Dave wrote:
So any attempts to measure the length of the line in
the black box, or figure out if it is a lumped circuit break the rules for
using it in the first place.

Ramo and Whinnery warn against even trying to calculate
the power dissipation inside the black box - something
that is regularly attempted on this newsgroup. The
calculated power dissipation inside a Thevenin equivalent
and a Norton equivalent can result in an infinite difference
even though they are both "equivalent".
--
73, Cecil http://www.w5dxp.com