Two years ago, I made the 74AC14 schmidt trigger TDR (the one from W2AEW's channel) and have been using it since. Today, it got a software upgrade 
It never crossed my mind that my USB scope does math channels. Now, with some startup settings, it reads out in Ω's (green trace)! That is looking down two pieces of old coax connected together with an antenna on the other end. For those with scopes that have math channels, the formula for this is Ω=-Zo*((1+(A-B)/B)/(1-(A-B)/B)) Trace A is the voltage of the TDR step response. Zo is the output impedance of the TDR. B is the voltage of the trace with a calibration load Zo attached.
It never crossed my mind that my USB scope does math channels. Now, with some startup settings, it reads out in Ω's (green trace)! That is looking down two pieces of old coax connected together with an antenna on the other end. For those with scopes that have math channels, the formula for this is Ω=-Zo*((1+(A-B)/B)/(1-(A-B)/B)) Trace A is the voltage of the TDR step response. Zo is the output impedance of the TDR. B is the voltage of the trace with a calibration load Zo attached.