___________________________________________________ Principles of Transmission Lines
3-132. At the even quarter-wave points in figure 3-33, voltage is minimum,
current is maximum, and impedance is minimum. Because 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.
3-133. 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.
EFFECT OF TERMINATION ON STANDING WAVES
3-134. 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
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.
3-43
Previous Page
