kollayliu,
I think I see your confusion. The reflection coefficient being zero for maximum power delivered is really true if Zs is entirely real.
The confusion depends on how you define your plane of reference.
to the real source impedance or complex source impedance.
Consider the simple example.
Zs=R
Zl=R+jX
matching it with a series capacitor Zc=1/sc = -j/wc where 1/wc =X
Matching plus load is Zl+Zc = R+jX-jX= R
So reflection coefficient is 0/2R at the sources plane of reference.
Try again but shift the plane of reference
Zs= R+1/sc where 1/sc =-j/wc =-jX
Zl=R+jX
Reflection coefficient (Zs-Zl)/(Zs+Zl) =- jX/R why? that plane of reference observes a transitional complex impedance. But the power is still being maximally delivered.
hope that helps
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