Build Your Own Time-Domain Reflectometer

Aside from the things camperbob identified, this is pretty neat. The tricky parts are getting the pulse generator right (you want a sharp leading edge and flat top, with no ringing) and having an oscilloscope with a flat response well into the GHz range.

One solution to the second challenge is to home-brew a sampling adapter, like this one from 2000[1]. This is a good fit for TDR because of two factors inherent in the TDR process: a) it is repetitive, so the usual drawbacks to "sub-nyquist" sampling don't apply; and b) you have ready access to a pre-trigger, so it is easy to see the leading edge of the signal of interest.

The first challenge requires a lot of cleverness or access to expensive test equipment.

[1] http://electronicdesign.com/boards/1-ghz-sampling-oscillosco...

   For all coaxial dielectrics, the relative permeability 
   is so close to 1 that we can just assume it is for our 
   purposes ( accurate to within 0.00000001%). Substituting 
   these values into our first equation we get
   
   Vp = 273,671,819.7 m/s or 10.7745 in/nS

   which means our signal will propagate at about 91.29% of 
   the speed of light, this percentage is known as the   
   velocity factor or VF.

Huh? That's nowhere near a typical coaxial velocity factor (most often around 0.66 for 50-ohm cables.)