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Transmit a high-speed edge pulse signal to one end of an unknown length cable. The signal will propagate along the cable to the other end. If the other end of the cable is disconnected, the signal will reflect back to the starting point after reaching the end point. Use an oscilloscope to measure the time difference between signal transmission and reflection, you can calculate the length of the cable. The wire or cable only transmits energy, and the wire itself does not consume energy or almost does not lose energy. When the radio-frequency signal reaches the end of the cable, there is no way to release the energy, and it will propagate back along the cable. Just like us yelling at the wall, the sound hits the wall and returns back to produce an echo.
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