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This is only true if the antenna is not matched to the TL line (leading to large reflections within the TL).

No idea what you mean by "intrinsic" vs "effective" efficiency, AFAIK those aren't established terms for power amplifiers.

Let's make up some example numbers:

Scenario #1:

PA output impedance Rout = 50ohm

Z0 of transmission line = 50ohm

Transmission line is a quarter wavelength long

S21 of transmission line = 0.9 (-0.46dB)

Antenna impedance Ra = 50ohm

Let's assume the PA has a Popt of 1W into Ropt = 50ohm, and its input is being driven appropriately for those conditions.

In this case, there's no reflections anywhere.

The power delivered by the PA is 1W, and the power absorbed by the antenna is 0.9W.

Also, this result is completely independent of the PA's output impedance Rout. If Rout were changed to 10ohms or 1000ohms then the antenna power would not change (so long as the other conditions listed above are still true).

Scenario #2:

PA output impedance Rout = 50ohm

Z0 of transmission line = 50ohm

Transmission line is a quarter wavelength long

S21 of transmission line = 0.9 (-0.46dB)

Antenna impedance Ra = 25ohm

Let's assume the PA has a Popt of 1W into Ropt = 93ohm, and its input is being driven appropriately for those conditions.

The impedance seen by the PA is 93ohm (not 100ohms, due to the TL loss), which is equal to Ropt. So the PA still delivers 1W.

However, due to mismatch at the other end of the line (gamma=0.333), the effective insertion loss of the TL is now 0.9*(1-0.333^2)/(1-0.9^2*0.333^2) = 0.879 (-0.56dB).

So the total power to the antenna is 0.879W. Again, this has nothing to do with the PA's output impedance. If Rout were changed to 10ohms or 1000ohms then the antenna power would not change (so long as the other conditions listed above are still true).