29. A section of transmission line acts like a resonant circuit.

a. Because of these impedance variations, a section of transmission

line can be used as a resonant circuit at ultra-high frequencies.

For

example, look at Part A of Figure 19 which shows the standing waves and

impedance variations on a short-circuited, quarter-wave transmission line.

the current is minimum at that point. A parallel-resonant circuit has the

same characteristics; that is, high voltage, low current, and high

impedance.

So the quarter-wave line is now equivalent to the parallel-

resonant circuit. Remember, however, that the quarter-wave line acts as a

parallel-resonant circuit only at the frequency which makes the transmission

line one-quarter wavelength long. At other frequencies, the line is not a

quarter wavelength, so the impedance decreases.

Figure 19. Voltage, Current and Impedance Variations

on a Quarter-wave Transmission Line.

b. Part B of Figure 19 shows voltage, current, and impedance variations

on an open-circuited, quarter-wave transmission line.

There is high

impedance at the open end.

But at the generator, the impedance is low

because the voltage is low and the current is high.

Low voltage, high

current, and low impedance are the characteristics of a series-resonant

circuit. So an open-circuited, quarter-wave line acts like a series-tuned

circuit.

30. Summary of the quarter-wave line.

a. A short-circuited, quarter-wave line acts as

a

parallel-tuned

circuit. It presents a high impedance to the generator.

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