Roger Sparks wrote:

Demo 5 is a demonstration of the pitfalls of dealing with discontinuities. The demonstration portrays equal periods of sequential open circuit conditions and short circuit conditions.

The discontinuity here is that a reflection factor of 1 is used if the returning voltage is less than the initial voltage, but a reflection factor of -1 is used for any returning voltage exceeding the initial voltage.

That's not correct. A voltage source reflection factor of -1 is always
used for the perfect source case, both in my analysis and in the
program. See my analysis of Dec. 28 http://tinyurl.com/ypfshd.

Demo 5 is a demonstration of why a Thevenin voltage source is much to be prefered over a simple ideal voltage source for continuous running circuits.

Ideal voltage sources are routinely and properly used in a great deal of
circuit analysis. The problem with this transmission line setup isn't
the use of perfect voltage source, it's that the entire system has no
loss. Putting a finite resistance at the far end of the line, for
example, results in a well-behaved system.

It sounds like you're confusing a Thevenin equivalent circuit with a
voltage source in series with an impedance. All Thevenin equivalent
circuits fit this description, but not all voltage sources in series
with a resistance are Thevenin equivalents. I haven't used a Thevenin
equivalent circuit in any of my analyses.

Another way of illustrating the problem is to say that the ideal voltage source overwhelms any voltage from a returning wave. If an external voltage less than the initial voltage is applied to the ideal voltage source, vr = 0, vf = videal. If the external voltage exceeds the initial voltage, then vr = 0, vf = 0 This is a discontinuity, which shows up in Demo 5.

There are no discontinuites in the mathematical analysis. The forward
and reverse waves always have a finite value or zero, and they always
sum to the source voltage. No infinite currents or voltages occur. You
can see from the program that the input voltage stays constant. (That
is, it generates a sine wave of constant amplitude.) The only
consequence is the oscillatory behavior of the line voltage and current.

A third way of illustrating the discontinuity problem is to say that you can not apply a voltage to two wires that are short circuited at the input. In Demo 5, the reflected wave returns to find the circuit short circuited. On the other hand, the ideal voltage source applied voltage to an open circuit. We can not have it both ways.

Again, there is no discontinuity.

The demo 5 system doesn't represent anything that can be built in real
life, and is basically just a curiosity. It was included mainly to
verify that the program works properly under this limiting-case
condition. It certainly has spurred some people to think about the
meaning of perfect sources and reflection coefficients and revealed a
lot of misunderstanding in the process, and I imagine it'll continue to
do so.

Roy Lewallen, W7EL