# RE: [SI-LIST] : Hi SI-gurus, One stupid question: Take a long (sa y 10 ft) long rod of perfect conductor (say copper) and connect one of i ts end to the +ve terminal of a battery through a switch. The other end of the battery is grounded to the earth. Now

From: Ingraham, Andrew ([email protected])
Date: Thu Mar 09 2000 - 05:55:05 PST

Um, that's one heck of a "subject" line!

> Does
>this mean that the impedance will be different when you put the rod
>parallel the the earth and when put perpendicular to the earth (because
>the distance from the earth and hence capacitance to the ground is
>different)?

When you say "transmission line" and "characteristic impedance," the first
assumption that comes to mind is that you mean a uniform two-dimensional
structure.

Generally, we only talk about impedances of transmission lines when the
problem is 2D (no variations in the 3rd dimension) or can be approximated by
a 2D geometry. When problem geometries aren't uniform, we may break them up
into shorter sections that are roughly uniform. But most folks would not
try to do this with your "perpendicular to the earth" case because it is
completely un-2D. The "transmission line" analysis breaks down; there is no
uniformity over the length of the line; the current paths are nowhere
parallel, no matter how much you break it down into smaller sections. It
looks more like an antenna than a transmission line. (After all, it IS an
antenna!)

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