[SI-LIST] Re: Transmission Line

>I know 
>that there are reflections on transmission line if there is a 
>impedance mismatch. I would like know how the reflections are 
>generated

Consider only two conductor transmission lines operating in TEM mode (for 
simplicity).
For a single, forward-going signal propagating down the line there is a certain
distribution of voltage between the conductors and current along the conductors
as a specific function of space and time.  Typically these distributions can
expressed as functions of (z-ct) or (z+ct) [assuming z is along the line, and 
depending upon which way the signal is propagating].  The ratio of these two
functions is the characteristic impedance of the line.  

Now introduce an impedance mismatch.  At that boundary, a wave can propagate
forward-only in the second line, but a reflected wave must now be generated
in the first line.  Why?  Because otherwise the voltage and the current at
the junction between the two lines canNOT both be the same in both lines,
but since these are the same physical point these two values must be the same.

The reflected wave in the first line is generated by that boundary mismatch.

In the extreme cases of an open-circuited end (infinite impedance for the
"second line") or short-circuited end (zero impedance for the "second line")
the entire wave is reflected; but with opposite senses so as to either cause
net cancellation of current for open circuit or cancellation of voltage for
short circuit at the boundary.  The continuation with a second line of either
higher or lower impedance is similar, but less extreme than are these two cases
respectively.

HTH
Kevin
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