Consider inductance. Each infinitesimal segment of a line has an L per
length, which includes both self and various mutual inductances to all its
nearby segments (of the same t-line). Together, they make up the net Lo per
unit length, for an infinite line.
Now truncate the line. You no longer have the mutuals off the truncated
end.
It is no longer a TEM wave near the ends of a line. All assumptions about
uniform Co and Lo are theoretically invalid. To an engineer's
approximation, it may still behave about the same; but not if you look
closely enough.
Regards,
Andy Ingraham
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