TC 9-64 _________________________________________________________________________
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.
Impedance for Various Lengths of Open Lines
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.
3-40
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