Hi Alan -

Reflections measured by a TDR are caused by physical impedance
discontinuities. Virtual impedances are defined by the superposition
of forward and reflected voltages in the steady state. Pulsed systems
offer the ability to study the transient effects of a system by
viewing reflections caused only by changes in the characteristic
impedance of the transmission line. Since TDR doesn't use CW (not to
be confused with Morse Code) it does not operate under steady state
conditions and can therefore neither prove nor disprove the claim for
reflections from virtual impedances.

73, Jim AC6XG

Hi Jim, not sure if my previous reply got through.
Yes, I have to admit to LNBF (Late Night Brain Fade) when I threw in the
rotary joint example. Of course it is CW in the pulse. I was trying to
underline that a stub puts a physical discontinuity on the TL which will
give a real reflection. But as you point out, this reflection is not the
same as the reflection from a virtual discontinuity for CW. However, if
the CW can be thought of as a series of pulses, then does that not mean
that real reflections occur and that the sum of the reflections for each
pulse looks like they have come from a virtual discontinuity?
If not, how would one go about proving or disproving the idea of
reflections from virtual discontinuities?
Alan