Richard Clark wrote:
"I will take it that you don`t know what the source impedance is as a
quantifiable either."

Terman wrote on page 76 of his 1955 opus:
"Alternatively, a load impedance may be matched to a source of power in
such a way as to make the power delivered to the load a maximum. (The
power delivered under these conditions is termed the "available power"
of the power source.) This is accomplished by making the load impedance
the conjugate of the generator as defined by Thevenin`s theorem. That
is, the load impedance must have the same magnitude as the generator
impedance, but the phase angle of the load is the negative of the phase
angle of the generator impedance."

Best regards, Richard Harrison, KB5WZI