when measuring the time reflected voltage for a LINAC in band S, I found that the reflected voltage varies depending on the length of the waveguide between the accelerator section and the source RF.

however the impedance is adapted as input and the VSWR of the iris at the level of the section is 1.3.

N.B: in frequency domaine following a simulation on HFSS the value of S11 is almost constant regardless of the waveguide length.