3-124. A parallel-resonant circuit has the following characteristics:

At resonance, the impedance appears as a very high resistance. A

loss-free circuit has infinite impedance (an open circuit). Other than

at resonance, the impedance decreases rapidly.

If the circuit is resonant at a point above the generator frequency (the

generator frequency is too low), more current flows through the coil

than through the capacitor. This happens because XL decreases with

a decrease in frequency, but XC increases.

3-125. A series-resonant circuit has these characteristics:

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

circuit acts capacitively.

If the circuit is resonant at a point below the generator frequency (the

circuit acts inductively.

3-126. Because 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.

3-127. 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.