[SI-LIST] Re: Transmission Line

Kevin,

Thank you for taking the time to explain the physical cause of 
reflections.  For folks like me who have been working with transmission 
lines for such a long time, its unfortunately too easy to forget 
something so basic.

     -Bill


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Kevin G. Rhoads wrote:
>>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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