Why microtrip T-junction does not show strong reflections similar to 90° bend line?
Why simple 90° bend shows poor S11, while 90° T-junction shows almost no reflections, how this may be explained?
The reflection in the unmitered bend can be understood if you look at current flow, for example using EM simulation: the current is crowding in the corner (=extra series L) and there is no current in the outer edge (=shunt C). That's why we cut away the corner, to reduce the shunt C.
For the T-junction, there are also parasitic effects and reflection if current needs to change direction. We usually use a simulation model to account for the electrical effect of the T.
It still is not clear why 90° T-junction have better S11 than 90° microstrip bend.
Do you mean that T-junction does not have outer edge, so shunt C is smaller than for 90° bend ? It makes sense
I don't think this general statement is true. What T-junction in what circuit context?
In context of Patch antenna arrays. T-junctions often designed without mitering, while 90 degree bend is always mitered.
I see what you mean. I would expect that these transformers including T-junctions are designed carefully, including the T-junction parasitic effects. Unlike bends, they are not placed somewhere in the middle of a line at random distances.
The context already suggests the answer. The T-junctions are part of matching networks with transmission line transformer, by nature small band design. Any deviation from ideal junction behavior hides perfectly behind the matching network characteristic. Worst case the effective junction point is moved by small amount.
This issue arise many times and some people keep saying that in antenna array case because the connected transmission lines are actually transformers, you don't need to compensate for any discontinuity.
In reality if you do an EM simulation you can find that, whatever situation you use a T-junction still needs (more or less) a discontinuity compensation.
Use in the EM simulator any of the discontinuity compensation example from below, and you will see the difference.
I'm more familar with the T-junction of a wilkinson divider. At low frequencies, the discontinuity is insignificant. At high frequencies and/or wide bandiwths, the discontinuity "could" become an issue.