3-38Q24.In figure 3-27, which waveforms on the left have a resultant wave of zero, and what is indicated bythese waves?Q25.On an open-ended transmission line, the voltage is always zero at what distance from each end ofthe line?TERMINATING A TRANSMISSION LINEA transmission line is either NONRESONANT or RESONANT. First, let us define the termsnonresonant lines and resonant lines. A nonresonant line is a line that has no standing waves of currentand voltage. A resonant line is a line that has standing waves of current and voltage.Nonresonant LinesA nonresonant line is either infinitely long or terminated in its characteristic impedance. Since noreflections occur, all the energy traveling down the line is absorbed by the load which terminates the line.Since 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 Z_{0}, no special tuning devices are required to effect amaximum power transfer; hence, the line is also called an UNTUNED line.Resonant LinesA resonant line has a finite length and is not terminated in its characteristic impedance. Thereforereflections of energy do occur. The load impedance is different from the Z_{0} of the line; therefore, the inputimpedance may not be purely resistive but may have reactive components. Tuning devices are used toeliminate 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 ortuned circuit.A resonant line is sometimes said to be resonant at an applied frequency. This means that at onefrequency 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.At the points of voltage maxima and minima on a short-circuited or open-circuited line, the lineimpedance is resistive. On a short-circuited line, each point at an odd number of quarter-wavelengthsfrom the receiving end has a high impedance (figure 3-31, view A). If the frequency of the appliedvoltage to the line is varied, this impedance decreases as the effective length of the line changes. Thisvariation is exactly the same as the change in the impedance of a parallel-resonant circuit when theapplied frequency is varied.

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