wrote
Electrically,
antennas are very linear (I believe) even when their current
distribution along the element length is not.

I stopped posting when I realized we were discussing two different
things.

In a linear system, there only needs to be a straight line function
between the input and output. The actual signals on the input wouldn't
be very useful if only straight line functions were allowed on that
input. The current distribution along an antenna element length only
ever approximates a straight line. The only requirement for that
current to obey the rules for a linear system is that it be a
linear function of the source current and it is at every point.
--
73, Cecil http://www.qsl.net/w5dxp