile) and on the frequency of the transmitted current. For an electrically short pair, the
parameters may be treated as distributed.
d. The characteristic impedance, ZO, of a line is equal to the impedance that must terminate the line in order to
make the input impedance equal to the terminating impedance. On a pair that is extremely long, the input impedance will
equal the characteristic impedance of the line irrespective of the terminating impedance.
e. The characteristic impedance of a pair depends on the parameters of the pair and on the frequency, but it is
independent of the length of the pair. The resistive component of the characteristic impedance is generally high at low
frequencies and falls off with increasing frequency, approaching a value equal to L/C at high frequencies. The reactive
component also starts out high at low frequencies and decreases at the higher frequencies. The characteristic impedance
of a pair may be obtained from the measured open-circuit and short circuit impedances by the formula,
ZO = ZOCZSC
f. Maximum power is transferred from a source to a load over a transmission line when the line is terminated in
its characteristic impedance.
g. Attenuation is the term used to express the power loss in a line. The attenuation of a line is measured in dB
and can be calculated by using the relationship,
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