In order to determine the characteristic impedance of a coaxial cable, I put a resistance on the one end and connected the other end of the cable to a signal generator and an oscilloscope.
This worked fine when using a sine signal as I just had to find the resistance, for which the returning signal was (almost) gone. However, I also wanted to try this with a square signal. The result was first a positive sharp peak decreasing similar to an exponential function followed by a negative peak of the same form.
Why does this happen? My first thought was a differentiator since the sine signal kept its form and the sides of the square signal resulted in sharp peaks with the constant middle part being changed to $0$ voltage. After some reading I found out, that an RL-circuit can be looked at as an differentiator. So my assumption is that the coaxial cable's inductance makes the experiment a RL-circuit. However, where do you draw the inductor into the circuit? My first guess would be to just add it in series with R:
Where V is the oscilloscope and the red parts are the coaxial cable. This also produces the next question: I am measuring the voltage over R and the inductor while for the differentiator the voltage is taken over the inductor only:
I only made a sketch of the seen image on the oscilloscope:
The coaxial cable was $40\; \mathrm{m}$ long.