Cecil, W5DXP wrote:
"There is nothing at the mouth of the stub capable of causing
reflections. All the reflections occur at the shorted end of the stub."

Correct, if the generator matches Zo. In any case, Zo limits current on
the transmission line until a reflection from the mismatched load
arrives back at the line input to change the impedance seen by the
generator. Then, the current delivered by the generator will depend on
the generator`s match to the new impedance caused by the reflection
after a few iterations.

Another view of the mismatched line is to ignore what the transmitter
can deliver into any load. Then, the forward and reflected waves simply
cause a current in each direction depending on the strength and the
current allowed by Zo.

Transmission lines are tyrannical in forcing current in a particular
direction to adhere to the Zo voltage to current ratio. That`s the
reason a junction between lines of differing impedances produce a
reflection. The line from the source contains a voltage wave and a
current wave conforming to Zo. The line of a different impedance
connected to the line from the source accepts alll the voltage or
current available to it which leaves a surplus of either voltage or
current that must be reflected back toward its source. The source line
is no less dictatorial about its Zo to the reflection than it is to the
incident wave.

Best regards, Richard Harrison, KB5WZI