I think I am using the 50Ω as a termination to the coaxial cable.
Then put it in between the coax center conductor and the drain of the Fet, with the Fet source tied to ground.
My question had to do with how is your "time domain reflectometer" supposed to work?
1. You charge the coax cable to 9V using a high-z source at the distal end.
2. You suddenly connect a 50Ω termination resistance at the proximal end.
3. The 50Ω termination creates a 2:1 voltage divider against the cable impedance, causing a downward step from 9V to 4.5V.
4. The step propagates to the distal end, and because the termination there is high impedance, the downward step reflects (almost 100%) and propagates back to the proximal end.
5. That causes the voltage at the proximal end to step downward to zero.
6. The time between the original half-step to the final half-step is proportional to the length of the cable, and is effected by the velocity factor of the cable.
In order to make this work right, you need to add a high-value resistor, say >10K between the 9V battery and the center-conductor at the distal end! Otherwise the reflection will not be 100%