Characteristic impedance equation - why is there a negative sign?

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sp2821

Joined Jan 8, 2014
23
Why is there a negative sign in below equation?

upload_2015-7-23_18-7-56.png

If I understand correctly, definition of characteristic impedance of a transmission line is a ratio of incident voltage wave to incident current wave OR reflected voltage wave to reflected wave current.

Since both reflected waves upload_2015-7-23_18-3-15.png and upload_2015-7-23_18-3-29.png are traveling in same direction,
shouldn't it be upload_2015-7-23_18-9-28.png?
 
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