3-43
At the even quarter-wave points voltage is minimum, current is maximum, and impedance is
minimum. Since these characteristics are similar to those of a series-resonant LC circuit, a shorted
transmission line whose length is an even number of quarter-wavelengths acts as a series-resonant circuit.
Resonant shorted lines, like open-end lines, also may act as pure capacitances or inductances. The
illustration shows that a shorted line less than 1/4 wavelength long acts as an inductance. A shorted line
with a length of from 1/4 to 1/2 wavelength acts as a capacitance. From 1/2 to 3/4 wavelength, the line
acts as an inductance; and from 3/4 to 1 wavelength, it acts as a capacitance, and so on. The equivalent
circuits of shorted lines of various lengths are shown in the illustration. Thus, properly chosen line
segments may be used as parallel-resonant, series-resonant, inductive, or capacitive circuits.
STANDING WAVES ON A TRANSMISSION LINE
There is a large variety of terminations for rf lines. Each type of termination has a characteristic
effect on the standing waves on the line. From the nature of the standing waves, you can determine the
type of termination that produces the waves.
TERMINATION IN Z0
Termination in Z0 (characteristic impedance) will cause a constant reading on an ac meter when it is
moved along the length of the line. As illustrated in figure 3-34, view A, the curve, provided there are no
losses in the line, will be a straight line. If there are losses in the line, the amplitude of the voltage and
current will diminish as they move down the line (view B). The losses are due to dc resistance in the line
itself.
