Gene Fuller wrote:
"I don`t know what happens to the source impedance when the load is
changed, because the system is undefined."

Yes. Maximum power transfer is accomplished by making the load impedance
the conjugate of the generator impedance as defined by Thevenin`s
theorem.

The value of the Thevinen impedance is that which might be measured by a
generator`s open-circuit voltage devided by its short-circuit current.
You don`t need to know the generator`s specifics other than, drop in the
output voltage is proportional to the current delivered.

The current which flows in a linear load impedance connected to a
Thevenin generator is the open-circuit voltage divided by the sum of the
generator`s internal impedance and the load impedance. These may be
complex impedances.

At maximum power transfer, internal and load impedances are equal in
resistance and their reactances are conjugate (opposite and equal).

There is no requirement that resistance in either the generator or load
be dissipative, and frequently, lossless resistance is a part of the
generator impedance so that we can get maximum possible power into the
load without losing 50% in the generator.

Best regards, Richard Harrison, KB5WZI