Input impedance of a transmission lines.

I need to use only Quarter-Wave transformers in order to have no retreating waves reaching the generator.
I first placed such tranfomrmer in lines (2) and (3).
That was easy since i knew that Zin(C) = R, Zin(B) = ZL.

Now i want to place a transformer in line (1), but i dont know what is Zin(A) equal?

Your statement:
"I need to use only Quarter-Wave transformers in order to have no retreating waves reaching the generator"

Where do you get this impression? It is not true.

If you only use quarter wave lines you will not be able to get rid of the reactive component in load to right of diagram.

As drawn you have 12.5 ohms in parallel with -j50 ohms (11.8 - j2.9) at point A.

Hey, Thanks a lot for the help.