AC Electric Circuits

Characteristic Impedance

17 questions By Tony R. Kuphaldt

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Question 1 of 17

Note the “50 ohm” rating printed on the jacket of the cable. What will an ohmmeter register when connected between the inner conductor and shield of the cable? What will an ohmmeter register when connected between opposite ends of either the inner conductor or the shield, from one end of the cable to the other?

Reveal answer

Resistance between inner conductor and shield = (infinite)

Resistance between ends of inner conductor = nearly 0 ohms

Resistance between ends of shield conductor = nearly 0 ohms

Notes:
It would be a great idea to have some samples of RG-58/U (or other coaxial cable type) available in your laboratory for students to measure themselves. There is nothing like direct, hands-on experimentation to make a point!
Question 2 of 17

If a battery and switch were connected to one end of a 10-mile long cable, and two oscilloscopes were used to record voltage at either end of the cable, how far apart in time would those two pulses be, assuming a propagation velocity equal to the speed of light (in other words, the cable has a velocity factor equal to 1.0)?

Reveal answer

53.68 microseconds

Notes:
Students should realize by the wording of the question that the voltage signal probably does not arrive at the far end of the cable instantaneously after the switch is closed. Although the speed of light is very, very fast, it is not instant: there will be a measurable time delay.
Question 3 of 17

If a battery and switch were connected to one end of a 10-mile long cable, and two oscilloscopes were used to record voltage at either end of the cable, how far apart in time would those two pulses be, assuming a propagation velocity equal to 69% the speed of light (in other words, the cable has a velocity factor equal to 0.69)?

Reveal answer

77.80 microseconds

Notes:
Students should realize by the wording of the question that the voltage signal probably does not arrive at the far end of the cable instantaneously after the switch is closed. Although 69% of the speed of light is still very, very fast, it is not instant: there will be a measurable time delay.