3-40
At resonance the impedance appears as a very low resistance. A loss-free circuit has zero
impedance (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 is
too low), then XC is larger than XL and the circuit acts capacitively.
If the circuit is resonant at a point below the generator frequency (the generator frequency is
too high), then XL is larger than XC 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 (figure
3-31, view C). An open quarter-wavelength of line may be used as a series-resonant circuit (view D). The
Q of such a resonant line is much greater than can be obtained with lumped capacitance and inductance.
Impedance for Various Lengths of Open Lines
In 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 the
generator for the various line lengths. The circuit symbols indicate the equivalent electrical circuits for the
transmission lines at each particular length. The standing waves of voltage and current are shown on each
length of line.
