TC 9-64 _________________________________________________________________________
and figure 3-30. Notice that the current and voltage standing waves are
shifted 90 degrees with respect to the termination. At the open end of a line,
voltage is maximum (zero if there are no losses in the line). At a short circuit,
current is maximum and voltage is minimum.
3-117. A transmission line is either nonresonant or resonant. A nonresonant
line is a line that has no standing waves of current and voltage. A resonant
line is a line that has standing waves of current and voltage.
3-118. A nonresonant line is either infinitely long or terminated in its
characteristic impedance. Because no reflections occur, all the energy
traveling down the line is absorbed by the load that terminates the line.
Because no standing waves are present, this type of line is sometimes spoken
of as a flat line. In addition, because the load impedance of such a line is
equal to Z0, no special tuning devices are required to effect a maximum power
transfer; hence, the line is also called an untuned line.
3-119. A resonant line has a finite length and is not terminated in its
characteristic impedance. Therefore reflections of energy do occur. The load
impedance is different from the Z0 of the line; therefore, the input impedance
may not be purely resistive but may have reactive components. Tuning
devices are used to eliminate the reactance and to bring about maximum
power transfer from the source to the line. Therefore, a resonant line is
sometimes called a tuned line. The line also may be used for a resonant or
3-120. A resonant line is sometimes said to be resonant at an applied
frequency. This means that at one frequency the line acts as a resonant
circuit. It may act either as a high-resistive circuit (parallel resonant) or as a
low-resistive circuit (series resonant). The line may be made to act in this
manner by either open- or short-circuiting it at the output end and cutting it
3-121. At the points of voltage maxima and minima on a short-circuited or
open-circuited line, the line impedance is resistive. On a short-circuited line,
each point at an odd number of quarter-wavelengths from the receiving end
voltage to the line is varied, this impedance decreases as the effective length
of the line changes. This variation is exactly the same as the change in the
impedance of a parallel-resonant circuit when the applied frequency is varied.