If you put a rising edge into a bit of coax via a 50 Ohm resistor, you will get a reflected edge coming back whose delay is proportional to the length of coax. If the coax has an open circuit at the end, it will be a positive edge, if the coax has a short circuit it will be a falling edge. I suggest you put a risiing edge into a 0 Ohm terminated length of coax and square up the returing negative edge. You can then very accurately control the width of the pulse by trimming the length of the coax. Square up the edges with a high speed comparator. This is transmission line theory.