Reflections at the source point of a transmission line

[miner_tom]

TL;DR

Is there always a reflection from the source after a reflection from the load?

I was using the Smith chart to determine the input impedance of a transmission line that has a reflection from the load. One can do this if one knows the characteristic impedance Zo, the degree of mismatch of the load ZL and the length of the transmission line in wavelengths. However, my question is: Consider the input impedance of a wave which appears back at the source after reflection from the load and has traveled for some fraction of a wavelength. The impedance of this wave as it appears to the source is not necessarily equal to the characteristic impedance of the transmission line, will there be a reflection even if the original source impedance is equal to Zo

Source: https://www.physicsforums.com/forums/electromagnetism.302/post-thread

 

[Baluncore]

The impedance of this wave as it appears to the source is not necessarily equal to the characteristic impedance of the transmission line, will there be a reflection even if the original source impedance is equal to Zo.

Reflections only occur where there is an impedance mismatch.

If the source is matched to the line, then there will be no reflection at the line input port. If there is a discontinuity somewhere up the line, there will be a reflection from there. When that reflected wave returns to the source, it will be absorbed in the matched impedance, without another reflection.

The voltage and current of the wave travelling up the line has a ratio fixed by the characteristic impedance of the line, v / i = Zo. The same is true for the reflected wave travelling back from a reflection. At any point on the line, the sum of the voltages and currents of the two waves travelling in different directions will make a standing wave.

 

[sophiecentaur]

If the source is matched to the line,

Exactly. Mr Smith says nothing about multiple reflections from both ends; the maths would be too hard and specific to every particular situation. Setting up a measurement has to involve matching the source to a very high standard and that's the only way you can measure what's going on at the other end.

[Edit: I'd reconsider that statement. I once had a colleague (way back) who could probably have done it. Bob Thoday - RIP - was very clever.)]