3-39Figure 3-31.—Sending-end impedance of various lengths and terminations.At all even numbered quarter-wavelength points from the short circuit, the impedance is extremelylow. When the frequency of the voltage applied to the line is varied, the impedance at these pointsincreases just as the impedance of a series-resonant circuit varies when the frequency applied to it ischanged. The same is true for an open-ended line (figure 3-31, view B) except that the points of high andlow impedance are reversed.At this point let us review some of the characteristics of resonant circuits so we can see how resonantline sections may be used in place of LC circuits.A PARALLEL-RESONANT circuit has the following characteristics:At resonance the impedance appears as a very high resistance. A loss-free circuit has infiniteimpedance (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 toolow), more current flows through the coil than through the capacitor. This happens because X_{L}decreases with a decrease in frequency but X_{C} increases.A SERIES-RESONANT circuit has these characteristics: