I am trying to realize the method in the literature:
A New Efficient and Stable 3D Conformal FDTD
and got the simulation result in the attached picture. The phase of S11 changed 180 degrees suddenly, what's the possible explanation for this? A bow tie antenna is simulated. In traditional FDTD, the phase changes gradually.
The phase of the reflection coefficient does not change 180 degrees suddenly in the plot you have shown, it changes 360 degrees. This is because your maths and plotting only gives results between -180 and 180, when actually the phase decreased smoothly from about -20 to -320. It has gone nearly once round the Smith chart. Your maths can't tell the difference between -181 degrees and +179 degrees, so it jumps to the top of the page.
Some codes have functions like "unwrap" which can stop this happening, or recover if it has happened.
In this exact example, one way would be to add 180 degrees to the phase of everything before working it out, which means it would start near the top and finish near the bottom, without a jump. Then set the scales to read -360 to 0 deg.
Phase can jump 180 degrees when the return loss is very large. In this case the complex return loss crosses the centre of the Smith chart (0,j0) and appears at the opposite angle on the other side of the centre, so a change of 180 deg. This only happens when the return loss is so big that the locus actually seems to pass through the centre. This depends on the return loss, and the spacing between your frequency points, and their exact value, and the accuracy of your maths.
Hi,
The phase of the reflection coefficient changes 360 degrees. Because in HFSS the result only shows phase values between plus-minus 180 degrees. You can also see the continuous phase angle using cang_deg plot instead of ang_deg plots. It unwraps the phase.
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