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

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

tuned circuit.

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

to some multiple of a quarter-wavelength.

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.