3-40At resonance the impedance appears as a very low resistance. A loss-free circuit has zeroimpedance (a short circuit). Other than at resonance the impedance increases rapidly.If the circuit is resonant at a point above the generator frequency (the generator frequency istoo low), then X_{C} is larger than X_{L} and the circuit acts capacitively.If the circuit is resonant at a point below the generator frequency (the generator frequency istoo high), then X_{L} is larger than X_{C} and the circuit acts inductively.Since the impedance a generator sees at the quarter-wave point in a shorted line is that of a parallel-resonant circuit, a shorted quarter-wave- length of line may be used as a parallel-resonant circuit (figure3-31, view C). An open quarter-wavelength of line may be used as a series-resonant circuit (view D). TheQ of such a resonant line is much greater than can be obtained with lumped capacitance and inductance.Impedance for Various Lengths of Open LinesIn figure 3-32, the impedance (Z) the generator sees for various lengths of line is shown at the top.The curves above the letters of various heights show the relative value of the impedances presented to thegenerator for the various line lengths. The circuit symbols indicate the equivalent electrical circuits for thetransmission lines at each particular length. The standing waves of voltage and current are shown on eachlength of line.

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