Page "Time-domain reflectometer" Paragraph 18

Consider the case where the far end of the cable is shorted ( that is, it is terminated into zero ohms impedance ).

When the rising edge of the pulse is launched down the cable, the voltage at the launching point " steps up " to a given value instantly and the pulse begins propagating down the cable towards the short.

When the pulse hits the short, no energy is absorbed at the far end.

Instead, an opposing pulse reflects back from the short towards the launching end.

It is only when this opposing reflection finally reaches the launch point that the voltage at this launching point abruptly drops back to zero, signalling the fact that there is a short at the end of the cable.

That is, the TDR had no indication that there is a short at the end of the cable until its emitted pulse can travel down the cable at roughly the speed of light and the echo can return back up the cable at the same speed.

It is only after this round-trip delay that the short can be perceived by the TDR.

Assuming that one knows the signal propagation speed in the particular cable-under-test, then in this way, the distance to the short can be measured.