___________________________________________________ Principles of Transmission Lines
Figure 3-32. Voltage, Current, and Impedance on Open Line
3-128. At all odd quarter-wave points (1/4λ, 3/4λ, etc.), the voltage is
minimum, the current is maximum, and the impedance is minimum. Thus, at
all odd quarter-wave points, the open-ended transmission line acts as a
series-resonant circuit. The impedance is equivalent to a very low resistance,
prevented from being zero only by small circuit losses.
3-129. At all even quarter-wave points (1/2λ, 1λ, 3/2λ, etc.), the voltage is
maximum, the current is minimum, and the impedance is maximum.
Comparison of the line with an LC resonant circuit shows that at an even
number of quarter-wavelengths, an open line acts as a parallel-resonant
circuit. The impedance is therefore an extremely high resistance.
3-130. In addition, resonant open lines may also act as nearly pure
capacitances or inductances. The illustration shows that an open line less
than a quarter-wavelength long acts as a capacitance. Also, it acts as an
3-41
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